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1. HIGH CURRENT REGULATED 5V
Diagram 1.0 High Current Regulated 5V
Since the introduction of the 3-pin regulator ICs in the market, the topic of designing power supply circuits has become less and less discussed. However, the same new ICs limit the types of power circuits being built in hobby labs to around 1 ampere units. In addition, power stages are required, because high current (5A or 10A) voltage regulator ICs are very expensive. This is the main reason why hobbyists usually opt for a discrete solution. The idea of cascaded power stages are actually not that bad. For this reason, we have used discrete components for the circuit featured above.
This regulator circuit was designed for microcomputer systems and other applications that require pulse resistant power supply while delivering high current output at the same time.
The 723 IC used in this design is widely replaced by the new 3-pin regulators. However, the 723 IC is adjustable and its technical data are better in some points. It is configured in the circuit to output voltage levels from 2V to 7V. The needed power is produced by doubling the voltage level through a voltage doubler circuit, rectifying it, and regulating it with a 3-pin regulator IC.
This seemingly complicated technique is being applied here to keep the voltage level as low as possible at the secondary winding of the transformer. This makes it possible to keep the power dissipation at transistors T1 and T2 as low as possible too.
The heat sink for T1, T2 and T3 must be properly dimensioned. A heat resistance of around 2°C (C F) is recommended. For the same reason, the resistor values for R1, R2 and R2 must be produced by wiring resistors in parallel. For R1 and R2: connect two 0.33 ohms (5watts) in parallel for each. For R6: connect two 0.22 ohms (5watts) in parallel by a current output of 6 amperes; or connect three 0.22 ohms (5watts) in parallel by an current output of 8 amperes. As additional safety measure, solder these resistors on the pcb with some distance from the board.
If one desires to remove the voltage doubling, just remove the components D1, D3, C3, C4 and move the diode D2 to the positive DC line as shown by the dotted line in the Diagram 1.0. Additionally, the output voltage of the transformer must be increased, and the values of C1, C2, R4 and R5 adjusted appropriately.
This power supply's voltage output drops, by a load of 0.85 ohms, from 5.5 volts to 5.32 volts. That results to a current output of almost 8 amperes! The voltage drop is just 3.3%! The ripple DC voltage is only less than 28 mVeff at the output.
--------- Figure 1.0 Printed Circuit Layout
Diagram 1.1 High Current Regulated
5V (AC rectifier part)
------------ VOLTAGE MONITOR
Diagram 2.0 Voltage Monitor
The MC3424 IC used in this circuit can protect two channels from overvoltage levels.
Additionally, it can detect over- and undervoltage levels on a 5 volts power line. This makes it very useful as monitor for disk drives and microprocessor power supplies. Each channel of the IC has input and output comparators. The input comparators check the voltage on a 5 volts line. Inside the IC, a constant reference voltage of 2.5 volts is generated. This reference voltage is continuously fed to the non-inverting input of comparator 1 and inverting input of comparator 2.
When the voltage at the power line drops under 2.4 volts, the input comparator of channel one changes its state. Pin 6 becomes logic 0 and the red LED (D2) lights up. D2 can also be used to trigger an interrupt routine in a computer to prompt it to save its data and switch over to an emergency power supply. When the voltage at the power line rise above 6.2 volts, the input comparator of channel 2 changes its state. Pin 10 becomes logic 0. The thyristor TH1 fires and shorts the power line to the ground line. Depending on how the thyristor was connected to the power line (shown as dotted line and points X,Y, Z in Diagram 2.0), either the fuse (F) of the non-regulated line or the fuse (F1) of the regulated line will be busted, effectively shutting off the power line. The diode 1N4001 must be added to the regulator IC of the power supply.
If the power supply has a fuse already (shown as F in Diagram 2.1), replace F1 with a jumper wire and short points Y and Z in Diagram 2.0 with another jumper wire.
Diagram 2.1 Connecting the voltage monitor to the power supply being monitored.
----------- Figure 2.0 Printed Circuit Layout
36V TO 12V CONVERTER
--------- Diagram 3.0 6V to 12V Converter
This circuits generates a 12 volts output from a 6 volts source. It was originally designed to be used in old cars with 6 volt batteries to enable operation of 12 volt powered car radios. No matter what your actual application is, this circuit delivers 12 volts output at 2 amperes maximum.
The current flowing through L1 controls the IC1 that switches on and off at a rate of around 45 kHz. The capacitor C3 gets charged with the counter-inductive voltage from L1 through D1. The LC filter at the output (C4/L2) suppresses any voltage spikes that might appear.
The efficiency of the circuit is around 70%. It is even higher at lower current consumption levels.
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Figure 3.0 Printed Circuit Layout
---------------- 45V - 3A POWER SOURCE
Diagram 4.0 5V - 3A Power Source
For most applications, a 7805 3-pin regulator IC is very easy to use. However, the maximum current output from such unit is low, typically 1 ampere. This low current output is not enough for some applications. To overcome such limitation, the current can be amplified with the addition of a power stage such as the transistor T1 shown in this circuit. At lower current levels, the 7805 IC behaves at its normal function. When the current rises above 15 mA, the voltage drop at R5 becomes high enough to trigger the transistor T1. The transistor T3, on the other, protects transistor T1 in case of a short circuit. At currents above 3 amperes, the voltage drop at R1 is too high that transistor T1 conducts. This limits the base-emitter voltage of T1 and the output current cannot increase any further. Transistor T3 is connected in parallel with transistor T2. The LED1 lights on when the over-current protection kicks in.
Resistor R4 works as a current limiter for the voltage regulator IC1. Without the R4, the IC1 would be heavily loaded in case of a short circuit.
One catch with this circuit: The higher current levels can only flow when the input voltage is higher than 10 volts. In contrast, a single 7805 can function with only 8.5 volts input level. Furthermore, the circuit is not protected from prolonged periods of short circuit.
In constructing the circuit, it is very important to place the transistors T1 and T2 on a properly dimensioned heatsink. A heat resistance of around 3 K/W is recommended. The 7805 IC does not strictly require a heatsink. However, it is a good practice to mount it on a good heatsink too. Certainly, protecting it from high temperature levels would not hurt.
Figure 4.0 Printed Circuit Layout
------------- 5. SYMMETRICAL POWER SUPPLY
Diagram 5.0 Symmetrical Power Supply
The first part of the circuit on the left side looks typical with its rectifier diodes and filter capacitors C1 and C4. The voltage regulator ICs IC1 and IC2 together with the zener diodes D2 and D3 work to deliver a stable DC voltage of +/- 21.8 volts. Capacitors C2 and C5 stabilize the regulator ICs.
Further into the circuit, it becomes unconventional. Both op-amps together with the driver transistors are wired as DC voltage amplifiers. The non-inverting inputs are connected to +2.45 volts through voltage dividers R2/R5 and -2.45 volts through R7/R10. A regulated symmetrical voltage extracted from the voltage divider P1/R1/R9 and subtracted from the fixed +/- 2.45 volts. The negative from IC3 and the positive from IC4. This results to a symmetrical voltage outputs from +/- 4.9 volts to 18 volts. Capacitors C7 and C8 are stabilizing capacitors. The resistors R6 and R11 are pull-down resistors to preload the outputs in case of an empty load.
The accuracy of the voltage symmetry is dependent on the resistor values of the three voltage divider circuits. If there is a non-symmetry in the output, it is probably due to one of the regulator ICs with its 10% tolerance rating. In such case, adding a 5K trimmer in series with R1 or R9 is recommended. Adjust the trimmer until a symmetry is achieved.
This circuit can deliver a current output up to 100 mA. See page 22 for increasing the current output capacity.
------------ Figure 5.0 Printed Circuit Layout
Increasing the current output capacity of the symmetrical power supply:
- Replace the IC1 and IC2 with 1 ampere type voltage regulator ICs (7815 and 7915)
- Mount the new IC1 and IC2 on a separate heatsink
- Replace D1 and D4 with a 1 ampere bridge rectifier (remove them from the pcb)
- Use a 18V-0-18V (1VA) center tapped transformer to supply the AC voltage
- Replace the capacitors C1 and C4 with 4700 µF/45V capacitors
- Use a slow-blow 1 ampere fuse
- Connect the center tap of the transformer secondary with the ground line of the pcb
- Connect the AC pins of the bridge rectifier to both outer taps of the transformer secondary
- Connect the plus pin of the bridge rectifier to the plus pole of C1
- Connect the minus pin of the bridge rectifier to the minus pole of C4 These changes will result to a current output capacity of maximum 1 ampere for the circuit.
