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Check Transistors and FETS in circuit for shorts or opens using a DVM ohm meter

Discussion in 'General Electronics Discussion' started by danny davis, May 30, 2012.

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  1. danny davis

    danny davis

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    May 9, 2012
    I'm trying to use a DVM meter in ohm mode to check transistors and mosfets in circuit

    All I get is resistance measurements in the Meg ohms

    Is there any other way to check or test transistors or mosfets in circuit?

    Cause I can't make head or tails if they are open since its reading in the meg ohm or if it's shorted because of the resistance reading

    The problem I'm having is that There is a SHORT or Multiple Shorts on PCB board circuits at my work and I'm trying to check the transistors and mosfets in the power supply section

    The SHORTS causes the resistance readings to be different on every PCB board I check

    I have Written down the resistance readings off of a GOOD working board a KGB, to reference off of.

    But that doesn't do any good, why? because the transistors and mosfets can still be operating fine even if the resistance is way off because the SHORT somewhere on the PCB boards has changed the resistance value

    So I have no Idea if the transistors or mosfets are GOOD or NOT , cause i can't make heads or tails because the SHORTS have changed everything

    I'm just trying to verify if the transistors and mosfets work

    How can I do this please?
     
  2. danny davis

    danny davis

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    May 9, 2012
    I think there is a Diode Testing on my DVM , Do I use this?

    But i think its just a continuity checker that beeps but there is a diode logo on it
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    A typical diode testing function on a DVM is like a resistor measurement but allowing for a higher measurement voltage. Thus you can measure the pass voltage on a diode which should be in the range 0.6 V - 0.8 V. This is what the DVM displays.

    Measuring electronic components in the circuit generally doesn't work too well. As you have already noticed the other components (and failures) influence the measurement. But you can't be sure in which way. A better way is to de-solder at least one lead before measuring.

    Harald
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you're expecting a diode junction and you see no conductance in either direction, you have a fault.

    Typically you'll see something due to other components in the circuit. Looking at the schematic at the same time (if it's available) can help
     
  5. danny davis

    danny davis

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    May 9, 2012
    Can I Use this to test Mosfets?

    So the Display should read be in the range 0.6 V - 0.8 V. This is what the DVM displays.
    for Diodes, transistors and Mosfets?

    Which Leg/lead should I Lift up?
     
  6. danny davis

    danny davis

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    May 9, 2012
    Whats the best pin to lift up on a Transistor? when it's in circuit to test with a DVM meter in Diode check? is it the base? emitter , or collector?

    What is the best pin to lift up on a MOSFET? when it's in circuit to test with DVM meter in Diode Check? is it the gate? source? drain?

    Ok I have my DVM meter set to diode check mode

    1.) I lift up the Base ( pin #1) of the transistor
    2.) I measure from base to Emitter it should read .7 volts?
    3.) I measure from base to Collect it should read open circuit?
    4.) I measure from Emitter to College it should read open circuit?

    Does the Polarity of the red and black probes make a different which way they go when measuring?

    I have my DVM meter set to diode check mode

    1.) I lift up the GATE ( pin #1) of a MOSFET
    2.) I measure from Gate to Source it should read .7 volts?
    3.) I measure from Gate to Drain it should read open circuit?
    4.) I measure from Source to Drain it should read open circuit?

    Does the Polarity of the red and black probes make a different which way they go when measuring?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Post may 30:
    You cannot test MOSFETs with a DVM's diode tester. As the name says (Metal Oxide Semiconductor) there is an oxide layer between gate and the source-drain channel. The oxide is a very good isolator so you can measure a resistance between gaet-source (or gate-drain) only if there's a breakdown of the oxide - which indicates a defect transistor, but is not so common a mode of failure.
    If you measure between source and drain you have to make a distinction:

    If it is an enhancement MOSFET (the much more common type), there will be no conductance between source and drain unless you apply a gate-source voltage higher than the threshold voltage of the transistor. Your DVM can't do that (at least not the DVMs I know). So you can onlky determine whether the transistor's source-drain channel is o.k., but not whether the gate control is functional.

