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Transistor tester


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:)here it is the in circuit transistor tester. ill re photo the circuit better and put it in as soon as i find the circuit

Steven, I would really like to put that transistor tester together 8). have you already posted the circuit for it somewhere, or are you still, to find it to post :).
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  • 4 months later...

It is a shame that you made it in ExpressPCB. No one will be able to make the board except ExpressPCB. Do you have them make all your boards or is there a trick to getting it to print out a board pattern so that you can make your own?
BTW- Your file did not attach.


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  • 11 months later...
  • 5 months later...

Hi all. As you can see this is my first post having stumbled across this site just recently. I am a relative newby to electronics as well and need some help with this tester. I had all the discussed problems ( 1 LED on all the time etc.) but this was completely solved by the changes given by audioguru. My problem now is that the circuit will identify an NPN or PNP tranny, but will not show whether shorted or open - on a confirmed shorted NPN tranny, the NPN LED still just flashes. Anyone have any idea if I have maybe damaged the 4027, or is this another fault in the circuit? Your feed  back will be very much  appreciated.

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Hi Steve,
If the tested transistor is shorted from collector to emitter, then two diodes in series will be across each LED. Two diodes limits the voltage to 1.3V but a LED needs 1.7V or more to light so the LEDs will not light.

If the tested transistor is open then the LEDs will flash like it is not there.

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Audioguru, steven thank you for your replies. I have exactly the results as you both describe if I physically dead short the legs - but I have a tranny that measures as follows with a multimeter:
NPN Tranny :B-E = .708V    B-C = .709  C-E one way = .5V    C-E the other  way = clear, B-C reverse polarity = 1.05V.  Surely B to C should only read one way? If I test this tranny in the tranny tester it behaves as if the tranny is good! Why is that?

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Hey I said I'm a newby - I don't know how to explain a leaky transistor between base and collector other than as a partial short. Anyhow, my real question has still not been answered - does anybody who has built this tester know if it does indicate whether a tranny is leaky or have I done something wrong with mine. As I said earlier, if I dead short the tranny at the legs it performs as indicated on the circuit diagram description - but with this transistor reading both ways between base and collector it doesn't pick it up. I know its not that important guys - but I'm trying to learn.
Any suggestions?

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