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In circuit transistor tester

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  • 3 months later...

:) thanks hot water wizard and audio guru its a nice pastic box with metal pannel at front with the 2 leds and switch the circuit box has the leads runing out the top end for the base ,emmitter and collector. so with the push down clips its easy to attach to the transister legs . at the top end of the plastic circuit box where the wires run out, the b for base and e for emmitter and c for collector are there so youll know which wires with the push clips go to what legs of the transister.

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:)audio guru the leds blink acording to the condition the transistor is in. look at the panel on the on the picture of the in circuit transister tester. it will show you . the white dots are for when the leds blink to indicate , wheather there npn or pnp and which legs are faulty arcording to the blinking lights. something like that.

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Hi Steven,
Yes, I understand that a transistor that is good will turn-off its blinking LED. But I think that your transistor tester will also show that a faulty transistor with a very low gain is good.
To simulate a transistor that has a very low gain, please test a good BC548, 2N3904 or 2N2222 with its emitter and collector leads reversed. Then let us know if its LED dims a bit or turns-off completely. Please try it with a PNP transistor too.

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Look at that one The design shown will test PNP and NPN transistors, diodes and SCRs both "in-situ" (equipment of course de-energised) and also by direct connection to a stand-alone component. It is a simple GO/NOGO test which can identify diode and transistor action and will indicate diode polarity and transistor type PNP/NPN, if this is unknown.


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Hi Hotwaterwizard,
Your 1st and 2nd transistor tester circuits are great and easy to make.
Sorry, but in the 3rd one, it tests the two transistor junctions separately, not like a transistor that normally has a reversed-biased C to B junction. Without any current-limiting, the transistor's base and/or the LEDs may blow. If the transistor is shorted, the LEDs will blow. If you accidently press both buttons without a transistor connected, say goodbye to all LEDs. ;D It uses a lot of batteries.
The 4th tester has an emitter-follower for the PNP, but a common-emitter for the NPN. Maybe both should be the same.

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