Mosfet -Testing k2996

Discussion in 'General Electronics Chat' started by imraneesa, Jun 26, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    4
    Hello sirs,

    I got this mosfet in the monitor which is used to drive the transformer.
    the number is k2996. the following points are noted.
    1. red probe at source and black on drain gave me 357 when the DMM is set on diode function.
    2. when I swap the probes I did not get any value.
    3. keeping the probes as it is I took the red probe up and touched the gate and then placed it back on Drain and I got the value 1550 and it did not stay soon it gone back to no value.

    I attached two lithium batteries to light a bulb using resistor in series. and I touched the gate the bulb on but as soon as I remove the voltage on gate the bub goes off. it is not staying on.

    So based on these is my mosfet good or bad? are there any mosfets when works only when gate is on all the time?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
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    Since the K2996 is an N-channel FET the gate needs to be kept several volts positive with respect to the source to keep the FET on.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,025
    3,236
    A MOSFET gate should never be left open, since the voltage on it can float to any value.
     
    cmartinez likes this.
  4. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Thanks @crutschow, you've just answered this question for me.
     
  5. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    so this is not mosfet? I saw some videos of mosfet n-channel. when they just touch positive voltage on the gate the mosfet get on and it wont get off until negative voltage is applied on gate again.
    then why in case of this we have t
    I am not telling to keep it open. I want to know if this mosfet is good with respect to the values I have mentioned. If this is good then I have to find some other things which have gone wrong in the monitor.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    If you use a basic continuity tester, you can charge the gate capacitance with its test voltage. On an N-channel MOSFET, if you charge the gate positive, you should get conduction from drain to source - if the gate charge leaks away while you're making the test, I'd treat the device as suspect. If you charge the gate negative, the drain to source resistance should be too high to measure - MOSFETs have a parasitic "body diode" from drain to source, that diode is normally reversed biased with the correct polarity drain voltage applied. The DMM diode check should give a typical diode Vf if you reverse the prods to D/S.
     
  7. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    UOTE="ian field, post: 875371, member: 182479"]If you use a basic continuity tester, you can charge the gate capacitance with its test voltage. On an N-channel MOSFET, if you charge the gate positive, you should get conduction from drain to source - if the gate charge leaks away while you're making the test, I'd treat the device as suspect. If you charge the gate negative, the drain to source resistance should be too high to measure - MOSFETs have a parasitic "body diode" from drain to source, that diode is normally reversed biased with the correct polarity drain voltage applied. The DMM diode check should give a typical diode Vf if you reverse the prods to D/S.[/QUOTE]
    So based on my readings what do you think about the mosfet? Is it bad or good. The current flows from drain to source when I apply positive voltage to gate. But as soon as I remove the positive voltage on gate, the current stops passing from drain to source.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    So based on my readings what do you think about the mosfet? Is it bad or good. The current flows from drain to source when I apply positive voltage to gate. But as soon as I remove the positive voltage on gate, the current stops passing from drain to source.[/QUOTE]

    Follow the instructions I gave and you can determine that for yourself.
     
  9. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    Follow the instructions I gave and you can determine that for yourself.[/QUOTE]
    OK thank you very much. My mosfet is not good. Becoz gate voltage is leaking. Thank you sir.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    OK thank you very much. My mosfet is not good. Becoz gate voltage is leaking. Thank you sir.[/QUOTE]

    Try it with a 9V battery with LED and 10k resistor in series as the continuity tester, the gate needs about 6 - 8V to turn it on, if its a bit marginal the drain conductivity could sink away during the test. But if the gate charge doesn't keep it conducting for at least a minute, I'd suspect the metal oxide gate layer may be compromised - it can only withstand about 18 - 20V or so, and can also be damaged if the device runs too hot.
     
  11. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    Try it with a 9V battery with LED and 10k resistor in series as the continuity tester, the gate needs about 6 - 8V to turn it on, if its a bit marginal the drain conductivity could sink away during the test. But if the gate charge doesn't keep it conducting for at least a minute, I'd suspect the metal oxide gate layer may be compromised - it can only withstand about 18 - 20V or so, and can also be damaged if the device runs too hot.[/QUOTE]

    hello,
    Good day. i have done as you told me to do. when i applied the 8v on gate. the led light glow and as soon as i removed the 8v on gate the light stayed for 2 seconds and disconnected.
    so i think metal oxide gate layer is compromised.
    that means that might be the problem with my monitor. i hope when i change it. my monitor comes back to normal. Thank you sir.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    hello,
    Good day. i have done as you told me to do. when i applied the 8v on gate. the led light glow and as soon as i removed the 8v on gate the light stayed for 2 seconds and disconnected.
    so i think metal oxide gate layer is compromised.
    that means that might be the problem with my monitor. i hope when i change it. my monitor comes back to normal. Thank you sir.[/QUOTE]

    A damaged MOSFET usually indicates some other problem that may have caused it.

    You should check/examine all the electrolytic capacitors in the general PSU area, there's often a small one - anything from about 1uF to 47uF associated with the regulation and/or the Vcc for the control chip, this should be replaced with a new part regardless.

    Sometimes faulty electrolytics will be obvious because the top bulges, but you can't absolutely rely on that. If they have been running hot the plastic sleeve may be discoloured - in fact if a capacitor is running hot at all it should be replaced.

    In the UK, most component supply houses for consumer electronics do capacitor replacement kits for popular TVs, set top boxes etc - but I don't know about monitors.
     
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