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Testing Solid-State Components
1. Testing a Diode
1. Connect the ohmmeter leads to the diode. Notice if the meter indicates continuity through the diode or not.
2. Reverse the diode connection to the ohmmeter. Notice if the meter indicates continuity through the diode or not. The ohmmeter should indicate continuity through the diode in only one direction.
NOTE: If continuity is not indicated in either direction, the diode is open. If continuity is indicated in both directions, the diode is shorted.
2. Testing a Transistor
1. Using a diode, determine which ohmmeter lead is positive and which is negative. The ohmmeter will indicate continuity through the diode only when the positive lead is connected to the anode and the negative lead is connected to the cathode.
2. If the transistor is an NPN, connect the positive ohm meter lead to the base and the negative lead to the collector. The ohmmeter should indicate continuity.
The reading should be about the same as the reading obtained when the diode was tested.
3. With the positive ohmmeter lead still connected to the base of the transistor, connect the negative lead to the emitter. The ohmmeter should again indicate a forward diode junction. NOTE: If the ohmmeter does not indicate continuity between the base collector or the base-emitter, the transistor is open.
4. Connect the negative ohmmeter lead to the base and the positive lead to the collector. The ohmmeter should indicate infinity or no continuity.
5. With the negative ohmmeter lead connected to the base, reconnect the positive lead to the emitter.
There should, again, be no indication of continuity.
NOTE: If a very high resistance is indicated by the ohmmeter, the transistor is "leaky" but it may still operate in the circuit. If a very low resistance is seen, the transistor is shorted.
6. To test a PNP transistor, reverse the polarity of the ohmmeter leads and repeat the test. When the negative ohmmeter lead is connected to the base, a forward diode junction should be indicated when the positive lead is connected to the collector or emitter.
7. If the positive ohmmeter lead is connected to the base of a PNP transistor, no continuity should be indicated when the negative lead is connected to the collector or the emitter.
3. Testing a Unijunction Transistor
1. Using a junction diode, determine which ohmmeter lead is positive and which is negative. The ohm meter will indicate continuity when the positive lead is connected to the anode and the negative lead is connected to the cathode.
2. Connect the positive ohmmeter lead to the emitter lead and the negative lead to base #1. The ohmmeter should indicate a forward diode junction.
3. With the positive ohmmeter lead connected to the emitter, reconnect the negative lead to base #2. The ohmmeter should again indicate a forward diode junction.
4. If the negative ohmmeter lead is connected to the emitter, no continuity should be indicated when the positive lead is connected to base #1 or base #2.
4. Testing an SCR
- Using a junction diode, determine which ohmmeter lead is positive and which is negative. The ohmmeter will indicate continuity only when the positive lead is connected to the anode of the diode and the negative lead is connected to the cathode.
- Connect the positive ohmmeter lead to the anode of the SCR and the negative lead to the cathode. The ohmmeter should indicate no continuity.
- Using a jumper lead, connect the gate of the SCR to the anode. The ohmmeter should indicate a for ward diode junction when the connection is made.
- NOTE: If the jumper is removed, the SCR may continue to conduct or it may turn off. This will be deter mined by whether or not the ohmmeter can supply enough current to keep the SCR above its holding current level.
- Reconnect the SCR so that the cathode is connected to the positive ohmmeter lead and the anode is connected to the negative lead. The ohmmeter should indicate no continuity.
- If a jumper lead is used to connect the gate to the anode, the ohmmeter should indicate no continuity. NOTE: SCRs designed to switch large currents (50 amperes or more) may indicate some leakage current with this test. This is normal for some devices.
5. Testing a Triac
1. Using a junction diode, determine which ohmmeter lead is positive and which is negative. The ohm meter will indicate continuity only when the positive lead is connected to the anode and the negative lead is connected to the cathode.
2. Connect the positive ohmmeter lead to MT2 and the negative lead to MT1. The ohmmeter should indicate no continuity through the triac.