6. REGULATED POWER SUPPLY
---------------------
Diagram 6.0 Regulated Power Supply
The most popular method of voltage regulation is either to use a 3-pin regulator IC or the transistor/zener diode combination. These techniques are sufficient in many cases. However, their current output capacity is limited to a maximum of around 1 ampere.
To get higher currents, the regulator must be supported by a power transistor. This, however, affects the regulation characteristics negatively. Ready-made high current regulator ICs (e.g. 5 ampere or 10 ampere types) are very expensive and not easy to find.
The transistor-zener diode combination has its limitations too. Its regulation by changing loads and its ripple suppression are not very good.
The above described problems are absent in the featured power supply circuit.
Additionally, it is an affordable alternative to expensive high current regulator ICs. At first glance, the circuit looks like the typical transistor/zener diode combination. The decisive difference is in the built-in feedback. This technique reduces the 120 Hz ripple by about 50 dB. This reduction level is not possible in conventional designs.
The reference voltage is supplied by the TL431 (D1 in Diagram 6.0). Diode D1 controls the base of transistor T1 in such a way that the voltage at R4 is a constant 2.5 volts. This makes it easy to calculate the output voltage by using the following formula:
Uo = 2.5 (1+(p1+R2)/R4) in volts
This particular circuit is dimensioned to deliver a 12 volts output. For other output levels, the divider network R2/P1/R4 must be re-dimensioned. Thereby making sure that the current flowing through the divider network does not go below 1 mA.
The power stage is the MJ3001 power transistor. It guarantees a 1000 current gain by 5 ampere collector current. A base current of around 5 mA is needed to control the MJ3001.
Figure 6.0 Printed Circuit Layout
The current through D1 is at least 0.5 mA. The current through R1 is around 5.5 mA. The value of R1 can be calculated using the formula:
R1 = (Uin - Ube - Uo)/IR1
where:
Uin is the lowest possible voltage input Ube is the base-emitter voltage of T1 Uo is the voltage output IR1 is the R1 current (approx. 5.5 mA) In practice, a slightly higher value for R1 than the calculated value must be chosen. This is due to the fact that the published current amplification factor of MJ3001 is an average value. This also reduces the power dissipation at D1.
The MJ3001 must be mounted on a heatsink. Select a heatsink that is good enough to handle heat dissipation from the transistor running at least 2 amperes. If one decides to supply a purely DC voltage to the pcb, remove the bridge rectifier and connect the dc supply lines to B+ IN and B- IN respectively. On the other hand, if an AC supply is chosen, use the bridge rectifier and connect the transformer secondary windings to the AC IN connections.
If the circuit is used for 5 ampere current consumptions: Mount the power transistor T1 externally on a bigger heatsink. Replace C1 with a 10,000 µF/45V capacitor. If this capacitor does not fit on the pcb, mount it externally. If the consumption stays at 5 amperes on long periods of time or continuously, mount the bridge rectifier externally on a properly sized heatsink. It is also a good idea to use forced air cooling with a mini air blower unit.
----------------
Figure 6.1 Parts Placement Layout for the Regulated Power Supply
7. SINGLE CHIP POWER SUPPLY
Diagram 7.0 Single Chip Power Supply
The 1077APA IC from Alfa Micro is a fully integrated power supply circuit. It can convert AC voltages from 18Veff up to 276Veff into DC voltages. The chip contains a DC bridge rectifier and a switching amplifier stage. Using this chip, a compact, lightweight and inexpensive power supply can be constructed with a minimum of external components.
The maximum output current is 50 mA, which confines this power supply to low current applications. A single zener diode (D1 in Diagram 7.0) sets the output voltage to a maximum of 70 volts.
The IC goes through a charge and discharge process with each AC cycle. At the start of the cycle, an internal switch connects the capacitor C2 with the internal DC rectified voltage. C2 gets charged to a level set by the zener diode D1. Then the switch opens and stays open until the begin of the next half cycle, while the capacitor C2 delivers current to the load. C2 gets recharged every half cycle. The charging cycle starts again when the DC rectified voltage rises to 1 volt higher than the capacitor voltage of C2. The input frequency can be freely chosen between 48 Hz and 200 Hz. The switching frequency and therefore the charging frequency is always the double of the input frequency.
One obvious disadvantage of the circuit is the total lack of galvanic isolation between the AC mains power supply and the connected load. Encase the entire circuit with a plastic box and use switches with plastic shafts and knobs.
8. SIMPLE SWITCHING SUPPLY
Diagram 8.0 Simple Switching Supply
This simple single chip switching power supply uses an IC from National Semiconductor.
This design enables the construction of compact power supplies. The LM2675 is available in different output voltages: 3.3V, 5V and 12V. There is also a version with adjustable output voltage. When applied like the featured circuit above, the chip can deliver a maximum current of 500 mA.
The switching frequency of the IC is 260 kHz. This seemingly high frequency makes it possible to use small coils and capacitors in the application. The efficiency rate is high and the overall dimension of the finished product is small. Under normal circumstances, it is common to achieve an efficiency rate of 90%. In some optimized applications, a 95% efficiency can be achieved.
The chip contains internal protection circuits against very high current outputs and thermal overload. Other positive features of the chip include: soft-start, optional use of an external clock generator to synchronize several voltage converters, and minimize noise.
The featured circuit delivers an output current of 500 mA by a voltage output of 5 volts.
Diode D1 is a schottky type with Urev = >45V and maximum current of 3 amperes.
---------
Figure 8.0 Printed Circuit Layout for the Simple Switching Supply
Figure 8.1 Parts Placement Layout for the Simple Switching Supply
9. COMPACT SYMMETRICAL PS
--------------
Diagram 9.0 Compact Symmetrical PS
A symmetrical power is usually needed for microphone amplifiers, audio circuits, impedance converters, and most circuits that use opamps. These applications often need only low currents. The circuit featured above offers a symmetrical voltage supply with low current output. It is designed to be compact too.
By an output of +/- 15 volts, it delivers a current of 25 mA. This can also go as high as 100 mA for very short periods of time. By using an transformer with the appropriate secondary voltage, one can use the circuit to produce +/-5V, +/-9V, +/-12V, +/-18V, and +/-24V outputs. The last two voltage levels might prove to be very difficult in finding the correct regulator IC. Due to its compact size, the circuit can be constructed directly inside the application being powered.
One disadvantage of small main transformers is their relatively high internal resistance.
This internal resistance results to a big difference between standby output voltage and the voltage under load.
For example, the transformer used in this particular circuit delivers a 32 volts at the input of the regulator IC on standby. Under some circumstances, it is possible that the voltage at the input of the regulator IC is much higher than the allowed level. It is important to note that the upper limits for the regulator ICs are:
5V = 30V 12V and 15V = 35V 18V and 24V = 40V
One way to avoid a very high standby voltage output is to add resistors in parallel with the transformer's secondary winding. The resistors provide an initial load preventing a very high standby voltage. This resistor, however, must not be low ohmic because otherwise the capacity of the power supply would be very limited, and the resistor would become very hot. In selecting the value of the resistor, a few milliamperes flowing through it is often enough to pull the voltage below the critical level.
If the transformer is rated with 3.5 VA and the C1 and C2 are replaced with 100µF capacitors, the power supply can be used for current loads up to 55 mA.
---------------
Figure 9.0 Printed Circuit Layout for the Compact Symmetrical PS
Figure 9.1 Parts Placement Layout for the Compact Symmetrical PS
10. REMOTE SENSING REGULATOR
----------------
Diagram 10.0 Remote Sensing Regulator
In some applications, it is very important to keep the voltage level constant and highly independent from varying output current levels. These varying current levels can, however, cause voltage regulation problems in many cases. If the load is connected to the regulated power supply with short and high gauge wires, the regulator should not have any problem in keeping the voltage output constant. On the other hand, if the load is connected far away from the power supply with long wires, the voltage drop at the connecting wires could be considerable. This voltage drop will not be sensed by the regulator resulting to a wrong voltage level at the point of the load. Furthermore, this results to an unstable voltage at the load although the voltage at the regulator output is stable.
