Re: [CAD_CAM_EDM_DRO] Bleeder resistor for power supply size?

John Guenther wrote:

>Good morning everyone,
>
>I know this has been asked before, but I can't find the answer so I will ask
>again.
>
>What is the proper size bleeder resistor for a power supply with 48 volts AC
>at 10 AMPS into the bridge rectifier, using a 15,000 uF capacitor? Your
>assistance will be greatly appreciated.
>
>
>

What is the purpose of the bleeder resistor? To discharge the cap
to a safe value in some time? What is the time?

Let's try it for 10 seconds. Assuming a straight-line discharge, which it
won't be (it will be an exponential decay) you get T=C/I, and I=E/R,
so T= E/(R*C). So, R = E/CT
C=.015 F, E = 67, T=10, R=67/(.015 * 10) = 447 Ohms

Assuming a 500 Ohm resistor, what power do we need?
P = E^2 / R = 67^2/500 = 9W. Use a 10, or better a 25 W resistor to keep
it from getting real hot.

For purists, the exponential decay of an RC circuit is
V = V0 * e ^ -(t/RC), and the capacitor will be discharged to
33% of the original voltage in one "time constant" or RC.
The time constant in this case is .015 * 500 or 7.5 seconds.

Jon