Re: [CAD_CAM_EDM_DRO] Bleed resistor AND Led

jbolt001 wrote:

> Sorry to rehash this againg but I'm not getting the whole picture. My
> PS is 24VAC at the secondary so this should be 33.6VDC at the CAP.
> The CAP is 25000uF 50V.
>
> What size bleed resistor do I need and could someone show me the math
> to figure this out.

I is current in amperes
V is voltage in Volts
R is resistance in Ohms
P is power in Watts

I = V/R
or
R=V/I

So, if you want to draw 100 mA (0.1A) at 33.6 V, you should use a
resistor of 33.6/0.1 = 336 Ohms, roughly. This will dissipate P = i ^2 * R
or 3.36 W. You can use a 5 or 10 W resistor.

> Can I connect the (-) side of the bleed resistor
> to the gound side of the normaly closed contactor used for the Gecko
> disable instead of across the cap or is this a bad idea? My thinking
> is to only have the bleed resistor active when the power is off.

If you want a fast bleed down to zero volts, this is a good idea.

> For the LED across the CAP what size resistor do I put in series and
> again the math to figure this out?

Just subtract 1.7 from the voltage, but you should probably try a current of
20 to 40 mA, so using the above, R = 31.9 / 0.02 = 1595 Ohms.
P=(0.02)^2 * 1600 = 0.64 W. SO, a 1 or 2 W resistor should work fine.

Jon