To solve this problem, the circuit feature above applies a remote sensing technique. The regulator has extra sensing wires connected directly to the load. Since the current flowing through these sensor wires are very low, the voltage drop on these wires are very low too.
This way, the regulator circuit can sample the voltage level directly at the load.
-------------
Figure 10.0 Printed Circuit Layout
Figure 10.1 Parts Placement Layout
However, most regulated power supplies do not have this remote sensing capability. It is also very difficult to upgrade existing power circuits with such technique. The circuit featured here, on the other hand solves the problem efficiently using the L200 regulator chip.
Output points + and - are the normal power supply outputs.
Points SENSE+ and SENSE are the remote sensing connections. The output voltage can be calculated using this formula:
U_out = 2.77V (I + Rp1/R1)
Using the resistor R2, the maximum current limitation can be calculated with this formula:
Imax = 0.45V/R2
This R2 must be rated to support the high power dissipation levels. The maximum input voltage allowable for the circuit is 40 volts. It delivers a maximum current of 2 amperes.
The IC must be mounted on a properly sized heatsink to protect it from high temperature levels.
11. CURRENT MONITORED SUPPLY
----------
Diagram 11.0 Current Monitored Power Supply
Here is a reliable power that indicates the actual current being consumed by the load.
Voltage regulation is done by the IC L200. Current monitoring is indicated by the LEDs D2D5 representing current values of 0.5A, 0.8A, 1.3A, and 1.8A. The circuit can supply currents up to 2 A and its output voltage is variable from 3 volts up to 30 volts.
Normally, LED5 is red colored to serve as a warning signal that the maximum current has been reached. The output voltage can be varied through P1. The circuit has a power-on delay composed of R3 and C1. Transistor T1 is the emergency shut-off.
-------------------
-------------------
Figure 11.0
Parts Placement Layout
2SA 970 2SC3622 2SA1136 2SC3245 2SA1137 2SC3245A 2SC3248
-------------- L200
-------------
Figure 11.1 Printed Circuit Layout
Parts List:
R1 = 3.9K C1 = 100µF/50V
R2,R17,R18,R19,R20 = 10KOC2 = 0.1/50V R3 = 1.2K C3 =
0.33/50V R4 = 1K D1,D2,D3,D4,D5 = LED R5,R8,R11,R14 = 68O D6,D7,D8,D9 = 1N4148 R6 = 0.47O/5W D10,D11,D12,D13 = 1N4148 R7,R10,R13,R16 = 22K T1,T3,T5,T7,T9 = 2SC3622 R9 = 0.33O/5W (2SC3245)(2SC3248) R12 = 0.15O/5W T2,T4,T6,T8 = 2SA970 R15 = 0.39 O /5W (2SA1136)(2SA1137) IC1 = L200
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12. VOLTAGE DOUBLER
Diagram 12.0 Voltage Doubler
This circuit produces a voltage that is twice its voltage supply. This is useful when a higher voltage level is needed out of a single but lower voltage power supply. Since the current consumption levels are low in such cases, the circuit can be built with minimal number of components.
The electronic circuit is basically a square wave generator using the common 555 timer IC. It is followed by a final stage made of transistors T1 and T2. The actual doubler circuit is made of D1, D2, C4, and C5 components.
The 555 timer IC works as an astable multivibrator and generates a frequency of about 8.5 kHz. The square wave output drives the final stage made of T1 and T2. This is how the doubler works: by a low amplitude of the signal, transistor T1 blocks while T2 conducts.
The minus electrode of the capacitor C4 is grounded and charges through D1. By a high amplitude of the signal, transistor T1 conducts while T2 blocks. However, capacitor C4 cannot discharge because it is blocked by D1. The following capacitor C5 is therefore charged with a combined voltage from C4 and the power supply (12V input).
On standby, the circuit delivers around 20 volts. The maximum load must not exceed 70 mA. The actual output voltage is around 18 volts giving an efficiency rating of 32%. On lower current ratings, the voltage is higher. If a stable voltage level is desired, a 3 pin voltage regulator IC can be added at the output. The regulator IC's own current consumption must be added to the total current consumption which must not exceed 70 mA.
13. PS REGULATOR
------------- Diagram 13.0 PS Regulator
Most modern power supply regulators are constructed using ICs and work more precisely. However some applications do not really require the precision of an IC. Sometimes discrete components are enough to provide the needed stability. As a hobbyists you are more interested in constructing circuits which present more challenge and chance to understand its working principles. Circuits with discrete components are more fun to work on than ready made, ready to use ICs.
The circuit featured here is designed to give an output voltage of 12V. What is special with this circuit is that it has a current limiter circuit and a constant current source. The current output is limited up to 0.5A. The constant current source (this function is done by a single FET T1) delivers a maximum of 18 mA to the power transistor T1. The output voltage of this power supply is variable through the potentiometer P1. The potentiometer must be wired outside of the circuit board. Be careful in connecting the potentiometer P1 to the circuit board. Never interchange the terminal connections. See Fig. 13.2 for wiring layout.
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2SB781 2SD1177 2SD1684 2N3819
------------------- Figure 13.0 Printed Circuit Layout
Figure 13.1 Parts Placement Layout
----------------- Figure 13.2 External wiring layout. Take note of the
connection of P1. Never interchange its terminal connections 2SA3112 2SA2675
---------------- 2N5486 SUBSTRATE 2N3823 2N5397 2N5398
14. VARIABLE POWER SUPPLY
------------- Diagram 14.0 Variable Power Supply
A stable power supply with an adjustable output voltage from 5 volts to 30 volts can be easily constructed with the regulator ICs UA78G or UA79G. These ICs differ from the common three-terminal regulators (which deliver fixed voltage levels only) since their output voltages are adjustable by a voltage level at their control inputs. The maximum current delivered by these ICs is 1 ampere.
The unregulated voltage must be at least 5 volts higher than the desired output level to maintain stability. The input voltage however must not exceed 40 volts. The maximum dissipation of the IC is 15 watts. It has a built-in electrical and thermal overload protection. The circuit featured in the diagram is designed to give a maximum voltage level of 28 volts. If P1 is replaced with 25K potentiometer, the regulator can deliver up to a maximum of 30 volts. Capacitors C1 and C2 stabilize the IC and they must be connected as close as possible to the IC terminals.
To calculate the correct voltage value of the Formula for Vts transformer's secondary, use this formula:
where Vts = the voltage output of the transformer's V_in
V_ts=secondary winding and:
Vin= is the needed voltage 0.7 input of the IC.
The IC UA79G delivers negative voltage level. Take note that the two ICs have different terminal connections.
Figure 14.0 Printed Circuit Layout for the Variable Power Supply
Figure 14.1 Parts Placement for the Variable Power Supply
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15. STABLE POWER SUPPLY
----------- Diagram 15.0 Stable Power Supply
The excellent voltage regulation of this circuit is achieved by feeding a constant current to the zener diode D7. The constant current source is composed of T1,D2,R1,R2. The output voltage can be selected between 3 volts and 47 volts. The output voltage is equal to Vz +2(0.7V). Vz is the zener voltage. The value 2(0.7V) comes from the base-emitter voltage of the two transistors T2 and T3. These two transistors work together as emitter follower.
When the circuit is to be used for devices which consume currents higher than 1 ampere, a 2N3055 transistor must be added to it as shown in Diagram 15.1. Transistors T3 and eventually the additional transistor T4 must be properly heatsinked.
----------------- Diagram 15.1 Additional 2N3055
Bottom view 2N3055
Figure 15.0 Printed Circuit Layout
------------ Figure 15.1
Parts Placement Layout Parts List:
R1,R3 = 10K R2 = 2700 R4 = 100O 2SC3622 2SA1286R5 = 470 2SC3245 2SB1288C1,C2 = 470µF/25V 2SC3245A 2SB1305C3,C4 =
100µF/25V 2SC3248 2SB1306D1,D2 = 1N4001 D3,D4,D5,D6= 1N4004 D7 = Zener diode(see text) T1 = 2SA1286 T2 = 2SC3622 T3 = 2SD826
-------------------- 2SA143 2SD826 2SD794
----------------- 2SD1682 2SD1685
16. STABLE ZENER VOLTAGE
Diagram 16.0 Stable Z-Voltage Source
The zener voltage of a zener diode depends on its bias current and can vary from type to type and from capacity to capacity. It presents a problem for circuits which need a highly stable reference voltage. The solution is to make the bias current constant so that the zener voltage also remains constant.
