Re: Bleed resistor AND Led

Jay,

It depends on what you are after. Do you want to light an LED or do
you want to discharge your capacitor rapidly?

If you want the latter, here are the considerations:

A resistor in parallel with a capacitor forms a "time constant" that
is measured in seconds. The time constant is the resistance in ohms
multiplied by capacitance in farads.

The capacitor discharges to 37% of the original voltage in 1 time
constant. After 2 time constants it is at 13% and after 3 it is at
5%. It takes the rest of eternity to discharge to 0% because the
decay is exponential. A capacitor is considered discharged after 3
time constants.

To discharge rapidly you need a small time constant. This means a
large capacitor needs a low Ohms resistor (t=RC). Low Ohms means
large currents and heat dissipation, so you start by determining how
much heat you want to tolerate (more heat = larger wattage resistor).

Let's it's 5 Watts. Knowing W=V^2/R and solve for R, R=V^2/W. In your
case V=34 volts and W=5, R=34^2/5, =1156/5, =231 Ohms. Use 270 Ohms
as the next largest standard 10% resistor value.

Now let's see what you get. Your time constant is t=RC, =270
times .025 Farads (25,000 uF), =6.75 seconds. Three time constants
are 3 times 6.75 seconds, or 20.25 seconds. You can consider your
supply discharged after 20 seconds.

Mariss

P.S. The equation for voltage remaining vs. time in seconds is:

V=Vo(e^-t/RC)

Where is is the natural logarithm, Vo is the original voltage, R is
Ohms, C is Farads and t is seconds.

--- In CAD_CAM_EDM_DRO@y..., "jbolt001" <js3mc@a...> 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. 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.
>
> For the LED across the CAP what size resistor do I put in series
and
> again the math to figure this out?
>
> Many Thanks,
>
> Jay