CHAPTER 16 ■ TRANSISTOR SWITCHES
Testing a Transistor When You Don’t Have the Datasheet
If you don’t have access to a transistor’s data sheet, then you’ll need to try various combinations of insertions
into the multimeter’s transistor test socket.
- Turn the multimeter dial to hFE (see Figure 16-2) or the appropriate function for
your meter. - Try each of the combinations of the NPN holes. On my multimeter, the transistor
can fit into the socket with EBC or BCE (E for emitter, B for base, C for collector).
Then, I can flip the transistor in the opposite direction and get CBE and ECB. This
allows for all combinations with either the base or the collector in the middle. - If you encounter the correct combination, the meter should display a number
between 10 hFE and 600 hFE, with the extremes being more atypical. Numbers
below 10 hFE or above the maximum of the meter usually signify the transistor is
installed incorrectly. - After trying the NPN combinations, try each of the combinations of the PNP holes.
- One of the connections in the NPN or PNP holes should have resulted in a gain
between 10 hFE and 600 hFE. The other combinations should have displayed
values below 10 hFE or above 600 hFE. The set of holes that worked tells you
whether the transistor is NPN or PNP, and which lead is the emitter, base, and
collector.
Even if you have the datasheet for a transistor, trying the other combinations of holes can indicate that
the transistor is damaged if it gains too much when installed in the wrong holes. Often, a damaged transistor
won’t provide useful gain despite being installed correctly in the multimeter’s transistor socket.
If the transistor’s gain isn’t within the reasonable range for any of the holes, it may be because:
- the transistor wasn’t fully inserted into the test socket
- the transistor isn’t a bipolar semiconductor (perhaps it’s field-effect)
- the transistor is damaged
- the transistor is designed for really high gain
- the transistor is designed for really low gain
Testing with a Multimeter that Has a Diode Test
If your multimeter doesn’t have a transistor test socket, you can still determine the transistor type (NPN or PNP)
and which lead is the base. For this experiment, you can pick either a PNP (like the 2907A) or NPN transistor.
I’ve produced test results for both types, so that you can compare your transistor to my samples.
- Turn the multimeter dial to diode test mode and prepare the test probes like you
were going to test an LED (see Figure 16-4).