Re-Forming Electrolytics

[Murray Leshner]

I've always taken a paranoid, or conservative approach to current limiting 
during reforming, depending on your point of view.

There are specs for leakage current for new caps, but they vary with many 
factors, as do the number of types of electrolytics.

I wouldn't expect old caps to perform as well as new, of course, but the new 
cap leakage current specs give one a reference point for perhaps choosing a 
current limiting resistor for the re-forming process...I feel better doing 
this instead of just guessing or not measuring.

I recall specs like 0.08*C*V - 0.1*C*V, in mADC.

So, by calculating how much this is for your given cap rating, and selecting 
a resistor that would limit 'short circuit current' to that amount, you know 
that you can't ever exceed a leakage current a happy cap can live with. This 
may be overly conservative...your mileage may vary etc.

For example, 125 uF @ 450 VDC for 0.08CV, 0.1CV and 0.2CV follow.

C, F	VDC	I, DC, A	Rlim	Pdiss
0.000125	450	0.0045	100000	2.025
0.000125	450	0.005625	80000	2.53125
0.000125	450	0.01125	40000	5.0625


The Power Dissipation above is calculated, not the rating of the resistor. 
You should consider doubling that for your resistor rating. The resistor is 
placed in series with either the + or - lead, NOT in parallel with the 
capacitor.

You migh twant to look at a capacitor spec sheet ,say, on Digi-Key.com or 
elsewhere and make sure I'm not off by a factor of 1000 for mA vs A....

Murray

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