The circuit featured here does just that. It uses a 6.2 volt zener diode. Zener diodes with different zener voltages can also be used as long as the values of R1 R4 are changed accordingly.
---------- Figure 16.0 Printed Circuit Layout
----------- Figure 16.1 Parts Placement Layout
2SA970 2SA1136 2SA1137 2SC3622 2SC3245 2SC3248
17. 3-WATT AMP POWER SUPPLY
------------ Diagram 17.0 3-Watt Amplifier Power Supply
The power stage of this circuit is composed of two transistors T1 and T2 in darlington configuration. The regulation is controlled by the zener diode Z1. The voltage of the zener diode must be 1 volt higher than the desired output voltage. Table 17.0 shows the component values for two voltage levels.
Table 17.0 Parts List:
R1= 270 ohms 12V 17V C1= 2,200µF/25V electrolytic C2= 100µF/25v electrolytic R1 270W 680W T1= 2N3055 Z1 13V 18V T2= 2SC2SC3622 (2SC3245) Tr1 12V 18V Tr= 220V/12V, 2A T2 2SC3622 2SC3245A D1D4= 2A/100V diode or bridge rectifier with similar specifications.
--------------
Figure 17.0 Printed Circuit Layout
------------ Figure 17.1 Parts Placement Layout
18. STANDBY SUPPLY
Diagram 18.0 Standby Supply
This circuit provides continuous power supply for memory circuits during sudden power break downs. When the main line loses power, the relay's magnetic field collapses. Since the contact is a normally-closed (NC) type, it connects the standby battery cells to the regulator circuit thereby providing continuous power.
--------------- Figure 18.0 Printed Circuit Layout
------------------- Figure 18.1 Parts Placement Layout
19. DC REGULATOR
------------- Diagram 19.0 DC Regulator
This circuit regulates a DC power output. It has a very wide application range. It can be used to control the speed of a motor, a pump, a toy train, the brightness of a LED or lamp, etc. Practically, it can be used in any application that uses a regulated DC power with pulse width modulation (PWM). It was originally used as a jumbo LED dimmer.
The circuit works this way: the A1 opamp functions as a square wave generator. At its noninverting input is a by product triangle wave signal (almost triangle anyway). The IC A2 following it functions as a simple comparator. A reference voltage is fed to the inverting input of the A2 IC via the potentiometer P1. The output of A2 is a square wave signal with a constant frequency of around 200 Hertz. This signal has a variable pulse width from 0 to 100%. The P1 sets the trigger point of the pulse. The transistor T1 works as the actual regulator by switching a relatively high current with a maximum of 5 amperes. The power supply voltage must be between 5 volts (minimum) and 30 volts (maximum). Take note that the lower the supply voltage is, the lower the efficiency of the circuit becomes.
20. SYMMETRICAL AUX PS
------- Diagram 20.0 Symmetrical Aux PS
This simple circuit provides a symmetrical power supply derived from the AC output of a transformer's secondary winding without center tap. It is very useful in applications where both positive and negative lines are needed but a center tapped transformer is not available.
The circuit is basically made of a bridge rectifier with additional components that function as the negative line. Take note however that this negative line can deliver less current than the positive line.
The resistors R1 and R2 are initial loads. They are added to make sure that the negative line is active even if the circuit has no load attached to its output lines. This trick is applied here because, otherwise, no negative output will be produced when the positive line has no load. In case you need the negative line to deliver more current than the positive line, just reverse the circuit.
To do this, just reverse the polarities of all diodes and electrolytic capacitors. It means all diodes and electrolytic caps, no exemption. The bridge rectifier will deliver more power in the negative line in such case.
The actual circuit can handle AC inputs up to a maximum of 18 volts AC. If you need a higher voltage level, change the voltage ratings of the diodes and capacitors to at least the double of the maximum AC input voltage. To select the correct diodes, see the appropriate table at the end pages of this guide.
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Figure 20.0 Printed Circuit Layout
Figure 20.1 Parts Placement Layout
Figure 20.2 External wiring layout for the symmetrical power supply. Take note that the negative line delivers less current than the positive line.
21. 0.1V-50V POWER SUPPLY
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Diagram 21.0 0.1V-50V Power Supply
This power supply circuit is highly stabilized that its output voltage will drop by only 0.005% even though the load changes from 0 to 100 %. Another excellent capability is that the output voltage will change only by 0.01 % if the input voltage fluctuates. The capability of the circuit to be adjusted from 0.1V up to 50V is due to the application of opamp IC CA3130 in the circuit. Transistor T4 raises the output voltage to higher level, and at the same time it separates the lower level opamp from the high level of the output voltage. The reference voltage is supplied by IC1. It is a temperature compensated transistor array with 5 transistors. Four of these transistors are used as reference diodes, and the fifth one sets the output impedance of the reference source.
The reference voltage is set through P1. The opamp CA3130 compares the reference voltage at its minus input to the output voltage at its plus input. The output voltage passes first through a voltage divider before it is fed into the plus input of the opamp. Transistors T1 and T2 work as darlington pair and amplifies the current. Transistor T3 functions as current limiter. The current limit is adjustable through P1, and the lowest current limit is 0.6 ampere. Once potentiometer P1 is set at maximum, current limiting is disabled.
22. DC TO DC CONVERTER
----- Diagram 22.0 DC to DC Converter
In some applications, one needs a voltage level that is higher than the one immediately available, like in programming EPROMS: a 25V is necessary, but common digital circuits have only 5V and 12V lines. This converter circuit solves that problem. It converts a DC voltage to a much higher level. To be able to obtain the desired output level, use Table 22.1 to know the correct value of Rx.
Table 22.1 Value of Rx L1 is a small coil with 85 turns of 0.2mm magnet wire
around a ferrite core. Its total inductance must be around 100 µH.
---------
Figure 22.0 Printed Circuit Layout
Figure
22.1 Parts Placement Layout
23. 3-AMPERE POWER SUPPLY
---------- Diagram 23.0 3-Ampere Power Supply
This is a compact power supply that delivers a stable voltage and currents up to 3 amperes.
The circuit is very conventional and its voltage output can be varied from 1.25 volts to 25 volts.
The main module is the LM350 IC which integrates a voltage regulator and a power stage.
It also has a built-in overload protection which activates at 30 watts of power dissipation.
The voltage output is set by connecting the “adj" pin of the IC to the voltage divider made of R1 and P1. The output voltage can be calculated using the following formula:
1.25 V x (1 + P1/R1)
where the P1 value is between 0 and 2.5 kiloohm. Capacitor C1 is a common ripple filter while capacitors C2 and C3 improves the regulation. The diodes D1 and D2 serves as protection for the regulator IC when the IC output is turned off. Resistor R1 is 120 ohm.
This ensures that the minimal load current for the IC (around 3.4 mA) is high enough to maintain good performance.
One thing that is most important in building the electronic circuit: provide adequate heatsink for the LM350 IC. The power dissipation at the IC is very high, around 85 watts.
----------------
LM350K
Figure 23.0 Printed Circuit Layout
Figure 23.1 Parts Placement Layout
Consider that the heat resistance of a TO-3 package is 1.5°C/W and the maximum allowable temperature is 150°C. If a heatsink is used with a heat resistance of 1.5°C/W, then the total heat resistance is 4°C/W. At 30 watts dissipation and 25°C outside temperature, the resulting internal IC temperature is 145°C. Once this dissipation level is reached, the internal protection activates shutting down the IC.
One way to avoid high dissipation levels is to use a lower voltage transformers when the needed output voltages are low. To put it simply: if you are using the circuit to supply voltages around 9 volts, do not use a 25 volts transformer but use a lower voltage one instead (e.g. 12 or 15 volts).
Figure 23.2 External Wiring and Heatsink Layout
24. 0-50V/0-2A POWER SUPPLY
Diagram 24.0 0-50V/0-2A Power Supply
This power supply uses two LM10 ICs which has an integrated reference source. The circuit is designed to be short-circuit proof with variable voltage and current outputs.
The output voltage can be linearly controlled through P1 and the current can be linearly controlled through P3. The maximum current limit is set by P2 and is adjusted only once.
The maximum current limit is 2A. The maximum output voltage on the other hand is set by a fixed-value resistor connected in parallel to potentiometer P1. Using a fixed value resistor for this purpose helps in reducing noise at the output voltage.
