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Testing of Mosfets. want to be sure.

Discussion in 'General Electronics Discussion' started by fastline, Aug 9, 2010.

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  1. fastline


    Aug 9, 2010
    Basically I am testing to possibly identical mosfets in which I think one is bad. I am using a 5V supply to the drain and source and then reconnect that same +5V to the gate to trigger it. One mosfet works like it should where it triggers an LED and then turns off when I touch the gate and source with my finger.

    The other one shows continuity from drain to source all the time BUT I decided to hook it up anyway and basically it shows continuity until I add the +5V on the gate and then it shuts the LED off while it is getting power. I am wondering if this is just because the current then shorts through the mosfet to complete the circuit rather than going from drain to source?

    Both oare IRF730s but there is a tiny little stamp (think it is a date stamp) at the bottom that is different. One is AE, the other is 8B.

    Just wanting to make sure this little guy is toast??
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The problem with touching the gate of a mosfet with your finger is that it may be sufficient to destroy the gate.

    It sounds like one mosfet is behaving like enhancement mode and the other like depletion mode. Since they're supposed to be the same device, that's not likely a feature :)

    I would suggest that the second device has failed, although I'm not aware of any failure modes that could cause such a weird change in behaviour.

    If you've been touching the gate terminal of them with your fingers, then there's a pretty good chance that both are toast UNLESS you've been taking anti-static precautions.
  3. fastline


    Aug 9, 2010
    Yes, I have an ESD blanket on the bench and a wrist wrap.

    I show one of the devices pretty much shorted in every way possible. I am just curious if they ARE actually identical components? All numbers on the face are the same other than that little stamped number which I assume to be a date code or something.

    There is also continuity from the "chassis" or metal tab to all 3 pins on the possible bad one and continuity only from the drain to chassis on the possible good one.

    I would like to learn MUCH more about mosfets, transistors, SCRs, etc in testing and such, can you recommend any links or documents for a good but brief read on them? I am also trying to test some NPN transistors which I find to be a totally different animal.
  4. Mitchekj


    Jan 24, 2010
    Sounds like your questionable one is toast, if all pins measure continuity to the tab. I would wager that all the current is going through the FET, and not through the LEDs, if you're using the same 5V supply for both gate and drain voltage.
  5. fastline


    Aug 9, 2010
    Yes, the same supply is being used so I think if I make another test circuit, I might use discrete supplies.

    Is a DMM with diode testing a decent way to test these? I was trying to learn how to bench test an NPN transistor and seems they can be tester pretty easy but not sure about a mosfet.
  6. shrtrnd


    Jan 15, 2010
    Be careful what family you're testing with a DMM, it generates it's own voltages and currents when it's testing, and you can easily blow sensitive devices with it.
    Most manufacturers show a simple test circuit to build for their devices on their data sheets (for each device). Most stuff is ok to use a DMM on for a quick check (usually but not always accurate,... some devices ONLY fail when under test, a lot of SCR's are like that, you have to reach the trip voltage, to actually ensure it's working or not).
    Anyway, my point is, don't zap a lot of MOSFETs or other sensitive devices, if you're unsure of the best way to test them. Use Mfgr test circuit on data sheet. Good luck.
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