An NPN transistor is electrically equivalent to two diodes with connected cathodes.
When an electronic circuit fails, it is not always readily apparent which component is to blame. If there are no obvious signs -- such as a smoldering or burnt component -- you will have to test each component in the circuit to make sure each is functioning properly. Active components like transistors require testing with special equipment. A diode tester is necessary to check a bipolar junction NPN transistor to verify it is functioning properly.
Instructions
1. Turn on the digital multimeter. Set the measurement scale to "Diode Test," or turn the scale selector to the triangle-shaped icon.
2. The base lead is usually the center leg on a transistor.
Place the red multimeter probe on the transistor's base lead. If the base lead is not clearly marked on the transistor, consult the specification sheet to determine which transistor leg is the base lead.
3. Place the black multimeter probe on the transistor's collector lead. Check the multimeter display. If the multimeter displays a "fail" message, remove both probes from the transistor legs and discard the transistor. If the multimeter displays a "pass" message, remove the black probe lead and proceed to the next step.
4. Place the black multimeter probe on the transistor's emitter lead. Check the multimeter display. If the multimeter displays a "fail" message, remove both probes from the transistor legs and discard the transistor. If the multimeter displays a "pass" message, the transistor is in working order.
Tags: base lead, multimeter displays, message remove, multimeter probe, multimeter probe transistor