The voltage regulation works this way:
The minus input of IC1 is connected to the output voltage and the plus input is connected to the combination R1/P1. This IC controls the base of transistor T1 so that any voltage difference between the two inputs are equalized. The collector current of this transistor causes a voltage drop at R6 which controls the output voltage through the darlington stage.
Pin 1 of LM10 chip is the reference output. Once the output has stabilized to the desired level, the difference between the two inputs is 0.
The voltage level at the junction of R1/P1 is then equal to the voltage level at the minus input of this opamp. Once the reference level is changed by adjusting the potentiometer P1, a voltage difference between the two inputs is sensed by the opamp. The opamp reacts by changing the control voltage until the difference voltage is pulled back to 0. The current is controlled by another LM10 chip. To stabilize the current, a reference level taken from potentiometer P3 is compared with the voltage at resistor R11. The current flows through this resistor. Since LM10 is not a fast opamp, the current is limited through T3. T3 limits the current to the maximum of 2 amperes.
------------ Figure 24.0 Parts Placement Layout; Figure 24.1 Printed
Circuit Layout
The minimum output is dependent on the load therefore R12 is added to the circuit as a fixed load. Using 470 ohm for R12 gives a minimum output of 0.4 volts. The maximum output voltage is fixed by R1b and can be set up to 50 volts. The value of R1b used in the circuit sets the maximum output to 45 volts. When it is desired to raise the maximum output to 50 volts, the following components must be replaced:
Transformer secondary 42V/2A, C5= 4,700µF/80V.
----------- 2SD1528 2SB764
------- 2N3055
25. SUPPLY FOR OPAMPS
Diagram 25.0 Supply for OPAMPS
This power supply circuit is designed to support a large number of opamp ICs like in mixer circuits and sound synthesizers. The regulation is done by the IC LM325. The output voltage is +/- 15 volts symmetrical. The output current can be up to 2 amperes.
Capacitors C9, C10, and C11 are tantalum types. The two power transistors T1 and T2 must be properly heatsinked. Figure 25.0 Printed Circuit Layout
Figure 25.1 Parts Placement Layout
------------ 2SD1049 2SD1840 2SD1841 2SD1842
----------------
26. PS w/ DISSIPATION LIMITER
Diagram 26.0 PS with Dissipation Limiter
An ordinary power supply normally suffers from high dissipation levels. It happens when the output voltage is set at low level while the input level remains at maximum. The resulting difference voltage then "fries" the regulator circuit's power transistor continuously. This shortens the life expectancy of the transistor. Also, the heat build up in such regulators is very high that an internal air blower is almost always needed.
Bottom view 2N3055
The power supply featured here does not know this kind of problem since it has an automatic dissipation limiter. The actual limiter circuit is composed of the components T1, T2, D5, D6, D7 between the transformer and the regulator stages. The triac T2 must be selected properly to handle the maximum current and dissipated power that will load on it.
You can use the table at the end pages of this guide to select the needed triac.
--------------
2SB781 2SD1177 2SD1684
The potentiometer P1 in the regulator stage is the normal output voltage adjuster. The output can be varied from 5 up to 50 volts.
The triac T2 and the power transistor T4 are to be installed on heatsinks outside the circuit board. The potentiometer P1 must also be installed outside the circuit board. Make sure to wire these external components correctly. Take note that two terminals of the potentiometer P1 are shorted together. See Fig. 26.2 for details.
Figure 26.0 Printed Circuit Layout
----------
Figure 26.1 Parts Placement Layout
Figure 26.2 External wiring layout
----------------
27. 5A DELAYED POWER-ON
Diagram 27.0 5A Delayed Power-On
This power supply has a short power-on delay. This short delay has two advantages: First, the initial short high current present during switching a power supply with high value capacitors and high efficient transformers is avoided. Second, this short delay prevents the over-voltage fluctuations produced during switching from reaching the load circuits. This is very important for load circuits which are highly sensitive to voltage spikes.
The regulator can be operated in either fixed output mode or variable output mode. Switch S1 selects between the two modes. Position B is the fixed output mode and the power supply delivers a well regulated 12V. Position C is the variable mode and the output can be varied from 12 volts to 15 volts. The output voltage can be varied through P1. The maximum current output of 78H12 is 5A and the maximum short circuit current is around 7A. The IC must be heatsinked properly.
------------
Figure 27.0 Printed Circuit Layout; Figure 27.1 Parts Placement Layout
28. TTL POWER SUPPLY MONITOR
-----------------
Diagram 28.0 TTL Power Supply Monitor
This simple to construct circuit monitors the 5-volt level TTL power line and gives a signal whether the supply voltage is outside or within the necessary range or "window".
The heart of the circuit is a low-current integrated window comparator. The center of this window is set at 2.5 volt +/- 0.005 volt by the band-gap reference diode D1 which is connected to pin 2 of LTC1042. The width of this window must be 20% (+/- 10%) of the reference voltage.
The reference voltage is reduced by 25% through resistors R4 and R5 and fed to pin 5 (width/2 input) of the IC. The monitored voltage is then fed to pin 2 (window center input) so that the green LED (D2) will light up when the voltage is within the desired range.
Otherwise, the red LED (D3) will light up signaling that the voltage is out of range.
---------- Table 28.0 Time vs Voltage
29. 6V to 12V CONVERTER
---------- Diagram 29.0 6V to 12V Converter
In some situations, one needs a 12 volts power supply but has only a 6 volts battery available. In such a case, a voltage converter such as this featured circuit comes very handy. This circuit is a simple converter made with an IC from SGS with several additional external components. The IC is a TDA2003 but it can be replaced with a TDA2002. The cost of building the circuit should be low enough to justify constructing it instead of modifying the entire equipment setup to work directly with a 6 volts power supply. The two principles of simplicity and low cost are applied in this electronic circuit.
It only uses two low cost AF amplifiers and functions properly without the need for a transformer.
The IC1 opamp functions as a stable power multivibrator. Its oscillation frequency is determined by the capacitor C3. Its oscillates at around 4 kHz at standby and increases in a loaded condition up to around 7 kHz. The output of the IC2 opamp is identical to the IC1 oscillator signal but in the opposite phase.
When the output of IC1 is at zero (saturation point of the output transistor to the ground), the capacitor C4 charges via the diode D1 up to the power supply level minus the voltage drop at D1. When the IC1 swings to the opposite direction, its output becomes positive.
The output voltage from IC1 adds up to the voltage stored at the capacitor C4 forcing the diode D1 to stop conducting. Capacitor C5 then charges via the diode D2 to a voltage that is double than the power supply level. Due to the opposite phase of IC2, the negative pole of C5 is connected to the ground through the IC2 at this moment. By the next swing of IC1, its output is again at zero potential. At this moment the capacitor C4 is recharged while at the same time the voltage at C5 is „lifted" up to the voltage level present at IC2.
Capacitor C5 delivers its voltage to the output capacitor C7 via the diode D3.
Theoretically, the output voltage has been multiplied by three. In practical application, however, the output voltage available from the capacitor C7 is lower than the theoretical value. This voltage level is also dependent on the load. Data gathered from testing the circuit showed that a 6 volts supply (from a battery) produces a voltage output of around 18 volts with no load to the circuit. This level sinks to about 12 volts when a load is applied that consumes a current of around 750 milliamperes. When the load current is about 400 mA, the voltage at the capacitor C7 is about 14 volts. These values are enough to power a standard car radio.
---------------
Figure 29.0 Parts Placement Layout
------------- Figure 29.1 Printed Circuit Layout
As mentioned in the preceding page, the theoretical output could reach the triple of the supply voltage. To guard against unnecessary voltage increases at low current consumptions, a limiter stage was added to the circuit composed of a 15 volt zener diode and a darlington transistor T1/ T2. This stage caps the output voltage to about 14.2 volts.
To filter out ripple from the output, the capacitor C8 was also added. This helps prevent the hum signal from being noticed on radio or audio devices.
In constructing the circuit, attach the ICs to a common heatsink close to the pcb. The transistor must be attached to a separate heatsink. The two ICs have built-in protection against short circuits and thermal overload. The TDA2002 can be used in place of TDA2003. The TDA2003 however, has better characteristics. To get a much higher current output from the circuit, the capacitors C4, C5 and C6 must be increased to 2200 µF.