    If it is a depletion MOSFET (the very rare type) you can measure the conductance from source to drain even without gate voltage. But this gives you no indication whether the transitor actually operates.

    Post may 31:
    For a bipolar transistor: Vbe = Vbc = approx. 0.7 V.
    If the transistor is NPN, then base= pos. (red) , emitter, collector = neg. (black)
    If the transistor is PNP, then base= neg. (black) , emitter, collector = pos. (red)
    In effect you're measuring the B-E and B-C diodes of the transistor.
    E-C = open circuit regardless of the polarity.

    For the MOSFET see my answer to yesterday's post.


    Harald
     
  8. danny davis

    danny davis

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    May 9, 2012
    So using a DVM meter in diode mode won't work for transistors or MOSFETS?

    How do u test MOSFETS than? using an ohm meter? or do you use an external power supply to power the MOSFET?

    How do u guys test MOSFETS?

    So I should measure .7 volts from base to collector?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    To test a MOSFET you need a suitable test circuit. Just a jury-rigged setup with a drain resistor, a power supply and variable gate voltage (as in a simple MOSFET inverter circuit.

    Or do you have access to a dedicated component tester?


    Harald
     
  10. danny davis

    danny davis

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    May 9, 2012
    What is the block diagram or schematic for this test setup?

    Like what kind? what are they called?
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    A test circuit could look like this:
    http://www.google.de/imgres?imgurl=...WVrIT_LCIIGn4gTw5sQl&ved=0CGYQ9QEwAg&dur=1172

    Well: component tester (or more specialized transistor tester)
    If you don't have one or access to one. don't bother buying one. They can be quite expensive. If you build a simple test circuit as in the first link, you can easily measure many parameters and determine the function of teh MOSFET.
    There is also a video out there: http://icvideo.blogspot.de/2007/02/how-to-test-mosfet-transistor.html

    Harald
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I have one of these

    It does NOT test semiconductors
    It does NOT work in-circuit
    All it does is identify what a component is (with certain significant limitations)

    Here is a device that tests semiconductors.

    Note that it does not work in circuit either.

    If a semiconductor is in-circuit, all you can do (generally) is test for certain faults but you MUST HAVE THE SCHEMATIC (full or partial) for the device. You need to know if there's (say) a resistor between the base and collector, or a zener diode from gate to source, or any number of other things that WILL affect your measurements.

    Here is a device that can do in-circuit testing, but beware that good components can show up as bad in certain configurations.
     
  13. danny davis

    danny davis

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    May 9, 2012
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    This could work, but you have to remove the component from the circuit for testing.

    Harald
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I can't really understand you, but in general you need to lift all but 1 pin (it may be easier just to remove it completely) to perform a full test/

    No. Amazon call it a tester, but it isn't. It does just enough to determine that a device is one of the things it can identify. This may include some fundamental testing but it does not guarantee it operates.

    The tests it can do are almost certainly better and faster than you can do with a multimeter alone.

    HOWEVER, with knowledge of the circuit and a multimeter, you may be able to do enough with just a multimeter.

    If you understand the circuit, this device as almost redundant because you already know what the component is.

     
  16. danny davis

    danny davis

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    May 9, 2012
    So I have to lift up the GATE pin and the Drain or source pin?

    Which 2 pins i have to lift up?
     
  17. davenn

    davenn Moderator

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    I have one of those, They work very well giving device type, gain, leakage etc

    never tried to test semi's in circuit with it tho I would still suspect other components may throw errors into the testing

    They are well worth the money

    Dave
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    any 2 it doesnt matter


    D
     
  19. danny davis

    danny davis

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    May 9, 2012
    Testing Transistor in circuit you have to lift up the BASE pin

    A lot of the transistor and FETS ,emitters/source are tried to ground or some of the transistors are collectors/drains tried to Vcc

    Would this throw off the DVM meter in diode mode because of this?

    Even tho the circuit is powered off when testing them
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Are you deliberately ignoring us?
     
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