Re: bleed resistor

Is this so that the power supply will discharge when not connected to
anything else? As someone else has indicated in this thread, if the supply
is connected to e.g., some Geckos, they will draw some current & will
therefore discharge the caps.

However, if you want to know what size of bleed resistor to use to
discharge the capacitors when the supply is switched off & is not connected
up to anything, then the answer depends on how quick you want the discharge
to be. If you connect a resistor across a charged capacitor, the decay
"time constant" is the resistance in Ohms multiplied by the capacitance in
Farads. The time constant gives you the time (in seconds) for the voltage
to drop to 63.2% of its original value. The capacitor is generally
considered to be discharged after 5 times this time constant has elapsed.

So, for your example, you have 0.0099 of a Farad of capacitance (3 times
3300 microfarads), so if you were after a discharge time of 1 second, you
would want a time constant of 0.2 of a second. Approx 20 Ohms across the
capacitors will give you this time constant; however, with an 80V supply
rail, the resistor will draw 4 amps all the time the supply is switched on,
which is a bit excessive & would be a massive waste of power, not to
mention needing a very beefy resistor to dissipate the heat. A more
reasonable choice might be to go for, say, discharge time of 100 seconds,
needing a time constant of 20 seconds, therefore a resistance value 2K
Ohms. This is still going to draw 40 milliamps at your 80V supply rail, so
will be dissipating about 3.2 Watts - still needing a 5 Watt rated resistor
- but not too bad.

Hope that helps -

Regards,
Tony

At 16:15 12/02/2002 +0000, you wrote:

>So I'm setting up my control box, Surplus geckos and a big ol transformer.
>My buddie and I have done all of the calculations to the best of our
>knowlege. One thing eludes me. I have a paralell array of three 3300uf
>capacitors and I was wondering if any of great electrical knowlege would
>happen to know what rating of bleed resistor I would need to set at the end
>of the array.
>
>Like I said 3300uf at 80V of capacitor rating.