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Help calculating capacitor value for a unique application.

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

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  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
    resistive 35-50 Ohm load.

    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...
     
  2. legg

    legg Guest

    Google for 'rectifier applications handbook'.

    RL
     
  3. Eeyore

    Eeyore Guest

    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 Bruhns

    Tom Bruhns Guest

    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
     
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