Supplying 0v to turn off mosfet.

Discussion in 'General Electronics Chat' started by cjdelphi, Jun 20, 2010.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    Basically the transistor which i'm using now a 2N222A all is good and when pin 3 supplies off to the transistor, the transistor turns off the flashing LED, really simple...

    But when swapped for an mosfet, IRF or anything similar, the capacitor in the mosfet keeps the output on to the LED (even though the signal from 555 pin 3 is off), what's the solution to this?
     
  2. sceadwian

    New Member

    Jun 1, 2009
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    Supply a path to ground for the current stored in the capacitance of the gate.
    IE use a pulldown resistor, you need to supply a circuit diagram to get any more specific than that because a 555 should be able to discharge the gate capacitance if it's used normally they can sink as well as source current.
     
  3. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    So a 10k resistor between ground and gate should be fine then.... cheers
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    We are just guessing, because there is no schematic to review. The 555 sources and sinks current at pin 3. Is that your output?

    A 10K resistor, as suggested, won't hurt. But, something else may be causing this problem.

    John
     
  5. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
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    Alright, turned out (by coincidence) that both my mosfets i had were fried, i tried a 3rd one and no longer had the issue, working just fine without the resistor..

    Thanks....
     
  6. sceadwian

    New Member

    Jun 1, 2009
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    Mosfets are finicky chips, even even without a visible/audible arc discharge it doesn't take much voltage to fry their gates, only a bit over 20 volts.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    These little jewels are also prone to be destroyed by ESD pulses that are so small that you will not likely feel it.

    An investment in a good ESD wrist strap will pay you back in dividends if you use it consistently.

    hgmjr
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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  9. sceadwian

    New Member

    Jun 1, 2009
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    hgmjr, sounds like what I said! =) I've never bothered using anti static straps though, there's a far easier way to avoid static build up, simply make sure there's metal objects around that are grounded to touch, and always touch them before you handle ICs. The case of the power supply on my desk is grounded I just slap my hands on that before I touch a FET, or if you're really paranoid hold one hand on the grounding source and use the other to handle the chips.

    I personally think the hazard of knocking something over with the wire or walking away from a bench with it on is more worrisome than static generally is if you take the proper basic precautions.
     
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