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Thread: Capacitor Safety

  1. #1
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    Capacitor Safety

    Im trying to understand some aspects of capacitor operation and safety.

    1. What is the proper method for discharging capacitors. Will a simple tap of a screwdriver across the terminals do the trick or do I need to involve a circuit with resistors to slow the discharge and reduce the current.

    2. What type of energy / charge can be expected from a capacitor. Take for example a 400V, 390 uF cap. What type of current / power is available from the discharge of a cap this size.

    3. If caps only pass AC and block DC, how and why are caps used on DC systems?

    Thanks
    Bryan P. Holland, MCP

  2. #2
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    Re: Capacitor Safety

    1. I have used a screw driver many times, but it is possible to damage the capacitor using this method. You could use a 5 or 10 ohm resistor rated at 10 Watts.

    2.Several joules for large DC filter capacitors. Use to know, but too lazy to look it up for you. Point is if you had some filter caps used in high voltage power supplies for tube circuits, enough to kill or throw you across the room.

    3. Filters and voltage doublers. One application is DC rectifiers. They are placed across the diode bridge to remove the pulsating AC current. Another application is simple high pass filters, blocking the lower frequencies and passing the higher.

  3. #3
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    Re: Capacitor Safety

    Thanks Dereck. :cool:

    Would a short piece of insulated wire be sufficent to discharge a cap? I wondering what kind of current am I looking at. Would there be any insulation / conductor damage? :confused:
    Bryan P. Holland, MCP

  4. #4
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    Re: Capacitor Safety

    By Dereck: They are placed across the diode bridge to remove the pulsating AC current
    Wouldn't that be DC current?
    I believe OSHA require's that a 1k ohm 10 watt resistor be placed across all external motor run or starting caps and if left around for storage your suppose to have a resistor or some kind of jumper on them. Maybe this is just something I heard through the grape vine?
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  5. #5
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    Re: Capacitor Safety

    The amount of energy a capacitor can deliver is based how quickly the energy is released. For instance, if a capacitor is discharged by shorting (screwdriver), it can release an incredibly large amount of energy in a fraction of a second. This is how flash cameras work.

    Or it can be discharged more slowly. The time it takes to discharge depends on the amount of resistance connected to it.

    Personally, I would use a resistor instead of shorting it.

    [ June 05, 2004, 06:48 PM: Message edited by: peter d ]

  6. #6
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    Re: Capacitor Safety

    A capacitor placed across a DC source will charge up and retain a charge similar to a battery. The larger it is the more energy it can store. Unlike a battery it does not produce energy on its own and will self discharge rather quickly (depending on design) as compared to a battery due to internal leakage. It is always a good idea to discharge a capacitor through a resistor, not just for the capacitor's sake but for the screwdriver's also!

    One application for capacitors that utilizes it's energy storage properties is for filtering in AC to DC rectifier supplies.

    The waveform right after the rectifier is pulsating DC. Picture a chopped in half sine wave. A capacitor (or capacitors) in parallel with the rectifier and load will charge up during the voltage excursions and then discharge (and try to carry the load) to fill in the gaps where the voltage drops to zero. This produces a smooth(er) DC.

    -Hal

  7. #7
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    Re: Capacitor Safety

    Originally posted by hurk27:
    Wouldn't that be DC current?
    Technically it is pulsating DC current, but it has an AC component that you are trying to remove.

    Well designed DC power supplies will have a bleeder resistor across the capacitor to discharge it after power is removed. Typically the size of the resistor in ohms is chosen to be 1/10-to-1/20 of the rated current output. For example if you had a 13.6 volt DC supply @ 30 amps, the resistor would be 4.7 ohms rated at 50 watts. For a power supply of this size the filter cap would be about 300K micro farads electrolytic capacitor (10K/amp) rated at 18 VDC.

    Caps discharge in five time constants (TC). A TC=R*C. So if R = 0 then theoretically the cap discharges instantly. So it is important there be some resistance. How much resistance depends on how long a discharge time you want verse the size of the cap. For example a 300 micro farad cap discharged with a 100 ohm resistor would discharge in 5 * .0003 * 100 = .15 seconds.

    The important thing is you do not want T or R to equal 0 , or R to be so large that it is impractical to use.

    Edited to add example.

    [ June 05, 2004, 11:25 PM: Message edited by: dereckbc ]

  8. #8
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    Re: Capacitor Safety

    Caps can keep DC a long time if there is no load,days or weeks.Before becoming an electrician i did TV repairs,and in the old tube sets 600 volts was not uncommon,and the picture tube itself was a large cap charged to 25 or 30 KV.Caps will charge to the peak voltage.
    Tampa Florida

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