Introduction:
Sooner or later you’ll encounter an unknown transistor to which you cannot find a datasheet for. Here’s a quick and simple tester you can build on perfboard or you can simply wire it up temporarily on your breadboard to identify those pesky pins!
The circuit:
Explanation:
- R2 pulls the base down so it’s not left floating whilst SW1 remains open.
- If the transistor is correctly connected and it’s of NPN type, when pressing the button the base would be forward biased and the LED should turn on brightly. Otherwise it may not turn on or it will be barely visible, this means C and E have been swapped.
- For PNP once power is applied the LED should turn on by itself if C and E are correct, because the base is pulled to ground by R2, if the button is pressed the base would be lifted from ground and the LED should turn off.
- If swapping C and E on both cases does not yield a conclusive result, you can assume this BJT is damaged or you incorrectly identified the base.
Perform your initial tests with known good devices. Make sure you swap C and E on purpose to visualize the outcome.
You may place an ammeter between VCC and C instead of an LED. You should normally get around ~10mA across it with a general purpose NPN as long as R1 is 330 Ohms, with a PNP this value would be nearly halved due to the pull-down resistor being an order of magnitude bigger than the pull-up.
However due to the nature of this circuit the resistor values are not critical, just respect (roughly) their ratios and you’ll be fine.
Test procedure:
NPN
- Identify the tra
nsistor base using your DMM in DIODE mode. The base is going to be the anode of both “diodes”. (See Fig:1) – If you can’t identify both diodes, this may not be a BJT or it’s been damaged; throw it away. - Connect base to the connector B from the transistor-tester and the rest to C and E (doesn’t matter which)
- Apply power, nothing should happen. Press the button, if the LED barely lights up or it doesn’t light up at all then the pin you connected on C may actually be E, if it lights up brightly then your assumption of C may have been correct.
- Turn C and E around even if you’ve got a result and go back to 3) to rule out a faulty device. If the transistor is shorted it would conduct either way around. With a good and properly connected transistor the LED should only turn on when the button is closed and should remain OFF for the rest of the time.
PNP
Identify the transistor base using your DMM in DIODE mode. The base is going to be the cathode of both “diodes”. (See Fig:2) – If you can’t identify both diodes, this may not be a BJT or it’s been damaged; throw it away.- Connect base to the connector B from the transistor-tester and the rest to C and E (doesn’t matter which)
- Apply power, if the LED lights up brightly then your assumption of C and E may have been correct.
- Press the button, the LED should go OFF, otherwise swap C and E and go back to 3) — If the results are inconclusive you may have a defective transistor. With a good and properly connected transistor the LED should only turn on when the button is open and should remain OFF for the rest of the time.
Conclusion:
With just a few parts from the junk box you can identify your unknown BJTs and test known ones for YES/NO fault finding. This circuit won’t identify damaged junctions; for that you’d need a curve tracer.
However it may come in handy if you’re constantly recycling components from old devices!
Have fun and don’t forget to share your findings with the community! — Likewise if you have any old datasheets don’t hesitate to scan them!
Cheers.