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discharging large capacitors

Discussion in 'General Electronics' started by stoj, Jan 29, 2004.

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  1. stoj

    stoj Guest

    hi all
    i have a home made power supply with a pair of 50v 4700uf capacitors
    (charged to around 45V depending to the transformer) and i was wonder what
    is the best to discharge them due to the fact that i have been zapped by
    them a few times?

    thanks in advanced
  2. JeffM

    JeffM Guest

  3. Quark Ng

    Quark Ng Guest

    Try this...

    Have a momentary pushbutton switch and a 4Kohm 1/2 Watt resistor
    connected in series.

    Then connect one end of that circuit to ground and the other end to
    the positive side of your capacitor.

    When you want to discharge the capacitor, just press the button for 5

    Note the time constant is tau = 1/RC = 1/(4K * 4700u) = 0.05319
    And 5 times tau = 0.265957 seconds.

    So you're really safe already just pressing the button for 1/4 of a
    second. But push it for 5 to be REALLY safe.

    Remember, 1/2 Watt is important. P = V^2/R = (45V)^2/(4Kohm) =
    0.506Watts. But since your V goes down quickly over time, it's ok to
    use 1/2 Watt.

  4. GPG

    GPG Guest

    Time constant = R*C= 18.8 sec for 4K.
    Wiring 10K .5W permanently across cap will reduce 45V to about
    12V after a minute. Vc = Vo*e^-T/R*C
  5. Quark Ng

    Quark Ng Guest

    OOPS, you are indeed correct. What on earth was I thinking of...
    And this here has just proven why engineering companies need at least
    two engineers to work on anything... one to check in case the other
    engineer didn't get enough coffee. :)

  6. Diverdan

    Diverdan Guest

    Hi to all you tech's, I am just a cabinet maker and want to remove an
    old Zenith TV out of a floor console. I am a little fearful of the HV
    in the CRT. What can I do to discharge the HV before attempting to
    remove the unit from the cabinet.
    Can someone help???
  7. JeffM

    JeffM Guest

    want to remove an old Zenith TV out of a floor console.
    I would be more worried about fracturing the back/neck
    of the picture tube and having it implode.

    Having said that,
    the way it is typically done is to hook a jumper lead to the chassis,
    hook the other end of the jumper to the blade of a thin screwdriver,
    slip it under the cup of the Second Anode lead where it contacts the CRT
    and touch the conductor (clip) in the center.
    Holding it there for 2 seconds is more than enough.

    Lift the lip of the cup and disconnect the Second Anode lead.
    Poke the shorting strap into the CRT's Second Anode (hole) once more
    just to be sure.
  8. Get yourself a 2.7 K / 2 Watt resistor and leave in in parallel with
    the main cap. The supply will be safely discharged in less than 10
    seconds after the power is shut off from it. The current draw will be
    very little while the supply is operating. Most supplies are designed
    with this type of bleeder resistor built in.

    If you want to have less current pull and use a smaller resistor you
    can go up to something like a 4.7 K / 1 Watt resistor. I would allow
    about 20 seconds after shut down, to regard the supply to be safely

    Jerry Greenberg
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