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New Here, Considering Capacitors.

Discussion in 'General Electronics Discussion' started by JoshuaE, Feb 6, 2018.

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  1. JoshuaE

    JoshuaE

    8
    1
    Feb 6, 2018
    This is my first post and I am just starting with electronics. My question is about discharging a capacitor. I read that with a very large capacity there could be problems shorting out it's terminals for discharging and really shorting a capacitor is something that should be avoided.

    https://electronics.stackexchange.c...r-determining-shock-hazard-of-capacitorsThere

    On the other hand, I have also read that a screwdriver can be used to discharge the capacitor by connecting the terminals. Or even a resistor or multimeter. Isn't this shorting too?

    Which one is correct? And is there any harm in discharging the unplugged circuit by touching these two leads with one of these three devices, and not my hands?

    And finally is it always safe to touch the actual capacitor, not the wires?

    Thank you,

    JoshuaE
     
  2. Minder

    Minder

    2,955
    624
    Apr 24, 2015
    I have a selection of screwdrivers that show blow marks on them from pulling that trick.
    The capacitor charge is only dangerous if it is charged to a high level.
    Over 50v is considered a dangerous level.
    M.
     
  3. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    For the most part, the capacitors you run across will not have enough voltage on them to harm you, but there are exceptions. For instance, do not open up a microwave oven and touch any capacitor terminals.

    The energy in a capacitor depends on its capacitance and the square of the voltage it is charged to. Shorting a 1000uF cap charged to 300V is dangerous. Shorting a 100nF cap charged to 10V is not.

    Bob
     
    Last edited: Feb 6, 2018
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,387
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    Jan 21, 2010
    A screwdriver presents a very low resistance (very close to zero ohms) to the capacitor. This it often referred to as a dead short.

    The only thing limiting the magnitude of the current when you place a dead short across a capacitor is the resistance. In this case it may be the internal resistance of the capacitor that is the highest resistance.

    Where a capacitor has little energy, the high current pulse may be so brief that nothing has the opportunity to heat up. However, when a large amount of energy is stored, it is possible to blow small pieces off the screwdriver or even to damage the capacitor. In addition to this, you'll hear a click or even a bang. You may even notice your screwdriver has slightly welded itself to the terminals of the capacitor.

    A resistor used to discharge a capacitor will do so in a more controlled manner. The choice of resistor in combination with the voltage across the capacitor will determine the initial (peak) current, and the relationship between the resistance and the capacitance will determine how fast the discharge occurs. Some care is required, too small a value can result in similar issues as with a screwdriver, too large a value can result in the capacitor discharging very slowly. If the resistor cannot handle the initial current it can blow like a fuse. If the resistor cannot handle the initial power it can get very hot and smoke, burst into flames, or fail like a smokey fuse.

    A multimeter on a voltage range is a high resistance and you may be able to see a low value capacitance discharge over time. The resistance is generally high enough that this rate of discharge may be very slow (sometimes so slow as to be unobservable).

    On a current range a multimeter has a very low resistance. Discharging a capacitor with this is like using a screwdriver, except that the high current pulse can either blow internal fuses or even damage the multimeter permanently.
     
    Cannonball likes this.
  5. CTP4500

    CTP4500

    21
    0
    Mar 3, 2018
    I saw this similar idea in a movie. But basically connect both negative, positive side to a lamp, and the lamp will glow a few seconds discharging the capacitor.
     
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