Figure 29.2 External Wiring Layout
30. VERSATILE POWER SUPPLY
-----------
Diagram 30.0 Versatile Power Supply
This circuit is called versatile power supply. It delivers 7.7 volts output converted from either a single 1.5 volt battery or from a 6.8V AC transformer. The output is higher than the input voltage. The voltage increase is done by the integrated chip TL496CP. Not only that, both an AC input and a battery can be connected simultaneously as the power source.
The AC input is controlled by a series regulator while the battery is controlled by a switching regulator.
As long as the series regulator delivers the necessary output, the switching regulator remains disabled. When a NiCad cell is used as a battery, it will be charged simultaneously by the AC input. Once the AC input is disconnected or when the AC line goes off, the battery automatically takes over the function of supplying the needed 7.7 volts. A really versatile power supply.
-------- Figure 30.0
Printed Circuit Layout
Figure 30.1 Parts Placement Layout
----- Figure 30.2 External wiring layout for the versatile
power supply
Parts List:
IC:
IC1 = TL496CP
Capacitors:
C1 = 470µF/15V
C2 = 100µF/16V Misc:
L1, L2 = 47µH/1A
D1 = 1N4004
Battery = 1.5 Volts
31. ROBUST 5V SUPPLY
----------------- Diagram 31.0 Robust 5V Supply
This 5V supply circuit is very durable and insensitive to voltage fluctuations. It can block voltage spikes up to 80V. Very short current interference does not affect the function of the IC. This circuit is specially designed to support microprocessor circuits.
The maximum current output is 500 mA. The voltage drop between the input and the output voltages is around 0.6V. The output voltage is 5V+/-2.5%. The IC is also protected from thermal overloads.
Parts List:
Resistors:
R1, R3 = 10K
R2 = 3300
Capacitors:
C1 = 0.1/50V
C2,C3 = 100µF/50V
Diode:
D1 = 1N4148 IC:
IC1= TEA7034
------------ Figure 31.0 Printed Circuit Layout; Figure 31.1 Parts Placement
Layout
------------------ 2SB1142 2SB1143 2N6414
------------------ TEA7034
32. VOLTAGE CONVERTER
------------ Diagram 32.0 Voltage Converter
This circuit converts an input voltage of 512 volts to a higher level of 1530 volts.
This is specially helpful in mobile applications where power supply levels are commonly limited to 12 volts supplied by batteries.
The circuit uses the IC L4973 as a flyback-voltage converter. The choke coil is 40µH/2A.
Capacitors C2 and C1 suppress voltage spikes. The maximum output current depends on the difference between the input and output voltages. This maximum is around 100 mA.
The ripple voltage is relatively low. The standby current is around 8 mA and the efficiency is about 70%.
The maximum output current is 100mA!
33. FUSE MONITOR
------------- Diagram 33.0 Fuse Monitor
This circuit monitors a DC fuse. Its LED lights continuously when the fuse is intact but blinks if the fuse is busted. The featured circuit is designed for 12 volts but can be modified for other voltages. To use the circuit for 6 volts, divide all resistance values by two. For 24 volts, double all resistance values.
The circuit is made of an astable multivibrator (AMV) and a LED driver. The biggest part of the circuit is before the fuse with the exception of R5. The AMV functions all the time as long as the power is on. The output of AMV is connected to the driver circuit via D2.
As long as the fuse is intact, current flows to the base of T3 via R3 and D1 and the LED lights. Once the fuse gets blown, no more current can flow to the base of T3. In this case, the AMV takes over the control of the LED driver and the LED blinks.
The circuit consumes around 25 mA. Most of the current is consumed by the LED. If one decides to use the circuit in battery operated modules, it is highly recommended to use a high efficiency LED and increase the value of R7 accordingly.
----------------
2SC734
34. LOW DROP REGULATOR
Diagram 34.0 Low Drop Regulator
----------------
2SC3622 2SC3245 2SC3248
This power supply regulator has the following characteristics:
a) very low voltage drop of less than 1V. b) negligible standby current from 20 to 30 µA. c) variable but very stable output.
------------------- bottom view MJ2955 2SB722 2SA1147
These characteristics make the circuit practical in some applications compared with the 3 terminal IC regulator which has high voltage drop and standby current consumption.
Specially in battery operated applications, the discrete regulator featured here is highly recommended.
The circuit is basically a series regulator that uses a normal LED to produce the reference voltage. The given component values deliver a variable output from 2V up to 8V. If you intend to increase it up to 16V, change R4 with 220 kilo-ohms. Eventually, you need to increase the value of P1. The maximum current that can be delivered by the regulator circuit depends on the type of the series transistor used and the difference between the input and output voltages.
Figure 34.0 Printed Circuit Layout
------------- Figure 34.1 Parts Placement Layout
Figure 34.2 External wiring layout of the low drop regulator. Take note that
two terminals of the potentiometer P1 must be shorted together. Transistor
T1 must be properly heatsinked.
35. AMPLIFIED REGULATOR
Diagram 35.0 Amplified Regulator
Normally, 78xx voltage regulators can only deliver up to 1 ampere. This circuit can deliver currents up to 10 amperes by adding a current amplifier to the common 78xx series voltage regulators. The current amplifier can be a single or two power transistors in parallel depending on the needed maximum current output.
Table 35.0 shows the necessary component values for every maximum current output. The transistors must be heatsinked to minimize voltage drop caused by increasing temperature.
When two transistors are used in parallel (additional T1° transistor as shown in Diagram 64.1), a resistor R1° must be added to each emitter terminal (see Diagram 35.1). The PCB layout for the higher current version is shown in figures 35.2 and 35.3. The transistor must be installed in a separate heatsink.
--------------- Diagram 35.1 Additional T1
-------------------
------------ Figure 35.0 Printed Circuit Layout; Figure 35.1 Parts Placement
Layout
-------------
25.0
Table 35.0 Component values
Figure 35.2 Printed Circuit Layout for the
higher current version
---------------------
Figure 35.3 Parts Placement
Layout for the higher current version 2SB1064, 2SB826, 2SB867, 2SB868, 2SB869, 2SB920, 2SB870
----------------- 2SB638 2N6285 2SB639 2N6286
36. 78XX REGULATOR MONITOR
------------ Diagram 36.0 78XX Regulator Monitor
Although 78XX 3-terminal regulators are known to be stable, problems sometimes arise in their applications. The input voltage of these ICs must be at least 3 volts higher than the output voltage. But sometimes these ICs breakdown due to overloads or internal defects.
The monitor circuit featured here continuously monitors a 78XX IC regulator whether if functions according to its specifications or not. The value of R1 in the circuit is computed for a 7805 regulator. If you use the monitor for other regulator IC types, change the R1 value by using the formula shown on the following page:
The input and output terminals of the regulator are connected to the inputs of IC1 which is configured as a difference amplifier. When the input voltage of the 78XX IC is low, the output of IC1 increases enough to charge C2 and turn on T1. In this case, the LED lights up. C2 holds the LED for about 10 mS longer before it finally turns off so that even short voltage collapses can be registered.
Figure 36.0 Printed Circuit Layout for the 78XX Regulator Monitor
Figure 36.1 Parts Placement Layout
Formula for R1
V_in R1= (-----) R2 V_regulator
37. EMERGENCY LAMP
Diagram 37.0 Automatic Emergency Lamp
This electronic circuit automatically turns on a lamp during brownouts. Its power source is a NiCad battery that is being charged continuously by the main power line when there is no brownout. The diagram shows the simplicity of the circuit.
The electronic circuit works this way: the voltage from the step down transformer is rectified by diode D1 and filtered by C2. The NiCad battery gets charged with about 100 mA via the diode D2 and resistor R1. The NiCad battery must have a capacity of at least 2 Ah to tolerate the charging rate. In a normal situation, the base voltage of T1 is positive in relation to its emitter due to the voltage drop at diode D2. Transistor T1 in this case does not conduct and the lamps stay unlit.
When the main power supply fails during a brownout, the charging current is interrupted.
In this case, a current flows from the base of T1 via resistor R2 which triggers the transistor to conduct and the two lamps light up. When the main power returns, the charging current flows again through D2 and the transistor turns the lamps off.
The "TEST" button S1 is used to check the function of the circuit. If the secondary coil of the transformer delivers a higher voltage level, replace the R1 with a higher value resistor to avoid exceeding the maximum charging current allowed for the NiCad battery being used.
