# Help calculating capacitor value for a unique application.

Discussion in 'Electronic Design' started by [email protected], Jul 16, 2007.

1. ### Guest

A little background on this process.

Take 208 VAC 60HZ. Hook up to a small SCR voltage controller for
variable voltage. Out to a full wave bridge rectifier. Hooked up to a

Here's the problem. I'm dropping to about 180VDC max at the load
because there is no ripple filtering. I need a much higher voltage. I
want to get this higher voltage by using a capacitor(s) across the
output.

However, I have no idea what value Capacitor to use. I've searched the
web and can't seem to find any calculations thet will help me. I know
it will be high uF due to the voltage and current ~4-5 amps. It
bascially is just a simple DC power supply but I can not find info on
a supply with this high of voltage.

Any ideas???????????

Thanks...

RL

3. ### EeyoreGuest

I suspect that's not going to work very well unless you also add some
inductance.You're going to get some huge current spikes every time the SCR turns
on otherwise.

Graham

4. ### Tom BruhnsGuest

i=C*dv/dt. You have 5 amps for i, and if your SCR controller gives
you a charging pulse every half-cycle, the maximum delta-t between
pulses is 1/(2*f) where f is the line frequency--generally 50Hz or
60Hz. Assume 50Hz: then max delta-t is 0.01 seconds. Now you have a
relationship between C and delta-v. If you want no more than 1 volt
sag, use 5*0.01 farads = 50,000 microfarads. If you can tolerate 10
volts sag (10V p-p ripple), use 5,000 microfarads.

But beware about the controller: if it triggers on a sine wave,
there's very little change between triggering at 90 degrees (the top
of the sine wave) and 0 degrees (the start of the sine), since the cap
charges to the peak voltage during the on state. Also, beware of
triggering the SCR only once; if the trigger happens before the input
voltage exceeds the capacitor voltage, no current flows and the SCR
never turns on that cycle.

Cheers,
Tom