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Transistor tester


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:)hi there,

it seems a few members have a problem having this project work. my xperience when i first built this project cost me 2 damage CD4027. the most common problem is the damage CMOS.

another member was asking if the series/parallel signal diode (IN4148) is presented correctl in the schem. yes, it should be connected as presented in the schem. remember that what the author had in mind when he designed this circuit that it can be used to test transistors in-circuit, so he has to employ such circuit configuration. am not trying to be the spokesman of the author but let me try to explain why this circuit is used in the design:

looking at the schem you will see that the 2 sets of series IN4148 are connected in series with the D.U.T. via its Collector terminal. it works this way, they allow only a current path thru the DUT as long as the transistor is fully saturated. thus insufficient current would flow thru parallel resistances resulting that 1 or both LED would be dark or unlit.

i hope this would make things clear.

in fact i have made revisions to this circuit using only 1 ic (CD4093) a quad 2-input NAND Schmitt Trigger in place of the 555 & 4027, but still using the same basic set up on the testing side, and it work just fine. i could check diodes, SCRs & transistors and it could also show the transistor type.

also pls remember that the output at pin 14 & 15 are at opposite of its other state. when 14 is high 15 is low which causes both LED to flash alternately. so if only 1 LED lights up continously, no alternate flashing, it means one of the output is not functioning. so what would be your logical conclusion. :)


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  • 2 weeks later...

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Yesterday I received a report from a user, read it:

I have an update for you regarding the transistor tester as I was having a few problems in getting it to work well it works now as a few modifications have been added such as connecting pin 8 of the 4027 to ground and then linking pins 3 and 9 of the 4027 together it then works.

once again many thanks mike it is a good site.

keep up the good work from kev Tellyaddict42[no-spam]aol.com .

Anyone that don't succed with this transistor tester can try this solution and post comments here so i will make any suggested corrections on schematic.


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  • 3 weeks later...

there is a simple transister tester i biult from a kit as handy as it is as a handy dandy peace of test gear , ive decided to biuld some myself , i have the decade counter tester, 4001 ic tester, 4011 ic tester, 555 and 556 ic tester and scr tester and reverse mode transformer tester . and 1458 ic tester and the 741 ic tester, and n channel mosfet tester, hv flyback transformer output tester, thats just some of it. [email protected]

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  • 2 months later...

i read out ur very good suggestions.iam also working o this project
i want to have some more details of this project(theory)
that what is happening internaly and what the way it works ?
on what conditions the flip flop turns how diodes behave etc
so any links ....or message..
i will be very much grate full to all of you people
and this is a grate site
keep it up

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  • 3 months later...
  • 1 month later...

Hello, I just built your transistor checker because I could not find one to buy. We manufacture our own tube gear and other electronics. After I built the unit with no transistor connected I get a solid LED2. What I understood was that both lights should flash. What should the unit do with no connection to and transistor? I have double checked the wiring and it's correct. All wiring is point to point, no board. Please let me know your thoughts.

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If you want the flip-flop to alternately toggle its output, then it must be clocked. The datasheet shows that the clock is pin 13, but the project's schematic has it grounded. So pin 3 of the 555 should connect to IC2's pin 13, not to pin 12. Pin 12 is IC2's reset, which should be connected to ground.
Also, the circuit will work better with a power supply bypass capacitor of about 100uF, connected from positive to ground.

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The only modification that was successfully done to this project was when Mozikluv replaced its 555 and 4027 with a 4093, similar to Steven's transistor multivibrator.
Mixos posted a successful "change" in bold red lettering that was to connect the ground (pin 8) pin of the 4027 to ground (as on the schematic), Duh!
Most of the posts say that one LED is continuously on, because the flip-flop isn't being clocked due to the error on the schematic.

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:)audio guru my in circuit transister tester dosent use intergrated circuits and i think the circuit i use works by shorting out the leds, something like that and the less parts it uses makes it really easy to biuld. mine works great .and gives great indication of any shorts between any legs of the transistor you test. its easy to revers engineer to.

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Hi Steven,
Your transistor multivibrator is the same basic circuit as this project that uses ICs. The transistor being tested shorts its LED.
Just think how poorly it would work if one transistor's emitter wasn't grounded so that it wouldn't oscillate, just like this faulty project.

I like yours because it is cheap and simple. If it was a digital document instead of its fuzzy photograph or scan, I would post it here again, for all to see.

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audio guru those clips are push down types that has a coper hook thing that enables me to clip them onto the transister legs, and if i test the 2n3055 npn transistor, because it only has 2 legs and the body is the collector i just push down on the third clip and touch the coper hook to the body of the transister to test.

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  • 1 month later...

Hi Steven,
Sorry, but its switch is labeled backwards. It is shown in the "NPN" position (NPN emitter-follower).

It is a waste of a 9V battery since most of its voltage is wasted in the 470 ohm current-limiting resistor. It could use cheaper and longer lasting two AA cells (3V), if it was rewired to be common-emitter, and its LED current-limiting resistor value decreased to match the lower voltage.

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