38. OVERVOLTAGE CROWBAR
Diagram 38.0 Overvoltage Crowbar
Simple but very effective electronic circuitry. This very simple circuit can protect a main circuit which is sensitive to overvoltages. The first circuit uses a zener diode in parallel to the power line of the protected circuit or device. Once the output of the power supply increases due to a malfunction, the excess voltage will be routed to ground by the zener diode. If the voltage will increase further, the total current consumption will exceed the capacity of the fuse. The fuse will blow at this moment and the current supply is broken.
The zener voltage of the zener diode must be 1 or 2 volts higher than the correct supply voltage and must have a higher current rating than the fuse. Of course the maximum allowable supply voltage of the protected circuit must be taken into account. For example if the supply voltage must be 15 volts and the maximum allowable voltage of the protected circuit is 18 volts, then a zener diode of 18 volts must be used.
------------
Figure 38.0.0 Printed Circuit Layout; Figure 38.0.1 Parts Placement
----------------------
Diagram 38.1 Overvoltage Crowbar Nr. 2
The second circuit can be used for higher current ratings. The low current zener diode triggers the SCR when the supply voltage exceeds the safe level. AS a result, the SCR shorts the fuse to ground thereby blowing it. The resistor R limits the trigger current and the zener current flowing through the diode. The approximate value of R can be found by using this simple formula:
R = Zener diode's voltage - SCR's trigger voltage SCR's trigger current Of course, you have to select the right SCR type for your particular application. Use the tables on the end pages of this guide.
Figure 38.1.0 Printed Circuit Layout; Figure 38.1.1 Parts Placement Layout
39. CONSTANT CURRENT
Diagram 39.0 Constant Current
There are different ways to construct a constant current source using a voltage regulator.
This time we use the µA723. This IC is sometimes labeled as TBA281. The excellent stability and temperature coefficient of this IC is well known. It is actually designed as a voltage regulator but can be used to regulate current as well. Two circuit designs are featured here - one which uses a 2N3055 power transistor and another which uses a MJ2955 transistor. The constant current I can be found with this formula:
The load is connected to pin 7 of the IC. The 2.2V maximum current load that can be handled I = safely by the IC is around 150 mA. Add a high R1 power transistor to the circuit to increase it current handling capacity.
Circuit 1 of Diagram 39.0 uses an NPN transistor while circuit 2 uses a PNP transistor.
Figure 39.0 Printed Circuit Layout for Diagram 39.0 (w/ NPN transistor); Figure
39.1 Parts Placement Layout for Diagram 39.0 (w/ NPN transistor)
---------------------
Figure 39.2 Printed Circuit Layout for Diagram 39.1 (w/ PNP transistor)
Figure 39.3 Parts Placement Layout for Diagram 39.1 (w/ PNP transistor)
40. DC MOTOR SPEED REGULATOR
------------- Diagram 40.0 DC Motor Speed Regulator
This speed regulator uses a single IC LM1014 to control the speed of a DC motor. It senses the increase in the motor-current when the rotation of the motor slows down due to a load. The IC then increases the motor voltage so that the original speed is recovered.
Potentiometer P1 varies the speed of the motor.
41. SYMMETRICAL POWER SUPPLY
Diagram 41.0 Symmetrical Power Supply
The compact 5 pin L165 IC generates a stabilized symmetrical power supply from a single asymmetrical power supply. The output voltage is however, half of the input voltage. One only needs to add the ripple filter capacitors C1,C2, C3 and C4 and some resistors for setting the symmetry. In constructing the circuit, place the capacitors C1 and C2 as close to the IC as possible. On the other hand, place the capacitors C3 and C4 close to the output jacks. Make sure that the circuit lines on the pcb are properly dimensioned to handle high current levels. Current levels up to 3 amperes can flow through the circuit. Additionally, provide a proper heatsink for the L165 IC.
The IC can also be viewed as a voltage amplifier. It amplifies the voltage appearing at the junction between R1 and R2.
One can also replace the IC with TCA1365. However, when using the TCA IC, pins 3 and 4 have to connected together. Also, connect a 220 pF capacitor between pins 5 and 6.
------------- Figure 41.0 Printed Circuit Layout
------------ Figure 41.1 Parts Placement Layout
----------------- L165
Pin configuration
1 = + IN
2 = - IN
3 = B- (- VS case)
4 = OUT
5 = B+ (+ VS)
42. 2N3055 DARLINGTONS
------------------- Diagram 42.0 2N3055 Darlingtons
Very high capacity is the business of these circuits. The featured transistor combinations can be used for power supply regulators or final amplifiers that require high collector voltage, high current, high power dissipation capacity, and high current amplification characteristics. Both circuits use a 2N3055 transistor for the NPN and the PNP functions respectively. Using an NPN transistor for the PNP function may seem unusual if not incorrect to you at first glance. Through the special configuration, however, the second circuit functions as a PNP transistor. Just consider the whole circuit as a single high capacity transistor. In fact, the three solder points are labeled E,B,C. They correspond to the normal terminals of a single transistor.
The gain of the circuits (both AC and DC) is around 1.5 million. The maximum power dissipation at 25°C is 115 watts. The maximum collector voltage is 60 volts and the maximum collector current is 15 amperes. The voltage drop at the NPN combination is around 2 volts. The voltage drop at the PNP combination is around 3 volts.
Figure 42.0 Printed Circuit Layout for the NPN function
-------------- Figure 42.1 Parts Placement Layout for the NPN
function Figure 42.2 Printed Circuit Layout for the PNP function
-------------- Figure 42.3 Parts Placement Layout for the PNP function
43. VOLTAGE MONITOR
-------------- Diagram 43.0 Voltage Monitor
This circuit provides an optical signal through a blinking LED to show whether the voltage level being monitored is lower or higher than a reference level.
The heart of the circuit is a 741 opamp which functions as comparator and oscillator simultaneously. The reference voltage is coupled to point B and the voltage being monitored is coupled to point A. As long as the voltage level at the non-inverting input is higher than at the inverting input, the output of 741 is 12V and the LED is off.
Once the voltage level at point A drops below the reference level, the output of 741 becomes 0 and the LED lights. This time capacitor C1 charges through R3 and the output of the IC1. At a certain charging level, the diode D1 turns off and the capacitor discharges to the minus input of IC1. The IC1 output increases back to 12 V and the LED turns off.
The capacitor continues to discharge until D1 turns on again and the process is repeated.
The LED therefore blinks as long as the monitored voltage is lower than the reference voltage.
-------- Figure 43.0 Printed Circuit Layout;
Figure
43.1 Parts Placement Layout
44. TEMPERATURE MONITOR
-------- Diagram 44.0 Temperature Monitor
High power amplifiers usually dissipate large amount of heat. It is therefore practical to monitor the level of heat from its power transistors and heatsinks and if needed, to automatically turn off the amplifier to avoid damage to the vital components. This can be done by using simple temperature monitors such as the circuit featured here. Since it is not required to monitor the temperature by strict degree resolution, this simple monitor will work very well.
The electronic circuit works by comparing the voltage drop of a „cold" diode (T1) with the base-emitter voltage of a „warm" transistor (T2). The transistor must be attached physically closest to the heat source. Ideally it should be attached to the heatsink of the power transistors. The diode (T1) must be positioned away from the heat source to ensure that it is always at room temperature. The circuit „measures" the heat difference between the transistor and the diode. The diode (T2) is connected to the power supply via R1. The base of T2 is connected to the anode of the diode via the potentiometer P1. The LED D1 and resistor R2 is connected in series to the collector of T2. Transistor T2 should not conduct as long as the temperature level being monitored is below the set threshold. The base-emitter voltage of T2 decrease by 2 mV per °C. Once this voltage is below the level set by P1, the transistor T2 conducts and the LED begins to light up. By a slow increase of temperature one will notice a dim light from the LED. This way, the user gets an „analog" indication of the temperature.
The values of R1 and R2 are dependent on the power supply voltage. The values can be computed using the following formulas:
R1 = (Ub/V - 0.6) / 5 kilo-ohm
R2 = (Ub/V - 1.5) / 15 kilo-ohm
For example: by 12 volts power supply, the R1 is 2.2 kilo-ohms while R2 is 680 ohms. The maximum current consumption of the circuit when the LED lights up is 20 mA. Take note that the transistor T2 must not become hotter than 125°C.
