[sdiy] How to discharge a capacitor ?

[harrybissell]

OK here's what I'd do...

Measure the voltage on the cap, or just use the printed rating on the
cap.

let's say its 50V

Pick some resistor value you have handy. It should ideally be a higher
wattage
resistor, like a wirewound type. Lets guess a 1K resistor to start...

50V / 1Kohms = .05A (peak)
.05A x 50V = 25W (peak)

So a 25W resistor would be absolutely safe for this discharge. The
voltage on the
cap will not be there constantly, it will exponentially decay. So the
power rating here
is estimated way on the high side.

The time constant for the discharge will be RC (4700 ^ -6 x 1000) = 4.7
seconds.
Assume 5 x RC for a total discharge = 23 seconds.

In real life most power resistors can take a 10X power overload for ten
seconds... so even a 1K 5W would probably be OK... BUT it will get hot
enough to burn your fingers.
Use clip leads and don't touch the resistor and it would be OK.

Most capacitors are NOT charged to the printed voltage, so if the
voltage was really
lower, you can use a smaller wattage resistor.

~or~ you could pick a higher VALUE resistor, which will cut the wattage
a lot, and
increase the discharge time (a lot).

If the resistor value was 10K, the wattage would be only .25W but the
discharge
(5 x RC) would take 235 seconds.

Now... a screwdriver might be .01ohms ???  So the current would be
50V / .01ohms = 5000A (peak)

and the wattage in the screwdriver would be 250,000 W (peak)

woah that CAN'T be right can it ?  Lets assume that the CAPACITOR has an

internal resistance (ESR) of .1 ohms.  Total R = .101 ohms

50V / .101ohms = 495 A (peak)

and the wattage is 24,752 W (peak)

woah that's a lot better... but 90% of that power was dissipated in the
capacitor.

Fortunately, after you do that ONCE, the internal resistance of the cap
will be
much higher (what did you expect of thin aluminum foil ???).  So the
next time
the power will be quite a bit lower... and the ESR of the cap will be
much higher.

So now you don't have to worry as much, because your capacitor is not
the man
it once was ???

Moral : Limit current in a discharge to a safe and reasonable value

H^) harry (who spent today repeatedly passing 360,000 W through a load
for
thirty seconds, once every five seconds.  Now how hot do you think THAT
resistor
got ?  After hooking it up to its own water hoses and giving it three
gallons a minute
of cold water, it did not turn the water to steam anymore :^)

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> wrote:
>
>      Y-ellow All.
>
>      At 03:30 PM 11/15/05 -0800, Harry Bissell Jr wrote:
>      >megaohm < 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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>
>