Capacitive voltage divider for AC circuit

Discussion in 'General Electronics Chat' started by Tammy1000, Oct 6, 2008.

Thread Status:
Not open for further replies.
  1. Tammy1000

    Thread Starter New Member

    Oct 6, 2008

    A long time ago, I read that if you use two capacitors as a voltage divider on an AC input circuit (instead of using a transformer) to supply a low voltage to a bridge rectifier, that the capacitors had to be of equal value.

    I have been web searching, and I can't find any information about why the capacitors would have to be of equal value.

    Can someone tell why this is true or where to look?

  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    Using capacitors isn't an efficient method to get a lowered AC voltage. If you need a very small current, then it can save a more costly transformer.

    I don't know why they would need to be equal value, unless you wanted to divide by two exactly. You can pick the reactances to give you whatever AC voltage you require.

    So, for example, if you have 120VAC and want 12VAC (both at 60Hz),

    120VAC- C1 -12VAC -C2 - GND

    C1 = 9uF
    C2 = 1uF

    You can treat the capacitance as resistors, as in the resistor divider formula.

    The output current capability is another calculation. As a rule of thumb, you should set the divider current to at least 10X the output current you require. So, if you require 10mA, then you need 100mA to flow through the divider.

    So, Vin = 120VAC
    Iin = 100mA
    Rin = 1200 ohms
    Xc = 1/(2*pi*f*C)
    C = 1/(2*pi*f*Xc)
    C = 1/(2*3.1416*60*1200)
    Ct = 2.21uF

    So, you'd just set the ratio the same as the above with this total capacitance.

    Power dissipation would be significant though, ~12W is wasted. For a total output of 120mW. You can set the 10X requirement a bit lower, but your voltage will droop significantly under load.

  3. SgtWookie


    Jul 17, 2007
    The use of such techniques is strongly discouraged here, as there is no positive isolation from mains power.

    Use a suitably sized transformer. They are vastly less expensive than a funeral.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    There is no substitute for a transformer for safety reasons.
Thread Status:
Not open for further replies.