Discussion in 'Homework Help' started by Siyabonga, Apr 7, 2011.

  1. Siyabonga

    Thread Starter New Member

    Mar 31, 2011
    why does a capacitor discharges when it is has been charged and if you swicth off the power source it is connected on.
  2. electronis whiz

    Well-Known Member

    Jul 29, 2010
    there are many resons why capacitors discharge. there can be a load on them for example an led,IC, motor, speaker, ETC. all draw current this can discharge a capacitor. or there may be a bleeder resistor conected in paralell to it. this will slowly discharge it when no power is applied. the biger the capacitor the longer it will take to discharge. phisical size as well as the capacitance make a diference. a 1 farad capacitor will provide power longer than a .01farad capacitor. they can hold charges for long times some even years. however i beleve they deteriate on holding time with age and use. this is why when working on divices you need to be carefull and discharge before touching. if you need to discharge you may use a screwdriver however i would not recomend it. the best way is to use a 100 watt light bulb to discharge it. i use a pencel sharpened on bothe ends and a clip lead the pencil acts as a many K ohm resistor makes small spark and it is dead.
    Siyabonga likes this.
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    Pencils are designed for writing and drawing. Their electrical characteristics are not specified, and so they should not be used to discharge capacitors.

    Amongst other things, anyone doing so risks electric shock, fire, burns, possible injuries to the eye or other bodily harm. There is a particular danger to eyesight because an arc may develop at the point of contact, which is just where the operator is likely to be looking.

    You may object that you may have done this many times without incident. That does not alter the fact that someone else may come to harm, perhaps using a pencil of different composition, or tackling capacitors containing more stored energy. The idea of some inexperienced person doing this to (say) the storage capacitors in a large photographic flash unit makes me shudder.

    As for using a screwdriver to apply a short-circuit, this is really unwise. The sparks produced can be dangerous. There is a real possibility of damaging any circuit so treated, and you may even ruin the screwdriver!

    Anyone wanting a tool for discharging capacitors should obtain well insulated probes, connected to resistors properly rated to cope with the maximum voltages and energies that the capacitors may hold.