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    allintitle:transistor testing

    How do i test a transistor with a ohm meter using ohms?

    By using ohms how can i tell which ones is the base,collector,emitter
    by the ohms reading?

    How can i tell if the transistor is NPN or PNP by ohms readings?

    What should the base and collect ohms reading be?
    What should the base and emitter ohms reading be?
    What should the Collector and emitter ohms reading be?

    Then if i switch the probe leads to change the reference it changes the ohms readings the other way

    Added after 7 minutes:

    Testing a transistor with a Ohm meter does this tell us that the transistor is "conducting" because we are measuring each terminal in ohms instead of having a voltage drop across it ?

    Some meters send out a voltage i think to the transitor to make sure the junctions have a .7 drop or so but measuring a transitor by ohms is that testing it if its conducting?

    •   Alt22nd August 2006, 00:33

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    Re: Testing a transistor with a Ohm meter

    First of all, if the ohmmeter's test voltage is below ≈650mV (for Silicone, and ≈300mV for Germanium), as far as transistors' testing is concerned - it is useless ..

    Before testing an unknown device, it is best to confirm and label lead polarity (of voltage provided in resistance or diode test mode) of your meter whether it be an analog VOM or digital DMM using a known good diode (e.g., 1N4007 rectifier or 1N4148 signal diode) as discussed below. This will also show you what to expect for a reading of a forward biased junction. If you expect any Germanium devices, you should do this with a Ge diode as well (e.g., 1N34).
    Quoted from:
    http://www.repairfaq.org/sam/semitest.htm
    where you can find detailed procedure on testing semiconductors - including transistors - with an ohmmeter ..

    Regards,
    IanP



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    Re: Testing a transistor with a Ohm meter

    Yes but if u buy a bunch of transistor even tho the beta gain u have to measure will be the same the junction to junction
    in ohms are all different from base to emitter, base to collector, emitter to collector and if u swap the probes around u get
    other ohm readings also from changing the reference i guess

    because at one job i had i had to test a bunch of transistors gains from a transistor beta test and put them each in different
    boxes and then i had a special box that tested the junctions to junctions in ohms because they all read different readings
    because of the doping processes it changes the ohms from junction to junction



    •   Alt22nd August 2006, 02:07

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    Re: Testing a transistor with a Ohm meter

    Quote Originally Posted by IanP
    First of all, if the ohmmeter's test voltage is below ≈650mV (for Silicone, and ≈300mV for Germanium), as far as transistors' testing is concerned - it is useless ..
    Yes but it is not a problem with those old analogue ohmmeters (or rather multimeters) and those modern digital ones usually have speciall range for diode testing, which of course can be used for transistors.



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    Re: Testing a transistor with a Ohm meter

    with the old analogue ohmmeters how can u test a transistors junction to junction in ohms and know its good please?



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    Re: Testing a transistor with a Ohm meter

    In all the time spent in discussion, you could have built a simple tester and learned more.



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    Re: Testing a transistor with a Ohm meter

    Quote Originally Posted by walters
    with the old analogue ohmmeters how can u test a transistors junction to junction in ohms and know its good please?
    You put one lead (let's say red one) to the base of transistor and another (black one) to collector and emiter. Then reverse leads. In one position you will have low resistance and in another very very high. Then check between emiter and colector in the same way. You should have high resistance both ways in MOST of the cases. Some transistors will show low resistance in one direction between emiter and colector, but it depends on manufacture procedure of a transistor and if it has incorporated diode or it is darlington type transistor.



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    Re: Testing a transistor with a Ohm meter

    Walters,

    PNP vs NPN

    The way you tell the difference between the two is by the dodes you see in operation WHILE you are measuring.

    An ohmmeter actually is a positive DC supply, like a battery. You measure (send current through the device under test) by connectng the leads to the device.

    A diode's anode and cathode can be determned by the resistance reading you get when you connect the positive (red) lead to one side and the negative (black) lead to the other. If you get a low R reading, you've forward biased the diode. Hence, you must have the red to the anode. If you get a high R (relative to the leads the other way around), then your positive (red) lead is on the cathode.

    (Do bear in mind that the above is correct ONLY if you have the red lead in the positive hole on the meter and the black in the negative.)

    Now the same way you measured the diode is the same as you do for the transistor, whether PNP or NPN.

    A diode looks like

    ------ P N -----

    A forward biased diode looks like

    positive lead ---- P N ------ negative lead

    An NPN transistor looks like

    ----- N --- P ---- N ----

    Now, about telling the difference between the collector and emitter. What I would recommend is this:

    For a 2n3904, an NPN Bi-Polar Junction Transistor, in a TO-92 package, with the flat part facing you, you have

    EBC

    Using this transistor (a very common device incidentally), measure it for yourself and see if you can tell the difference between the diodes. (Do it with several 2n3904's so as to compare.)

    The 2n3904's complement (opposite, or PNP) is the 2n3906, and it has the same pinout.

    Just ask if you need more help. (...assuming I have time)



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