Testing transistors

Discussion in 'DIY' started by avionic, Dec 30, 2008.

  1. avionic

    avionic " Black Knights " Subscriber

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    Testing BJT transistors for shorts or opens.

    The majority of "Bad" transistors are due to shorted junctions. This is my attempt to simplify the routine for those that may not already know the process.
    Photo 1
    Transistor lead identification - If this is unknown you will need to find a spec sheet on the transistor under test.Sometimes it is provided on the schematics " legend" or even silkscreened on the printed circuit board.
    Photos 2 & 3
    Testing the Base-Collector Junction***With your DMM in the Diode Check function -Attach test leads as shown.One direction should show -OL(open) - and in the other direction a similiar value to the one depicted in the photo ,depending on whether its a PNP or NPN determines the direction of conduction.
    Photos 4 & 5
    Testing the Emitter-Collector Junction*** As seen in photo. Both directions should show (open) in a good transistor reguardless of NPN or PNP.
    Photos 6 & 7
    Testing the Emitter-Base Junction*** Again depending on NPN or PNP. One direction will show (open) and the other direction a similiar value to the example depicted in the photo.

    NOTE:The transistor in the photo is a " serviceable " example. ONsemi MJL4302A PNP

    If your meter at any time - beeps continuously - and/or reads a low value close to zero . First verify your test leads are not touching.If they are not touching ---You have identified a faulty junction and a unserviceable transistor.

    Good luck..I hope this is helpful..

    Dave
     

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    Samokan, kjello, tmsears and 2 others like this.
  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I think this is the first time I have seen pictures of a transistor connected to a DMM with the correct DMM reading showing in the picture for the transistor under test.

    Thanks for taking the trouble to illustrate this,

    John
     
  3. Ausjoe

    Ausjoe Super Member

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    Nice work!
     
  4. roggom

    roggom Super Member

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    Great tutorial, this question comes up from time to time. Very clear and concise.
     
  5. Chazb11

    Chazb11 Vintage Human

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    Many thanks avionic! I already had this info somewhere but this is much more easy to follow. I'm going to print the post and keep it on my bench.
     
  6. thedruid

    thedruid Greetings Earthlings

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    You can take this a step further and test IC's in much the same manner. With the DMM in the diode setting place the + lead on the ground pin of the IC. Then place the - lead on each of the other pins and you should get a reading pretty much the same as the transistor junctions, anywhere from 500-700 is usually a good reading.
     
  7. tensleep

    tensleep Addicted Member

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    Thanks for the photo tutorial, Avionic; I will be using this in the near future.

    So, how about testing the transistor in situ? Desoldering/soldering can have adverse effects on the circuit board.
     
  8. premiumplus

    premiumplus Vintage Junkie

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    Very good advice, but be aware that some types of transistors, particularly TO-3 cased ones, are actually darlington pairs on one die, and they will have different voltage drops, as will germanium transistors. And some actually have built in resistors that will read 100 ohms or so, but they are actually good devices. A spec sheet should always be consulted if you get bizarre readings.
     
  9. avionic

    avionic " Black Knights " Subscriber

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    Testing a transistor with a DMM needs to be done out of circuit..period!
     
  10. 2chanman

    2chanman Well-Known Member

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    To tensleep:
    Regarding testing transistors in situ: There will be circuit paths through the components connected to the transistor that will affect ohmmeter readings. The only sure way to be sure is to remove the transistor from it's circuit location when attempting to determine it's condition.
     
  11. tensleep

    tensleep Addicted Member

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    Gotcha! Thx.
     
  12. avionic

    avionic " Black Knights " Subscriber

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    Roger that...Some transistors have built in diodes as well..
    I will also test a new transistor before installing in the circuit. It has happened that I have gotten defective "New" transistors..:yes:
     
  13. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    Ahhh, so thats what them little devils look like.:D
     
  14. EchoWars

    EchoWars Hiding in Honduras

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    Might add that, simply because a transistor passes a diode-junction test, doesn't mean it is a good one. Simply means it isn't shorted or open (meaning that it apparently has no gross failures).
     
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  15. avionic

    avionic " Black Knights " Subscriber

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    Very much agreed. So I edited thread topic..In reality one should do a little research and reading as to the theory of operation for transistors,If you don't know how its suppose to work-How in the hell can you find the problem.....
     
  16. jaymanaa

    jaymanaa AK Subscriber Subscriber

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    What about those little sockets on some VOMs (labeled NPN and PNP)? and what about matching pairs up?:scratch2:
     
  17. avionic

    avionic " Black Knights " Subscriber

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    Thats another thread Jay...I'll get right on it...:D
     
  18. Hokieman

    Hokieman Super Member

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    Thank you kind sir!
     
  19. chadbang

    chadbang Well-Known Member

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    I thought I would add this for determining the base, emitter and collector

    Most Digital Multi-Meters (DMM's) have a diode test function the symbol looks something like this >| (picture the greater than sign as a black triangle pointing to and touching the redline).

    Initially you should practice on a known good transistor, so you'll be able to tell if you have a bad one.

    1) Ensure the transistor is out of circuit.
    2) Set the DMM to the diode function.
    3) Touch the positive (red) and negative (black) probes to any two leads on the transistor until you get a reading other than infinity.
    4) Once you get a reading leave one of the probes on one of the leads (it doesn't matter which one).
    5) Take the other probe and touch the third lead. If you don't get a reading other than infinity go back to step 4, this time leave the other probe on the lead it was originally on, when you got the reading other than infinity.
    6) Now take the other probe and touch it to the third lead, you should get a reading other than infinity. If not, go back and repeat steps 3-6, but start with 2 different leads, until you can leave 1 lead in place and get a reading other than infinity on the other 2 leads.
    7) If you kept the Positive lead in place you have an NPN transistor. If you kept the Negative lead in place you have a PNP transistor.
    8) The probe you left in place denotes the Base.
    9) On the probe that you swapped, the lead with the lower reading is the Collector and the lead with the higher reading is the Emitter.

    Read more: http://wiki.answers.com/Q/How_do_yo...itter_terminals_of_a_transistor#ixzz1Kt5eZEr1
     
  20. avionic

    avionic " Black Knights " Subscriber

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    If your going to be testing a lot of transistors. Which I do. I love my little DCA.
    [​IMG]
     

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