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Becoming The Path Of Least Resistance!

Discussion in 'Electronic Basics' started by Richard Harris, Apr 2, 2006.

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  1. Hi,
    Im trying to modify a digital camera, so I disassembled it and as I was
    so got one mother of a shock, have the blister on my finger to prove it. The
    very large capacitor in the camera discharged on me and it hurt something
    bad, my question is how can I saffely discharge the capacitor so I can work
    on the dam thing without getting zaped. I cant tell exactly what the cap is
    other than its electrolytic and fairly large.

  2. Don Bruder

    Don Bruder Guest

    Take a medium-large resistor, and connect it across the pins of the cap
    "for a while" - How long "a while" is will need to be figured based on
    the value of the cap and the resistor, but a minute or three should be
    adequate for all except the largest resistor/cap combinations.

    And count yourself lucky - Caps can be more dangerous than
    straight-from-the-plug juice.
  3. info

    What would you call medium to large 500K to 1M ?

    Also I was thinking, If I connected my self to an Earth with a high resistor
    in between would this not stop me from getting a bitch of a shock.

  4. Don Bruder

    Don Bruder Guest

    That would be a reasonable starting point.
    That answer would be a resounding "MAYBE".

    A "maybe" that I wouldn't count on as far as I can throw an elephant.

    Bleed your big caps like those in flash units or power supplies down to
    zero before doing anything. And once you've bled them down, short them
    out so they can't bite you with "bounceback". Don't even consider any
    other method unless you don't care about whether you live or die in the
    process, and your life insurance is paid up. It doesn't take much to
    stop your heart if you manage to get a cross-chest discharge to happen.
    You might get lucky a few times, but sooner or later, the timing is
    going to be "just wrong", and you'll die waiting for someone to find you
    and call the ambulance.
  5. Cool thanks,
    Ill bleed them for a long time, and then short and test with a multimeter.
    Also Ill disassemble the thing wearing insulated gloves this time.

    Thanks alot for the help

  6. John  Larkin

    John Larkin Guest

    Something like 2K to 5K, 10 watt wirewound, for a few minutes.
    That won't help at all.

  7. Wow, what kind of voltage and capacitance are we talking about?
  8. John  Larkin

    John Larkin Guest

    Photoflashes run roughly 300 volts typically. Cheap ones store tens of
    joules, and a really juicy pro flash might run 100 j or so.

    Each microfard stores about 1/20 of a joule at 300 volts.

    E = 0.5 * C * V^2

  9. ehsjr

    ehsjr Guest

    Go to a local store that processes film, and sweet talk
    the clerk out of one or more of the disposable camera
    "empties" that are left after they remove the film. Some
    nice low Vce xsistors can be gotten that way, plus the cap.
    Nothing special about it - just (relatively) high voltage.
    Discharge the cap during disassembly! Getting bitten by
    the ~300 V is not likely to be a favorite activity. :)

  10. Also I was thinking, If I connected my self to an Earth with a high resistor
    You got the shock because you were holding the case, i.e. the electric
    "ground" for the camera's electronics and touched the high voltage end
    of the flash strobe capacitor.
    That won't really help as the capacitor will still be charged when you
    start doing any modifications to the camera's electronics.
  11. Yeah, I was recently tinkering with something similar. I had an 820uF
    cap charging to ~350V (roughly 50J) and dumping it thru a 4 ohm
    electromagnet coil from a Ford starter solenoid using a TYN612MFP SCR.
    Pretty good at launching computer case screws across the room. ;-)
    Lately I've been using that same coil to listen to the stepper motor
    impulses of my quartz watch, while timing them to the microsecond with a
    PIC (jitter free even). Problem is my watch has a better crystal than
    the PIC.

    At any rate, AIUI my setup could be discharged from 350V to 9V in right
    at 15 seconds using a 5K resistor. The OP wouldn't really need to wait
    for a few minutes, would he?
  12. Bob Masta

    Bob Masta Guest

    Better yet, clip the meter across the resistor leads and watch
    the cap discharge. Be sure to set the meter to a high voltage range
    to start with. If it looks like the cap voltage is going down too
    slowly, use a smaller R.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  13. John Larkin

    John Larkin Guest

    Lytics have a bad habit of recharging themselves from dielectric
    absorption. If you did the quick discharge to, say, 9 volts, it might
    well charge back up to some 10's of volts if you removed the resistor
    at that point. That won't zap you, but you'd still get a nice little
    spark if you shorted it later. Minutes of discharge will pretty much
    clean it out.

    It would be fun to get a *big* sheet of copperclad (as in *big),
    drill/cut/etch a low-impedance pattern, and start collecting caps from
    disposable cameras, and adding them over the years. Around a kilojoule
    or three, things start to get interesting.

  14. Owww...thanks! That gave me an idea for an inductive pickup.
  15. Det var smuk og solrig dag da Don Bruder skrev
    i sci.electronics.basics:
    More like a NO WAY. He would have to be sure that he isn't connected to
    ground anywhere else. Then when he is sure that he isn't connected to
    ground anywhere, why would he connect himself to ground through a resistor?

    To quote the subject:Becoming The Path Of Least Resistance! That path would
    not be through the resistor, thus he would get zapped when he touch ground
    somewhere(of course by mistake).
  16. "DecaturTxCowboy"
    They're great, and they only cost about $5 brand new. Just be careful
    when removing the coil from the casing; if you break the inside lead off
    too short, you're gonna have a real hard time getting some more slack in
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