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Testing FET's in circuit with an ohm meter and an external DC power supply

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 2, 2013.

  1. Billy Mayo

    Billy Mayo Member

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    At work , I ran into a guy that said how he test FET's in circuit

    You said that he uses a ohm meter ,

    he puts it across the Source and drain and it should be 200ohms to 700ohms if its GOOD,

    If the FET is open from Source to drain , the ohm meter will measure in the MEGohms

    To test if the GATE is working, you can use an external DC power supply and look up the FETS datasheets on how many DC voltage the GATE to be turned ON or OFF

    I'm guessing you do this when the circuit is OFF

    The FETS are in circuit well testing them

    I Tried using the Diode checker mode on my DVM meter to check the FETS , but the voltage will intergrate up or down depending on the polarity of the red/black meter wire you have on the FET's pins

    In Diode check mode on my DVM meter, when the FET is shorted , it will make a BEEP sound
     
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    So i guess then you can use a shorted FET to test the meter (he he).

    I like the external DC power supply test because that will check the FET much better than an ohm meter.
    Try that and see how it goes. You can also check that you get good results with a large value resistor on the gate.
     
  3. Billy Mayo

    Billy Mayo Member

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    What value resistor, and how do I use it to test the gate?

    Do you know what DC volts I can use to check if the FET's gate is working or not? I don't want to damage the FET
     
  4. dave

    Dave New Member

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  5. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Are you talking about testing FET's or MOSFET's?

    John
     
  6. Billy Mayo

    Billy Mayo Member

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  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    When a Jfet or a Mosfet is turned off then its drain to source measures megohms, not 200ohms to 700ohms. It should have its resistance measures out of the circuit.
    The drain to source in a Mosfet has a diode built-in that conducts if the polarity of the power supply is backwards.
    When they are turned on then you must look on their datasheet to see if the drain-source resistance is 1k ohms or a fraction of an ohm.
     
  8. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    That will make a difference when you test the gate function with your power supply. For example, with an N-channel mosfet, one test is to charge the gate relative to the source (forward bias) and see whether drain to source conduction is turned on. Since gate drives often have a turn-off resistor or other high leakage path to ground, that test is done with the gate disconnected from the circuit. With a JFET, you want to be sure not to forward the gate with your power supply.

    John
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    Well since the FET has a high input impedance you should easily get it to work with a 100k or even 1 meg resistor on the gate in a static test.

    As others have indicated, depending on what kind of device you are testing the test results could be quite different. That's because the FET's and MOSFET's have a wide variety of specifications that you need to review before testing the device so you know what to look for.

    For example, for a normal FET you might see 500 ohms across drain to source when it is turned on, but for a power MOSFET you could see much less than 1 ohm like 0.050 ohms even, but then other MOSFET's might show 10 ohms but they are still good because they have different specifications.

    In general regular FET's would be tested at lower current levels than power MOSFET's.

    To find out the min and max gate voltage, you really do have to look at the data sheet for that particular device part number. There's no other way because here we see a wide variation too from maybe 5v up to 20v or something like that, and if you apply 20v to a 5v part it might blow out, and if you apply 5v to a 20v device it might not turn on fully. So you have to know what you are dealing with before you begin any tests.

    Also as audioguru pointed out, you need to test the device out of circuit for best results. If you intend to test in circuit then you need a schematic of the entire circuit so you know what to look for during the test.

    When you use a DC power supply to test you dont need an ohm meter. You can use a resistor on the output as load and test that way. It will usually be much better that way.

    Because there are so many variations here your best bet is to review the data sheet first and if you still cant come up with something then you'll just have to ask here for more help for each device. If you only have a few devices you want to test that wont be too hard.
     
    Last edited: Dec 3, 2013
  10. Billy Mayo

    Billy Mayo Member

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    What is a static test?
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    A static test is a test where we would apply a DC excitation and measure DC quantities.
    A dynamic test would be a test where we apply either AC sine waves and measure AC quantities or we apply transients and measure transient phenomena.
     
  12. Billy Mayo

    Billy Mayo Member

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    Mr. Al

    How do you test FETS and MOSFETS , in circuit or out of circuit

    If you had all kinds of FETS and MOSFETS in a circuit , how would you approach testing them?

    Would you print out all the datasheets for each one

    Or how did you test FETS and MOSFETS in the past , from your experience?
     
  13. ChrisP58

    ChrisP58 Well-Known Member

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    There is no single way of testing mosfets (or really, any other component) in circuit. That is because you you usually can't isolate that component from the rest of the circuit, so your measurements are affected by all of the other components it is connected to.

    The guy you quoted in post one was probably describing his test of a specific fet in a particular part of the circuit, and he has learned that by experience. A different part in another section will have it's own local good/bad behavior.

    To figure out how to test ANY component in circuit, you need to look at not only that parts datasheet, but also the schematic to see what it is connected to, and how those components will affect the circuit measurements with the component of interest in it's different good/bad modes.
     
  14. Billy Mayo

    Billy Mayo Member

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    True I agree

    Ok I remove the FET or MOSFET out of the circuit

    But do you use the DVM diode checker mode to test and check FETS and MOSFETs?

    Or do you use an ohm meter? how do you check if the GATE is working or not?
     
  15. ChrisP58

    ChrisP58 Well-Known Member

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    For a quick bench test, I use an ohm-meter, not a diode checker.
    I test the gate with a 10V power supply through a 10K resister.

    For power mosfets with low RDSon, I use a bench power supply set to limit the current to ~1 amp, then measure the D-S voltage with a kelvin connection to the mosfet pins when I stimulate the gate.
     

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