[sdiy] How to discharge a capacitor ?

[Seb Francis]

The resistor you use depends on the voltage of the cap.  use this formula:

Power = V*V/R
R=V*V/Power

So if you are using a 0.5W resistor for 15V charge then you need to use 
at least 450R resistor to avoid melting it.

Then there is the question of discharge time .. the capacitor will 
discharge logarithmically so in theory it never completely discharges, 
but in practice if you wait for 5*R*C seconds it will be pretty much 
discharged.

So taking the above example with a 4700uF cap it would take about 10 
seconds to discharge.

A higher power resistor will be quicker, especially for higher 
voltages.  Also bear in mind that you can probably exceed a resistor's 
power rating for short periods, and depending on how fully you want to 
discharge the cap you may not need to wait a full 5RC seconds.  e.g. 3RC 
seconds will get down to approx 5% of original voltage.

Seb




megaohm wrote:

> I'll try and be more specific:
>  
>   Let's say I want to discharge a 4700uF electrolytic capacitor. There 
> are a couple in the power supply portion of the circuit. Do I take one 
> end of the resistor and attach it to the negative leg of the cap, then 
> take the other end of the resistor and attach it to the positive leg 
> of the cap? What size resistor do you suggest? What power rating 
> should the resistor have? There are also some larger and smaller caps, 
> will the same resistor work for them? Or should it be larger, smaller, 
> different power rating?
>  
> peng
>
>  
> On 11/15/05, *Batz Goodfortune* <batzman-nr at all-electric.com 
> <mailto:batzman-nr at all-electric.com>> wrote:
>
>     Y-ellow All.
>
>     At 03:30 PM 11/15/05 -0800, Harry Bissell Jr wrote:
>     >megaohm < megaohm1 at gmail.com <mailto:megaohm1 at gmail.com>> wrote:
>     >>    Sort of an "idiot's guide" to discharging caps. This would be
>     >> immensly helpful.
>     >That'd be EASY.
>     >
>     >Discharge them with a screwdriver...  ;^P
>
>     And if you don't have a screwdriver, the old standby of sticking your
>     tongue across the terminals is often workable.
>
>     I have a mate who once claimed he could measure up to 50 volts
>     with his
>     tongue. His tongue was accurate to within about 1% but got a bit
>     non-linear
>     after about 35 volts. Mind you he was also the engineer who claimed...
>             "There is no part of the human body that cannot be used in the
>     pursuit of electronic engineering."
>
>     And I'll leave the rest to your imagination.
>
>     NOTE: Leave the bleed resistor on the cap for a while, they have a
>     nasty
>     habit of self charging. Something else Nige could tell you about
>     at length.
>
>     Be absolutely Icebox.
>
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>