Current limiting AC using capacitors, how does it work?

Discussion in 'The Projects Forum' started by rudyauction8, May 9, 2014.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    I've been trying to build a lead-acid charger for quite a while. Burned a few transformers, tried bulb in series with the battery: not enough current. Today I pulled a capacitor from an induction motor. I wired that in series with the battery and get quite a bit of current and the battery voltage stays within it's limits so it's not a direct short.

    What I don't understand is how the extra voltage is dropped.

    Say I feed in 120 volts and the current is 5 amps. The output is near 24 volts (2x 12v batteries in series). Would the current be multiplied and the battery taking in 20+ amps or would the battery see only 5 amps? The capacitors became warm but not hot. I don't have a functioning amp meter so I can't figure this one out on my own.

    Side note: the AC is rectified by a full wave bridge rectifier, so the battery gets charged with DC current. My current setup with the capacitor seems to charge the battery very quickly, but not dangerously fast.
  2. bertus


    Apr 5, 2008

    Can you post a schematic?
    As you might know, we do not allow transformerless powersuplies.

  3. DickCappels


    Aug 21, 2008
    Sounds like you need to add an isolation transformer.

    Two small items: Capacitors vary widely in how power they dissipate when handling AC, and without a series resistance or inductance in series, the inrush current if the circuit were to be switched on near the peak of the power cycle can be high enough to destroy components.
  4. MrChips


    Oct 2, 2009
    Sounds like you need to discontinue doing what you are doing and get a proper battery charger.
  5. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    Looks like I need to go to another forum for help. I'm not about to stop building this circuit.
  6. MrChips


    Oct 2, 2009
    We're doing this out of concern for your own safety.
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Yet another Darwin Award entry?
  8. wayneh


    Sep 9, 2010
    This approach can work but is unreliable and unsafe. The capacitor completely charges and discharges at 120Hz. The average current is limited by the capacitance, that's how it "works". (Note that the average does not respond to the state of charge of the battery, so it should not exceed the safe trickle current for the battery.)

    But the peak or instantaneous current that the capacitor and the battery sees is limited only by the rectifier diodes, wire resistance and the battery internal resistance, and any resistance in the capacitor itself. These are all "unknown" and variable with temperature and other factors (battery state of charge, for instance). It's just a matter of time until the weakest link manifests itself.

    Aren't high voltage pulses used for desulfating lead batteries? I don't think that's a process you want to use all the time.
    Last edited: May 9, 2014
  9. #12


    Nov 30, 2010
    I've seen a lot of line pulsed battery chargers, but not on this website. Try Electro-tech-online