Re: Capacitor safety questions

> > >
> > >I just ordered a 12,000uF x 100V capacitor for my power supply.


> Jon, I have a very similar setup and got buzzed a few times and
> like to find out more on the "bleeder resistors" you mensioned
> my system retains power when unpluged for more then 24 hours and
> I think this may be the answer to my problem.
> What I am looking for is something that will drain power after
> shutoff in about an hour or earlier so I can work on it.
> Will this bleeder resistor reduce power output during normal
> opperation?
> What size partno. where to get and how should it be wired up.
> Thanks for any info you could provide.

There are a few ways to bleed the energy stored in the caps.

A bleeder resistor is simple, just use Ohm's law to figure out how
much energy you want to use/lose.

At 100 volts, if you used 1,000 ohms, you would bleed off 0.1 amps,
or 10 watts. pretty big resistor, and it would get pretty hot.

10,000 ohms, 0.01 amps, 1 watt.

Another method a lot of people use is to power LED's or lights or a
fan or other device from the DC bus. Whatever you connect, it will
use the energy until the cap bleeds down.

As far as energy used, it will add to the total load, but unless you
are at the bleeding edge of the transformer, the use will not detract
from your available power.

LED's are not fast bleeding devices as a typical LED will be rated
for 20mADC.

As far as discharge speed, the cap loses it's power exponentially.
Theoretically, it will never discharge completely. Since the
discharge it not a simple linear rate, there is a method of
calculating discharge rates based on time constants.

The question really becomes how fast you perceive the need to bleed
of the energy.

My drivers (Gecko's) remain powered, and hold the motors in position,
therefore they effectively bleed the cap down fast enough for me.

To figure out the way you would prefer, fan, resistor, lights, etc,
let us know how fast you want to lose power. This will be in seconds
or minutes.

Also, in the files section, Mariss put a circuit to bleed down the
caps when you shut down. Just in case you want to go a more
elaborate route.


Dave