Cap bleeding

Discussion in 'General Electronics Chat' started by thegman, Sep 11, 2010.

  1. thegman

    Thread Starter New Member

    Sep 11, 2010
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    what size resistor to discharge 112,000 uf @25 volts cap. Not being smart, but Tired of waiting for cap to discharge or dead short it for my power supply. Just need to know if someone could tell me bleed size for say 10 minutes?
    Thanks
    G
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Tired of notches on your screwdriver blade? Put in a switch to place a 10 ohm 25 watt resistor across the capacitor. Any toggle switch can handle the 2.5 amp initial current (assuming you are running at the edge of destruction right at 25 volts). You really need some voltage headroom with a cap that size - they can wreck the equipment they are installed in if they get leaky.
     
  3. thegman

    Thread Starter New Member

    Sep 11, 2010
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    I'll just be darn a straight answer! Reason I say that is because some gentlemen forget they had to learn as well and tend to get a bit nasty. I certainly thank you for that answer. I was out of this for awhile and forgot my savy on what and how, ok ok I got lazy..LOL I've got a 900 amp supply I have not fired up in years and I just built this 200 amp and couldn't remember my math and how on the bleeder....LOL I was into my antennas and radios and forgot about the power supply stuff. But anyhoot thanks a million
    Mr G
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Just be sure you keep track of that bleeder switch. The resistor wattage is very light for the initial current and voltage, but the charge comes off the cap pretty quick, so the 25 watt resistor should only get a bit hot. It will die horribly if the supply is turned on, though - the dissipation for that is 62.5 watts. If you might forget the switch, use a 100 watt resistor.
     
  5. PackratKing

    Well-Known Member

    Jul 13, 2008
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    For work on photo- studio strobe equipment, I made up a bleeder with a 100 Ω 50 watt ceramic wirewound, in series with a 10 amp SPMC pushbutton, housed in a piece of sched. 40 plastic waterpipe, and potted with high-temp rtv silicone, 12" #18 leads w/ ratshack alligators.......

    Capacitors in those monsters can run 350 volt, as high as 2K μF, and usually a gang of up to six is the largest I've seen / worked on. Normal consumer EFU have main caps anywhere from 200-1200 ΩF

    One does not want to run afoul of them.......lightning just plain hurts :D
     
    Last edited: Sep 12, 2010
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Going with Beenthere's idea, you might use a N.C. relay (or the NC side of a SPDT relay) whos' coil is powered by ON switch.

    When power to the primary side of the transformer goes on, the relay coil is energized and the contacts open up.
    When primary power is turned off, the relay contacts close. Use the resistor in series with the contacts to bleed the charge off quickly. That way you won't forget.

    Another idea is to use a couple of automotive-type incandescent lamps in series; say a couple of 1156 bulbs. The advantage of using incandescent lamps is that they have a non-linear resistance characteristic; when the filament is hot, it has a high resistance; as it cools down the resistance drops. This non-linear curve will discharge a capacitor very quickly, and you also get a positive indication of the charge level by the intensity of the lamps.

    Here's what I'm talking about (parts to add highlighted in red):
    [​IMG]

    If you REALLY want to keep it simple, just replace the power on/off switch with a DPDT switch, and use one side of it to switch power on to the primary, and the other side to switch the bulbs or resistor to ground.
     
    Last edited: Sep 12, 2010
    PackratKing likes this.
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You should see some of the capacitor banks I've seen back when I worked around lasers. Most were housed separately from the actual supply and triggering/driving circuitry and had safety interlocks. If you had to bleed one it was an overnight situation.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    And then you had to replace some power transistor mounted on a beryllium oxide (can you say brittle?) insulator.
     
  9. PackratKing

    Well-Known Member

    Jul 13, 2008
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    :p
    I can certainly appreciate that....one thing I get a charge out of [[ OK ... lousy pun ]] is the current crop of "Fast & Furious " wannabes with the 1000 + watt "ghetto blasters " they install in their hotrods. Part of the system to power the subwoofers in these monstrosities, are caps in excess of a farad or more :eek:

    On that note, have you ever seen the caps related to diesel-electric locomotives ?? And then there are the U.S. Military "ground-wave" transmitters..........

    I enjoy messing with the storage ability of large caps....in a saner range of say 50v 100K μF.............the right cap will keep an led glowing, or a hobby motor spinning for a loooooong time after the power is cut off :p
     
    Last edited: Sep 12, 2010
  10. thegman

    Thread Starter New Member

    Sep 11, 2010
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    I went with a mon push switch and since I had a bunch of 100 ohm 5 watters ( fire proof) around I stacked ( 8) of them together in a piece of pvc and when I want to get'er done I push that button. This is only because when I change RF Amps I don't want to forget this thing is still alive in my upper years now. LOL Plus I want to make it a few more years at least long enough to throw the kids off of my will trail..
    Next project is getting this Big Ole regulator for my 900 amp'r down to a smaller size...
     
  11. thegman

    Thread Starter New Member

    Sep 11, 2010
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    I meant to say special thanks to Packratking in which I used that idea except I used a momentary switch in order to release a little at a time or hold it as long as it takes....
    SgtWookie I use that method in my 900 amp'r but I use two relays. Been working for 14 plus years that way.. You stay out of a certain things for awhile you tend to forget the things that really matter.
    Again thanks
    Guys/Girls
     
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