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Best way to discharge caps?

Discussion in 'Electronic Basics' started by Richie, Dec 26, 2003.

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  1. Richie

    Richie Guest

    i was recently working on a guitar amp (not plugged in, or hooked up to
    any power source) and got a nasty shock. I always heard putting a metal
    screw driver with a non conductive handle between the two prongs on
    the power chord will discharge whatever voltage that is left in the
    caps. I found that this was not correct, and got a 100-200v shock from
    a totallly dissconnected circuit board (ouchie!) that was off for a few
    hours.. I do not plan on repeating this experience (im lucky i only got
    a minor shock) a 2nd time..

    Whats the best way to discharge these caps? i measured hours later and
    there was still about +180v DCV being stored.. Is there a easy to do





    "The only thing better than sitting outside and
    playing a banjo is sitting outside playing a banjo
    made of the skulls of people that made fun of you in
    elementry school."
  2. That was bad advice! You need a 1000 to 10,000 ohm resistor and a set
    of insulated alligator clips. Ground one clip, then connect the other
    clip to the positive terminal on the capacitor. I would use a 2 watt (or
    higher) resistor in case you run into a higher voltage. Leave it
    connected for a minute, then check the voltage. If it is gone, or down
    to a couple volts, it is safe to unhook the resistor.

    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida
  3. Don Bruder

    Don Bruder Guest

    While the screwdriver-across-the-prongs method can be used, there are
    two considerations: first, precisely which prongs? Not the power cord
    prongs, but the actual "guilty party", the capacitors. The power cord
    may drain SOME capacitors when shorted like that, but I wouldn't want to
    be making any bets on it getting them all (or even the most dangerous
    ones) Second, I feel it probably does bad things to the long-term life
    of the capacitor to "spark it off" like that. I could be wrong, but...
    <shrug> that's the way I feel, so that's the way I do it, and it gets
    the job done just as well.

    Find the biggest caps on the board. Use whatever suits you to grab onto
    a large-ish wattage resistor rated around 10K (not critical - just want
    to have a decent bit of resistance in the circuit, and hopefully of a
    power-handling capability that won't burn up or explode in the process.
    If you don't have a 10K, go with whatever you've got that's kinda
    close.) and use the leads of the resistor to jump the capacitor
    terminals. Hold it there for a second or two. Now do it again, just to
    be sure. Be aware that depending on the size of the cap and ratings of
    the resistor, the resistor may get rather toasty. Or it may stay cool.
    Which is why I suggest you use something to grab the resistor, rather
    than just grabbing onto it bare handed.

    Repeat on any capacitors you don't trust for whatever reason, then play
    on the board to your heart's content.
  4. John Larkin

    John Larkin Guest

    If the cap is 10,000 uF (and many are bigger) and you use a 10K
    resistor, the discharge time constant will be 100 seconds, so there
    will be almost no voltage reduction in a second or two. Use a 50 or
    100 ohm power resistor and hold it for 10 seconds.

  5. The OP mentioned 180 volts, so I doubt there is a 10,000 µF
    Merry Christmas!

    Take care, and God bless.
    Michael A. Terrell
    Central Florida

  6. I like this method myself. I usually use a 15 ohm or maybe 100 ohm 25W
    wirewound power resistor. With such low values a couple of seconds is
    adequate to do the job without doing any calculations first.

    But the real advantage to using such a low resistance is it will usually
    make a small spark when you make initial contact. When you see the spark
    you can be pretty confident you successfully discharged the capacitor,
    whereas without a spark it could be there is a small insulating film or
    oxide layer on your terminals and you don't make any contact at all. As
    such the capacitors remain charged, and even if you try to measure the
    voltage afterwards it is also still possible the meter electrodes will not
    make contact due to the film/oxide layer and you may mistakenly believe they
    have been drained.

    Of course, the amp should have some bleed down resistors on it. If it
    doesn't have any then it would probably be prudent to install some.
  7. John Larkin

    John Larkin Guest

    Bleeders can be nasty. With lots of stored energy, as in a big power
    amp, a bleeder that has a decent time constant (say, not too many
    minutes) can wind up dissipating an impractical amount of power. On
    our bigger NMR gradient amps (running at up to +-180 volts, tons of
    caps) we have a DISCHARGE pushbutton or rocker switch and LEDs to show
    voltage on the caps. We fuse the discharge resistors in case somebody
    decides to press the button while power's on!

    In a lot of cases, you can use a single discharge/bleeeder resistor
    from V+ to V-.

  8. Richie

    Richie Guest

    Wow i really like how i got so many responces over a 24 hr period and
    not one of them was flaming me saying i was dumb for even shocking
    myself in the first place! thats what i was expecting, but instead i
    got nothing but helpful responces! you guys rock! :)

    I'll try using a big wirewound resistor like mentioned.. i have a few
    huge ones lying around that i pulled out of a old tv set..

    and someone mentioned a spark when putting the leads onto the negitive
    and positve pins. Thats exactly what happened when i first went to
    meausure the 180v with my DMM.. scared the shit outta me!!!!

    And for this is for all the people who replied with helpful ideas, the
    cap in question is only 22uF but rated at 450v.. So i guess im lucky
    that i didn't get a way bigger shock, or mabye i took 200v or so off
    the caps when i accidently shocked myself :)



    "The only thing better than sitting outside and
    playing a banjo is sitting outside playing a banjo
    made of the skulls of people that made fun of you in
    elementry school."
  9. DarkMatter

    DarkMatter Guest

    Snipped good response.

    The best way is to use a resistor to ground. The industry calls
    them "bleeders" or "bleed resistors" in this application.

    The best resistor is calculated for a given discharge time, which
    depends on charged voltage and capacitance available, and how hot the
    resistor will be "allowed" to get for a given environment (discharge
    stick, or part of circuit).

    Our HV products bleed off inside 5 seconds IIRC. It is easy to
    calculate the resistance required to bleed off a given capacitance at
    a given voltage in a given time frame.

    The problem is that if the capacitance is too high, the resistor
    will heat a lot more than our HV modules with only nF capacitances in
    their output stage.

    In such cases, longer bleed times should be used, and or higher
    wattage rated resistors, Which can also be figured out.

    It all depends on the conditions that the bleed resistor will be

    In HV circuits, bleeders are almost required by law. At least UL.

    In LV circuits, they can rob power, cause circuit heating, or both,
    so calculating the correct value is always good, particularly if you
    are installing it to be a added circuit element, instead of merely
    discharging your banks at the bench during test and run procedures.
  10. DarkMatter

    DarkMatter Guest

    You surely did reduce voltage if your shock had any duration to
    speak of.

    You should measure it just after a power shutoff, or even while

    That voltage, and the known capacitance should allow you to
    determine if your resistors are going to see a lot of power during
    their capacitor discharge events.

    Since your capacitance is fairly low, I'd say that you won't heat
    'em up to much, if they re not to low in wattage and value.
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