Looking for a small transistor tester circuit

[shellcode]

Hi. I'm looking for a small transistor tester circuit. All it should do is tell if a transistor is PNP, NPN, and whether it works or not. Since I'm a beginner this should be very simple. Thanks.

 

[IanP]

Re: Transistor Tester

Here are examples of transistor testers:
http://www.discovercircuits.com/PDF-FILES/xistrtst.pdf
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Regards,
IanP

 

[shellcode]

Re: Transistor Tester

Thanks for the reply.

http://www.discovercircuits.com/PDF-FILES/xistrtst.pdf
I had already found this one. The problem is that it does not say that values of the elements. For example next to the capacitor it says 0.047. Is this in picofarads, nanofarads, microfarads, farads...? Also, I prefer an LED to light up rather than a speaker to sound when testing the transistor.

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This one is too complicated. I don't need gain and whatnot to be tested.

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This one looks perfect. Simple and effective. One problem though:
1. I don't have a 100nF capacitor. I have a whole value pack of electrolytic capacitors and ceramic capacitors but none of these is 100nF (and I don't think that an electrolytic will work). What kind of capacitor is 100nF? Mylar, something else?

Thanks.

 

[IanP]

Re: Transistor Tester

These capacitors do not need to have exactly 100nF. Also, check your ceramic caps if you don't have any laballed 104 (it is code for 100nF capacitance, 103 will be 10nF, 105 is 1µF, etc). In most cases 100nF capacitor is a ceramic cap, but you can find tantalum, polyester, polypropylene and other types.

The value 100nF together with R=39kΩ sets the switching frequency of this multivibrator. As it is not crytical in this application, try different capacitors starting from 10nF +++ and up to small electrolytic caps, say, 2.2µF, maybe even 10µF ..
Remember of connecting the negative pole(s) of electrolytic caps to base(s) and positive pole to collector(s) of the other transistor ..
Good luck and regards,
IanP

 

[shellcode]

Re: Transistor Tester

Ok. I've found two ceramic capacitors labeled 103M in the value pack. My multimeter says they are 10nF so I guess I will try those but what does the M stand for? Why are they not just labeled 103 instead of 103M?

Also, I don't have a 3.9kΩ resistor (Rt in the circuit). Is it ok to use one with a fairly close value like 3.3kΩ or should I use a couple in series/parallel to create a 3.9kΩ equivalent?

Thanks.

 

[IanP]

Re: Transistor Tester

The letter M stands for +/-20% tolerance.
(more on this you can find here: **broken link removed** ).
So in your case 103M = 10nF +/-20%.

Resistors:
Also their values are not critical, so because you will be using 10 times smaller capacitance, you can try anything from 30kΩ to 100kΩ (Rb) and 470Ω to 2kΩ (Rc).
Rt value is also not very important, you can try from 1kΩ to 10kΩ.

Regards,
IanP

 

[shellcode]

Re: Transistor Tester

Ok I've breadboarded the circuit. I've checked and rechecked and every connection and they all seem fine to me. Both LED's stay on when the switch is not hit. According to the chart provided with the schematic, this means that the transistor under test is ok. However, when I hit the switch both LED's continue to stay on. According to the chart, one should turn on and the other off indicating whether the transistor is NPN or PNP.

Without seeing my breadboarding, what do you think could cause both LED's to stay on regardless of whether the switch is hit or not?

 

[IanP]

Re: Transistor Tester

If both LEDs are on it means that your tester generates square wave as planned. If it didn't both LEDs would be OFF.
For basic test of your circuit you can use any silicone (or other) diode and connect it between C and E instead of tested transistor. Diode connected A(node) to the left and C(athode) to the right (see drawing) will simulate NPN transistor in ON stage, and the lower LED should be OFF. If you reverse the polarity of this "test" diode the top LED should be OFF.

If both situations described above have worked correctly, you can try to test a transistor. If the switch is on and both LEDs are ON, this indicates a faulty transistor..

Regards,
IanP

 

[shellcode]

Re: Transistor Tester

Aha! It works! Wonderful.

I've realized the problem. I had the collector and emitter inverted. Prior to hitting the switch, the tester will only tell you if the collector and base are inverted. Both LED's staying on when the switch is hit, then, either indicates a faulty transistor OR that the collector and emitter are inverted right?

Now, what would be the easiest way to make this circuit permanent?

Thanks a lot for your help.

 

[IanP]

Re: Transistor Tester

Some transistors may work with inverted E and C..
But generally, if both LEDs are ON then the tested transistor is faulty.

From my experience temporary arangements last for ever, so ..
Try to find a box and fit this circuit neatly inside, install test terminals on the top and .. done.
Now you can build yourself adjustable power supply ..
Good luck and regards,
IanP

 

[shellcode]

Re: Transistor Tester

Well I could do that but then I wouldn't be able to use this solderless breadboard for any other circuit. Is there a type of board that would allow me to maintain (for the most part) the positioning of the elements on my solderless breadboard? Thanks.

 

[IanP]

Re: Transistor Tester

You can use so called VEROBOARD and solder all components on the board.
Some advices on this sort of boards you can find here:
https://www.smspower.org/smsreader/prepare.html
Regards,
IanP

 

[shellcode]

Re: Transistor Tester

That seems good. I'll try to build it. Thanks a lot for your help. It has been very valuable

 

[emadi]

Re: Transistor Tester

You don't need tester circuit.
You can do it by a multimetetr by measuring B Vbe Vce