Re: [CAD_CAM_EDM_DRO] bleed resistor

Scot Rogers 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.

Well, the 80 V is the most important thing. So, you can put a 5 K Ohm
resistor across it, and it will draw 16 mA. You can put an LED in series with

this and have a caps still charged indicator. The time constant
(RC) = 49.5 seconds, so the caps will drain to 36% of 80 V in 49.5 seconds.
That will be 28 V. That will be safe to touch, but not safe to accidentally
short
out. When the LED is out, it will be safe to short it out, too.

Anyway, with a 5 K Ohm resistor, power will be 80 * 80 / 5000 = 1.28 W
Use a 5 W wirewound power resistor, and everything should be fine.

You could go with a slightly lower resistance to get a faster discharge time.
You could also use a much lower resistance with a relay operated by the
power line. This would be connected through the normally closed contacts,
so the resistor only drained the caps when the relay was de-energized.
A 100 Ohm 10 W resistor should work there, and will discharge the cap
bank in about 3 seconds.

Jon