------------------
Figure 44.0 Parts Placement Layout
Figure 44.1 Printed Circuit Layout
Figure 44.2 External wiring of the T2 temperature sensor and placement to the heatsink being monitored
45. POLARITY INVERTER
------ Diagram 45.0 Polarity Inverter
This circuit is used to reverse the polarity of a supply line. A negative potential is sometimes necessary in computer or battery powered circuits. Since the current consumption for such a need is mostly low, a single IC can be used as a polarity converter.
The TL497A IC is originally designed for downward or upward transformation applications but is used here as polarity converter. The design does not need any transformer or bridge rectifiers.
Coil L1 can be between 100 and 500 µH.
The output voltage (Vout) can be found -1.2 with this formula: Vout =
R2 The load current must not exceed 500 mA.
------------ Figure 45.0 Printed Circuit Layout for the Polarity
Inverter
Figure 45.1 Parts Placement Layout for the Polarity Inverter
46. ELECTRONIC FUSE
2SA1705 2SA1706 2SA1137 2SA970 2SA1136 2SA1137 2SB764 2SB1116
--------------
Diagram 46.0 Electronic Fuse
Of course you know what a fuse is and how it functions. This circuit is the electronic version of a fuse. The electronic fuse cuts off the current (like a normal fuse does) and lights up a LED (not with a normal fuse) when a certain current level is exceeded.
The advantage of this fuse in comparison to a conventional one is that it can be reset and re-used (not to mention the cute lighting of the LED). The circuit functions this way: The current flowing to the connected load flows first through R1. Once the voltage drop at R1 reaches 0.5 volts, transistor T1 conducts and switches off T2 and thereby cutting off the current going out of the circuit. The electronic fuse is at this time "blown" and can only be re-used after pressing S1. You then have enough time to check what is wrong with the device connected to the fuse before resetting it.
The value of R1 in the circuit is selected so that the circuit will "blow" when the current is around 500 mA. The circuit cannot deliver currents above 500 mA due to limitations of the transistors used. If you want to change the limit of the circuit change the value of R1 using this formula:
R1= 0.4(I) Maximum output current is 500 mA.
47. HEATSINK MONITOR
--------- Diagram 47.0 Heatsink Monitor
The heat dissipation from power transistors and diodes in electronic devices are typically absorbed by heatsinks and further dissipated into the environment. The heatsink dimensions are based on the maximum allowable crystal temperature of the semiconductor as published by the manufacturer. However, the actual temperature reached by the heatsink is not clearly known. Many people test the heatsink temperature with their fingers. It seems that as long as it does not burn their fingers, it is accepted to be safe.
Obviously, this method is not accurate that is why the electronic circuit featured here was designed.
This heatsink monitor signals via three LEDs when the temperature exceeds two boundary levels.
When the heatsink temperature is below 50 60 degrees centigrade (122 140 degrees fahrenheit) the green LED lights. The yellow LED signals that the temperature is within 60 and 70 degrees centigrade (140-158 degrees fahrenheit). The red LED signals that the temperature has reached beyond 7080 degrees centigrade (158-176 degrees fahrenheit). At this high point a relay can be triggered to break the power supply or otherwise protect the electronic device by any other means (e.g. turning on a cooling fan).
As shown in the diagram above the electronic circuit is made of two opamps that are set as window comparators. The input voltage level is set by the thermo-sensor IC LM335.
Figure 47.0 External Wiring Diagram. The LM335 heat sensor is clamped to the
heatsink being monitored.
This voltage level increases linearly by a factor of 10mV/°centigrade. If this voltage is below the voltage level set by the potentiometers P1 and P2, the opamp outputs are almost at ground potential (0 volts) and the green LED lights up. If this voltage is within the set boundaries, the output of A1 is "high" and the yellow LED will light up. If this voltage exceeds the voltage level set by P2, both A1 and A2 outputs are "high" and the red LED lights up while the yellow LED turns off. At the same time, the load at T1 is reduced to prevent further increase in the heatsink temperature.
To set the desired temperature levels: put the thermo-sensor LM355 together with a thermometer into a pot of water which is being heated slowly. Turn P1 to its lowest point and turn P2 to its maximum point. The switching point from green LED to yellow LED (5060°C or 122-140°F) is set with P1. The switching point from yellow LED to red LED is set with P2. Watch the thermometer while the water is being heated and adjust the potentiometers accordingly. Once the desired temperature boundaries are set, attach the thermo-sensor to the heatsink.
48. COMPUTER CONTROLLED POWER REGULATION
---------- Diagram 48.0 Computer controlled power regulation.
This computer interface can control an electrical load like a lamp, electric drill, etc. in 255 steps. The maximum power handling capacity of the circuit is 400 watts. The power regulation is done by controlling the voltage level at the connected load (see Diagram 48.1).
Optocouplers are used to transmit the computer control to the power line part of the circuit. With this technique, the computer is electrically and galvanically isolated from the main power control circuit. This isolation is very important to protect the computer from possible damages caused by high voltage peaks. The phase angle of the voltage that triggers the triac is dependent on the value of the byte delivered by the computer. The higher the byte value, the higher the phase angle becomes, and the shorter will be the time when the triac remains conducting.
Calibration: Turn P1 while the input of the counter sees FFhex (255dec) until the voltage output is 0. The data 00hex produces the same output as the data FFhex. Full power is delivered by data 01hex. The triac must be rated twice the maximum load consumption.
Connect to Diagram 38.0
-------------------- (From Diagram 38.0)
Diagram 48.1 Computer Controlled Power Regulation 2SA 970 2SC3622 2SA1136
2SC3245 2SA1137 2SC3245A 2SC3248
-------------------
TIC206 TIC216 49 CURRENT ALARM
-------- Diagram 49.0 Current Alarm
This electronic circuit is used to monitor the current flow of a device. Once the current flowing through D1 and D2 drops significantly, transistor T1 conducts and the relay closes. The relay then switches on whatever alarm is connected to it.
This circuit has a very wide application area. For example you can monitor exhaust blowers or water pumps. Once the blower or pump motor breaks down, the alarm turns on.
Two PCB layouts are available for this circuit. If you use the first PCB layout, you must install the transistor on a heatsink near the PCB. On the second PCB, you can install the transistor directly on the PCB board. Use a U-shaped heatsink which can be sandwiched between the transistor and the PCB.
----------- Figure 49.0 Printed Circuit Layout #1 for the Current
Alarm; Figure 49.1 Parts Placement Layout #1 for the Current Alarm
----------------
2SC2124 2SC2125 2SD621 2SD838
Note: Bottom view Parts List:
Resistors: Transistors:
R1a= 470 ohm
T1= 2SC2124
Capacitors: Replacements:
C1= 0.22/400V ceramic
2SC2125, 2SD621, 2SD838 C2=
10µF/63V electrolytic
Diodes:
D1, D2= 1N5401
D3,D4,D5,D6,D7 = 1N4004
------------
Figure 49.2 Printed Circuit Layout #2 for the Current Alarm
Figure 49.3 Parts Placement Layout #2 for the Current Alarm
50. BATTERY LINE BREAKER
Diagram 50.0 Battery Line Breaker
This circuit is specifically designed to automatically turn off any battery operated device when its user has forgotten to switch it off within a preset period of time. This helps enormously in preserving the battery's life.
How does the electronic circuit work? When the device connected to this circuit is turned on (S1), the capacitor C1 gets a voltage of +9 volts through the diode D1. Since C1 is discharged at the beginning, this voltage goes through to the gate of T3 making both T2 and T3 conduct. The connected device is now turned on as in a normal case. Capacitor C1 then slowly charges via resistor R2. After two or three minutes, the gate voltage of T3 is much reduced to a level that T3 turns off. At this time, T2 turns off the power output to the connected device.
The transistor T1 functions as a quick discharger of capacitor C1 when the user manually switches off the device. It works this way: When the switch S1 is opened (turned off), the base of T1 receives a voltage from C1 via R1 and R2. T1 conducts thereby discharging the capacitor C1. The circuit is again ready for the next switching on. Without this add-on circuit, the user would wait for another minute or two before he can use the device.
Using the circuit is simple. To build it in the device, the positive line of the power (battery) to the device must be cut and the circuit (S1 and output) connected in series to this positive line. The ground line of the circuit must be connected to the minus line of the device.
This circuit can handle current consumptions up to 150 milliamperes. Originally, it was used to control a multimeter device.