Re: [CAD_CAM_EDM_DRO] Power Supply Guidance Needed

Larry, Bill,
I've designed some pretty hefty linear power supplies at work before
(in a past life). I always made sure my caps were discharged in less
than 15 seconds (can't remember if it was CSA of TÜV standard). But my
bleeder resistors were driven off of a relay. I ran the relay (usually
24V AC coil, sometimes 120V) directly off of the transformer. That way I
wasn't wasting power, and generating heat while the supply was running.
Since it's DC I rated the contacts 2x the peak current the resistors
would see (make sure you're using a relay rated for DC). And I usually
didn't over rate the power dissipation of the resistors much since the
duty cycle was so low.

Darren Clark



woodwelder@... wrote:

>Hi Larry,
>
>Bleeder resistors just discharge the stored energy in the caps after a power off cycle, and the number I've always used is "discharge the caps in one minute (60 secs)", because usually the caps are inside a box and you can't get to them to shock yourself in less time than that.
>
>So, if you use the formula I = C (dv/dt), a good discharge amperage for a 24,000 uf cap with a 100 volt charge would be:
>
>I = C (dv/dt)
>I = 0.024000 farads ( 100 volts / 60 seconds )
>I= 0.040 amps
>
>And a proper resistor, by Ohms Law (R = E/I)
>
>R = 100 volts / 0.040 amps
>R = 2500 ohms
>
>And the power rating of the resistor should be:
>
>P = I * E
>P = 0.040 amps * 100 volts
>P = 4 watts (absolute minimum, 10 W recommended)
>
>But 4 watts continuous dissipation just for a bleeder resistor seems too wastefull to me. Perhaps making it harder to access the capacitors and use a larger value resistor would be a better design.
>
>Regards,
>
>Bill Newell
>
>