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Capacitor charge/discharge

  1. Jul 7, 2011 #1
    I have a capacitor from a camera. It is rated at 330v 150uF. I want to charge this capacitor with a 9volt battery. It would be preferable if I could leave the battery attached to the circuit until the cap is discharged. I want as much power to flow out during discharge as the cap can handle.
    What value of resistor for charging is safe but provides for a quick charge?
    Is it OK to leave the battery attached to the cap for longer than is needed to charge?
    How much resistance should be in the circuit during discharge to prevent damage to the cap?

    Thank you,
  2. jcsd
  3. Jul 8, 2011 #2


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    Science Advisor

    If you charge this capacitor directly from a 9 volt battery, it will only charge to 9 volts.

    150 uF is not a very big capacitor so if you shorted it out after charging, you would just get a small spark.

    The discharge current depends on the internal resistance of the capacitor as well as the resistance in the discharge path.

    It is OK to leave the capacitor across the 9 V battery, but shorting the capacitor while the battery was there would also short the battery and this may reduce the life of the battery if it persisted for more than a second or so.
  4. Jul 8, 2011 #3
    That raises a few questions.
    1. Why was a 330v capacitor in a camera with a 12v battery?
    2. That cap makes a flash go off, are there other parts helping out the flash?
    3. I want to launch my estes with this cap, will it be enough to power an estes ignition?
  5. Jul 8, 2011 #4
    There's a circuit in the camera that elevates the battery voltage to maybe 300 volts to charge the capacitor.
  6. Jul 9, 2011 #5
    Yes, you need the entire camera flash circuitry. It has a DC/AC converter, a step-up transformer, and a rectifier which turns the AC 300V back into DC to charge the capacitor.

    DANGER, BE WARNED. The capacitor voltage is lethal. It's more unsafe than playing around with 120VAC line voltages. You can zap yourself very badly, and perhaps trigger some lethal heart fibrillation.

    You can get the strobe charger circuitry out of a cheap disposable camera. ALso I think these entire devices are commonly sold by surplus mail-order. But DON'T mess with these unless you already know how to use high voltage without killing yourself.
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