Reforming a 250V cap

Discussion in 'General Electronics Chat' started by tom66, Aug 13, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
    I have some 250V capacitors here which need to be reformed. Problem is, I do not have a 250V power supply, which would normally be used to reform them. What can I do to reform these?
  2. #12


    Nov 30, 2010
    Build a 250 V power supply. I use a variac to feed a microwave oven transformer. Then rectify, filter, and a resistor in series with the victim.

    Get creative. Are you a wizard, or not?
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
    It would be cheaper for me to buy capacitors that don't need reforming.

    A variac is on my wish list as is an isolation transformer..
  4. someonesdad

    Senior Member

    Jul 7, 2009
    Do you have a suitable transformer with the requisite ratio? You could probably get away with reversing the intended use and step up the line voltage a bit (as long as the ratio isn't too large), then run it through a full wave bridge and filter capacitor.

    As long as transformers are involved and things are isolated, you could use two transformers/rectifiers as mentioned and put them in series to get the desired voltage.

    Aren't you a student? If so, your school might have an old tube power supply laying around that you could use for this.
  5. SgtWookie


    Jul 17, 2007
    Have you seen Ronald Dekkers' "Flyback Converters for Dummies" page?

    That's a basis for creating a HV cap reformer. You'd need to limit the current through the cap while charging to ~10mA until the cap reached 250v.
  6. Adjuster

    Well-Known Member

    Dec 26, 2010

    Warning: 120V AC 60Hz is not universal!
    If the OP is correctly declaring his location as being in Basingstoke, England, he does not need a step-up transformer. The local mains will be about 230V 50 Hz, which comes to over 325V peak. A 1:1 isolating transformer would therefore give more voltage than he needs, and a slight step-down would be more to the point

    On the flyback approach, one "el-cheapo" method might to be to take the guts out of a disposable camera with flash. You might want to replace the big flash capacitor with something smaller, say a few tens of μF rated 450V. The circuit could be run from an adjustable low-voltage supply set to get the right volts out. Add a series resistor of maybe 47kΩ 2W to limit the current, - the voltage drop across the resistor will also let you check the leakage. NB this is the sort of thing only to be attempted by adults with a fair amount of experience: capacitors charged to hundreds of volts can kill.