A transistor is a semiconductor that allows current to flow through it under certain conditions, and interrupts current when other conditions are met. Transistors are usually used as switches or current amplifiers. You can test transistors with a multimeter that has a diode test function.
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Method 1 of 4: Understanding Transistors
Step 1. A transistor is basically 2 diodes sharing one end
This common end is called the base and the other 2 ends are called the emitter and collector.
- The collector accepts input current from the circuit, but cannot deliver current through the transistor until it is allowed by the base.
- The emitter sends current out into the circuit, but only if the base allows the collector to deliver current through the transistor to the emitter.
- The base acts like a gate. If a small current is applied to the base, the gate opens and a large current can flow from collector to emitter.
Step 2. Transistors can operate with junctions or field effects, but they come in two basic types
- NPN transistors use a positive semiconductor material (P type) for the base and a negative semiconductor material (N type) for the collector and emitter. In the circuit diagram, the NPN transistor shows the emitter with the arrow pointing outward.
- PNP transistors use N-type materials for the base and P-type materials for the emitter and collector. The PNP transistor shows the emitter with an arrow pointing inward.
Method 2 of 4: Setting the Multimeter
Step 1. Insert the probe into the multimeter
Plug the black probe into the common terminal and the red probe into the terminal to test the diode.
Step 2. Turn the selector knob on the diode test function
Step 3. Replace the probe tip with an alligator clip
Method 3 of 4: Testing If You Know Which Base, Emitter, and Collector
Step 1. Determine which ends are the base, emitter, and collector
The end is a round or flat wire that extends from the bottom of the transistor. On some transistors, all three may be marked or you can determine which end is the base by checking the circuit diagram.
Step 2. Clip the black probe to the base of the transistor
Step 3. Touch the red probe on the emitter
Read the display on the multimeter and see if the resistance is high or low.
Step 4. Move the red probe on the collector
The display should give the same reading as when you touched the probe to the emitter.
Step 5. Remove the black probe and clip the red probe to the base
Step 6. Touch the black probe on the emitter and collector
Compare the readings on the multimeter display with the readings previously obtained.
- If the previous readings are both high and the current readings are both low, the transistor is in good condition.
- If the previous readings are both low and the current readings are both low, the transistor is in good condition.
- If the two readings with the red probe are not the same, the two readings with the black probe are not the same, or the readings do not change when the probe is changed, the transistor is in bad condition.
Method 4 of 4: Testing If You Don't Know Which Base, Emitter and Collector
Step 1. Snap the black probe on one end of the transistor
Step 2. Touch the red probe to each of the other two ends
- If the reading shows high resistance when each end is touched, you've found the base (and you have a good NPN transistor).
- If the reading shows two different results for the other two ends, clip the black probe to the other end and repeat the test.
- After clipping the black probe on each of the three ends, if you don't get the same high resistance reading when touching the other two with the red probe, you have either a bad transistor or a PNP transistor.
Step 3. Remove the black probe and clip the red probe at one end
Step 4. Touch the black probe to each of the other two ends
- If the reading shows high resistance when each end is touched, you've found the base (and you have a good PNP transistor).
- If the reading shows two different results for both ends, clip the red probe to the other end and repeat the test.
- After clipping the red probe on each of the three ends, if you don't get the same high resistance reading when touching the two ends of each using the black probe, you have a bad PNP transistor.