3. Using a jumper lead, connect the gate of the triac to MT2. The ohmmeter should indicate a forward diode junction.
4. Reconnect the triac so that MT1 is connected to the positive ohmmeter lead and MT2 is connected to the negative lead. The ohmmeter should indicate no continuity through the triac.
5. Using a jumper lead, again connect the gate to MT2. The ohmmeter should indicate a forward diode junction.
Identifying the Leads of a Three-Phase, Wye-Connected, Dual-Voltage Motor
FIG. 1 Standard terminal markings for a three-phase motor.
FIG. 2 Low voltage connection.
The terminal markings of a three-phase motor are standardized and used to connect the motor for operation on 240 or 480 volts. FIG. 1 shows these terminal markings and their relationship to the other motor windings. If the motor is to be connected to a 240 volt line, the motor windings are connected parallel to each other as shown in FIG. 2. If the motor is to be operated on a 480 volt line, the motor windings are connected in series as shown in FIG. 3.
As long as these motor windings remain marked with the proper numbers, connecting the motor for operation on a 240 or 480 volt power line is relatively simple. If these numbers are removed or damaged, however, the leads must be re-identified before the mo tor can be connected. The following procedure can be used to identify the proper relationship of the motor windings:
1. Using an ohmmeter, divide the motor windings into four separate circuits. One circuit will have continuity to three leads, and the other three circuits will have continuity between only two leads (Figure #1).
Caution: the circuits that exhibit continuity between two leads must be identified as pairs, but do not let the ends of the leads touch anything.
2. Mark the three leads that have continuity with each other as T7, T8, and T9. Connect these three leads to a 240 volt, three-phase power source (FIG. 4).
(Note: Since these windings are rated at 240 volts each, the motor can be safely operated on one set of windings as long as it is not connected to a load.)
FIG. 3 High voltage connection.
FIG. 4 -- T7, T8, and T9 connected to a three-phase, 240 volt line.
3. With the power turned off, connect one end of one of the paired leads to the terminal marked T7. Turn the power on, and using an AC voltmeter set for a range not less than 480 volts, measure the voltage from the unconnected end of the paired lead to terminals T8 and T9 (FIG. 5). If the measured voltages are unequal, the wrong paired lead is connected to terminal T7. Turn the power off, and connect another paired lead to T7. When the correct set of paired leads is connected to T7, the voltage readings to T8 and T9 will be equal.
4. After finding the correct pair of leads, a decision must be made as to which lead should be labeled T4 and which should be labeled T1. Since an induction motor is basically a transformer, the phase windings act very similar to a multiwinding autotransformer.
If terminal T1 is connected to terminal T7, it will operate similar to a transformer with its windings connected to form subtractive polarity. If an AC voltmeter is connected to T4, a voltage of about 140 volts should be seen between T4 and T8 or T4 and T9 (FIG. 6).
If terminal T4 is connected to T7, the winding will operate similar to a transformer with its windings connected for additive polarity. If an AC volt meter is connected to T1, a voltage of about 360 volts will be indicated when the other lead of the voltmeter is connected to T8 or T9 (FIG. 7).
Label leads T1 and T4 using the preceding procedure to determine which lead is correct. Then disconnect and separate T1 and T4.
5. To identify the other leads, follow the same basic procedure. Connect one end of one of the remaining pairs to T8. Measure the voltage between the un connected lead and T7 and T9 to determine if it is the correct lead pair for terminal T8. When the correct lead pair is connected to T8, the voltage between the unconnected terminal and T7 or T9 will be equal.
Then determine which is T5 or T2 by measuring for a high or low voltage. When T5 is connected to T8, about 360 volts can be measured between T2 and T7 or T2 and T9.
6. The remaining pair can be identified as T3 or T6.
When T6 is connected to T9, a voltage of about 360 volts can be measured between T3 and T7 or T3 and T8.
FIG. 5 -- Measure voltage from unconnected paired lead to T8 and T9.
FIG. 6 -- T1 connected to T7.
FIG. 7 -- T4 connected to T7.
Ohm's Law Formulas
Standard Wiring Diagram Symbols
Electronic Symbols
