oscillation in amplifier

Discussion in 'General Electronics Chat' started by misael.mata, Oct 30, 2014.

  1. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    hello there im having a problem with a kicker amplifier where my mosfets get instantly hot, i had thought i found a short in a ceramic capacitor but i pulled it out and seems okay, its in the same circuit as the rectifiers. how could i eliminate this short or is it like this, rectifiers check out ok

    this is the amplifier its a kicker zx350 couldnt find a schematic

    and this is the ceramic capacitor that seems shorted only in circuit, its next to the resistor. irfz44 getting instantly hot. please help
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,027
    In my limited experience, when MOSFETs fail, they fail to a short and destroy themselves. Perhaps yours are bad? You can check for a dead short across the pins in-circuit, but you have to remove them for better testing. If you do, be careful to guard against static and be sure to control the gate voltage before deciding on the MOSFET. The gate can hold a charge to hold itself "on" while disconnected.
     
  3. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    Thank you very much for the response, but the fets are new. How would I be able to control the gate voltage?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    You simply tie it to either power rail, positive or negative, so that it doesn't float. You can do it directly but using a 1K resistor, for instance, will prevent further damage if the MOSFET has failed and its gate is shorted to drain or source.
     
  5. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    0
    thanks but where would i have to tie it down to. i just solder in the resistor on the gate?
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    What sort of oscillation? - is the amplifier impersonating an oscillator, or have you scoped it and found parasitic oscillations on signal peaks?

    A favourite trick is putting "stopper resistors" in series with MOSFET gates, about 100R is a good starting place.

    More for RF work, but sometimes a ferrite bead slipped onto the gate lead can help.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    We're talking about a MOSFET that has been removed from the circuit board. I would use test leads with alligator clips to make the test connections. Or a quick dab of solder if that's handy.
     
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