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?

 

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?

 

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?

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..

 

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.

 

Tom,
Have you seen Ronald Dekkers' "Flyback Converters for Dummies" page?
http://www.dos4ever.com/flyback/flyback.html

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.

 

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.

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.