Electronic systems with transistors can be tested with an ohmmeter. You can remove transistors from their sockets and perform some simple tests. Determine which transistor lead is the emitter and which is the collector (the base is always in the center) by referring to a semiconductor data reference manual. Perform checks for shorts and opens and also polarity (whether the transistor is "positive negative positive" or "negative positive negative").
Things You'll Need
- Test leads
- Semiconductor data reference manual
- Dynamic transistor tester (optional)
Simple Transistor Testing
Identify the transistor emitter and collector by referring to its identification number in alpha-numeric code in the semiconductor data reference manual.
Turn your ohmmeter to the diode position. For all PNP transistors, connect the black (-) lead to the base and Red (+) lead to the emitter and then to the collector. Read both as forward based diodes. Your reading will be between 0.45 and 0.9 VDC for a good transistor.
Reverse your meter leads and check to see that both PN junctions do not conduct. Your ohmmeter will indicate open or an open circuit and display a reading of OUCH or OL.
Read the resistance from emitter to collector and check for an open circuit. Reverse leads and verify collector to emitter open circuit as well. A short circuit can exist from emitter to collector even if the individual PN junctions work properly.
Conduct a dynamic transistor test in a special tester designed to measure the emitter current gain (amplification factor). This parameter is also called Hfe or Beta. (See Reference 2) Sometimes a transistor becomes sensitive to temperature and can be detected by changing the ambient temperature and watching for failure. Direct replacement is another test.
Tips & Warnings
- A transistor should be removed from a printed circuit prior to testing as other components in the circuit may affect its readings.
- Care should be taken to avoid harming a good transistor by adjusting temperature externally via a soldering iron or freeze spray.
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