Re: Capacitor safety questions

Hello,

Lotta good info JJ!

Let me add:
Generally, a capacitor is considered "fully" discharged after 5 time
contants.

Safety organisations like UL set requirements for the voltage level
remaining after 3 time constants. (That is, it must be to a "safe"
value within this time.) Bleeder resistor calcs should bear this in
mind.

Hope this helps,

Ballendo


--- In CAD_CAM_EDM_DRO@yahoogroups.com, John Johnson <johnatl@m...>
wrote:

> The discharge time is given by the formula:
>
> Time = R C
>
> So if you have a 1 000 ohm resistor, and a 12 000 micro Farad
> capacitor, it would be:
>
> 1 000 x 0.012 = 12 seconds
>
> (0.012 is 12 000 micro farads)
>
> Here is the discharge rate:
>
> Time Remaining charge
> 12 37%
> 24 13.5%
> 36 5%
> 48 2%
> 60 <1%
>
> So, for a 100v power supply, after 36 seconds, it will be 5v or

less.

>
> To determine the size (power) resistor you need, use this formula:
>
> E^2
> ------
> R
>
> that is:
>
> voltage x voltage
> ------------------------
> resistance
>
> So in our 100v power supply example:
>
> 100 x 100
> -------------- = 10 watts
> 1000
>
> I would add a 20% or more safety factor and get, say, a 15 watt
> resistor.
>
> To calculate how much current the resistor will use from the supply:
>
> voltage
> ----------
> resistance
>
> or
>
> 100
> -------- = 0.100 amps
> 1000
>
> Connect the resistor directly across the capacitor. I crimp ring

lugs

> on mine and connect directly to the screw terminals on the cap.
>
> Also, when buying a capacitor, always add a safety margin to the
> voltage rating of the cap. For a 100v supply, I would use 125v or

more.

> Since the output voltage of a transformer can rise above the stated
> value when there is no load, blowing a fuse might cause the

transformer

> output to rise above the rating of the capacitor. You don't want to
> blow or stress your cap if this happens. Of course you want to be

extra

> careful about the polarity of the cap too. Connecting one backwards

can

> cause quiet a mess when electrolyte spews everywhere.
>
> Regards,
> JJ