Transistor testing

If you are seeing .02 v it is as if the transistor is shorted. Try testing in both directions with the meter set as an ohm meter. I assume you are using a digital meter which usually will not have enough current/voltage to bias the transistor into conduction. It will probably show shorted or very near zero ohms in both directions. An analog meter will usually bias the transistor into conduction so it will show low resistance in one direction and if the leads are reversed , a high resistance.

Seems that you are using the meter like a diode tester. That is fine.

For any type of a a bipolar transistor put one lead on the base. Then put the other lead on the emitter, then the collector. It should show around .2 to .4 volts if the older germanium and .6 to .9 if silicon. Then reverse the leads and do the same . The meter should show an open circuit or OL . Then put the leads on the emitter and collector. They should show open OL on in both directions.

A common transistor will test like 2 diodes in series or back to back with the base as the common connection point.

The meter I am using is an old Fluke77 digital from RS about 30 or 35 years ago, and I have tried x5 BC547 tansistors. Using the voltage selection, I get 0.0v with negative lead on the collector and positive on the Emitter, and swapping round I get 0.25v Using the Ohm selection, I get OL in both directions, and the same using the diode selection with the negative lead on the base of the transistor

I don't have an analogue meter now.

Thanks

Well, then your transistors look fine as far as you could test them.

The voltage test is useless, a transistor does not generate a voltage. What you might see is the diode of the B-E (or C-B) rectifying noise from the environment. Useless as an indicator. The ohms range test is also useless, it normally test with 0.2V which is not enough to bias the diode and will give an OL reading in both directions. As you properly described.

You _MUST_ use the diode testing range! The B-E test should be about 0.55V with + on B side, and OL in the other direction. Same for the B-C test: about 0.55V with + on B side and OL in the other direction. The C-E test should be OL in both directions.

Regards, Arie

I may have misunderstood what you said about using the voltage selection. The diode selectionis what I would have thought you were using. That puts a voltage on the probes, then reads how much voltage is dropped across the leads so to speak. The way I remember the first posting was 0.025 volts. It could have been a typo and you ment the

If you get a voltage (number showing up on the meter readout) with the leads in one direction and the OL in the other direction while in the other direction going base to emitter and then base to collector, and OL in both directions emitter to collector, the transistor is probably good.

The main thing is not the total number,but a large difference in each direction with the exception of collector-emitter which should be very large resistance in both directions or OL in the diode poaition.

You must put your meter in diode-test mode, (on the fluke 77 it looks like that's also continuity beeper mode) it should say OL with the leads unconnected and 0.00V with them toucheing eaco other. then do the above measurements.

I'm guessing operator error, they should still be good after only a few years.

Putting the meter in diode-test mode,it reads OL when leads are unconnected and 0.00v when touching each other. Then connecting the positive to the Emitter and negative lead to the Base the meter beeps, then reads 0.666, the same reading when the positive is connected to the collector.

?? (Over

that means the tr is dropping 0.666v as each junction conducts. Black lead is +ve on diode test so you have an npn tr behaving normally.

NT

Thanks, and by accident I found that the transistor is working correctly. I have put together a IR circuit with transmiter led and and photo diode, using x2 transistors and a 555. I couldn't get to work until a wire from the ground rail of a breadboard touched a leg of a 220ohm resistor. Then, for some unknown reason, it works in reverse!!

Update: The circuit works correctly now, as I swapped another transistor, and put in the wrong way round, Doh!

For 8 bux, one of these component testers would be good for your fooling around ... er ... experimenting:

Thanks, but we don't do bux in the UK.

It is also available on eBay. Some are Chinese sellers, so if they ship to the US, they probably also ship to the UK.

Do you get PE (formerly Everyday Practical Electronics)? Available from yo ur favourite bookshop for ?5. For the past few years, they have had a series of articles on how much they like the cheap electronics modules t hat are coming out of China; they may even have reviewed this tester. Also , the March 2020 issue has an article on how to build your own Arduino-base d graphical diode tester.

Having said all that, I suggest that you rig up your own with a 9v battery and a 10K resistor (values not critical, and +/- 50% will work OK). Wire i n series and apply to your diode (or two of the transistor leads) and measu re the voltage across the DUT. This is simple enough that you will eventua lly learn what to expect and why. After you have some e practice, try chan ging the resistor value and/or the battery voltage to see how that changes the answer. To me, the fancy test devices are like using the OBD when the check engine light comes on: the diagnostic may be meaningful to those with skills and experience, but can be gibberish to the uninitiated.

on how to build your own Arduino-based graphical diode tester.

why. After you have some e practice, try changing the resistor value and/or the battery voltage to see how that changes the answer. To me, the fancy test devices are like using the OBD when the check engine light comes on: the diagnostic may be meaningful to those with skills and experience, but can be gibberish to the uninitiated.

Here is one for 12.49 pounds or what ever that funny looking L is in the UK. It is in a nice caes and has leads . You can get them for about 7 pounds out of the case and probably no leads.

They are mainly the same as the Peak testers that sell for around $ 100 US. Not sure what that would be in Pounds or Euros. Probably somewhere in the general price range.

If one can not understand what these testers display, they should probably not be messing with the electronics at the component level. All you do is hook up any of the 3 leads to a device and look at the screen. It will tell you what leads are connected and the value of the component.

That is a lot different than the car computers. They do not actually test a specific component to see if it is good or bad or the parameters of it. Just like the tire pressure sensor on some of the cars. The warning light will come on,but not tell which tire or what the pressure is or if the sensor is bad or not. Some cars do tell the pressure of the individual tires, but will not tell if the shown pressure is accurate or not.

Thanks

Ha! That's a different sort of "Tube" circuit in the background.