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Reforming Electrolytic Capacitors
  by Bill Degnan - 04/11/2009 15:12
Originally posted on Midatlantic Retro Yahoo Group:

- In midatlanticre----@---groups.com, Mike Loewen wrote:
>
>
> I tore apart my IMSAI in order to remove the four large electrolytic
> capacitors for testing/reforming. There are two 9500uf-30VDC caps and two
> 95000uf-15VDC caps. Here's the rig I put together for bringing them back
> to life:
>
> http://sturgeon.css.psu....h/IMSAI/Reforming-L.jpg
>
> I put this together after reading many documents about testing old
> capacitors. Hidden behind the cap is an 8K power resistor in series with
> the cap. The small meter is measuring the voltage output from the the
> power supply (a HP 6443B 0-120VDC/2.5A unit), in this case 25.0 volts.
> The large meter is measuring the current flowing through the cap, 0.11ma.
> The voltage across the cap is 24.7V at this point.
>
> I started out by raising the voltage by steps, from 3-6-9-12-15-20-25,
> watching the current and making sure it went no higher than 0.5ma. At
> each step, I'd let the cap charge up until the current was down to about
> .05ma then increase the voltage. Once I got to 25.0 volts, I let it sit
> for a while until the current was down to .03ma.
>
> So far I've done one of the 9500uf caps. I'm hoping this procedure is
> effective. How about it, Dan and the rest of the electronics gurus?
>
>
> Mike Loewen mloewen@...
> Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/

Hi Mike,

I use a simpler, more effective method. Power supply negative to cap
negative. Power supply positive to 30kohm resistor to cap positive. Voltmeter
across 30kohm resistor. Cap voltage determines the increments I raise the
supply voltage in. For low voltage caps like yours, I just raise it smoothly
until voltage hitting the cap is its rated voltage... but never exceeding 1ma to
get there. If the cap can't be slowly raised to rated voltage without going
over 1ma, the leakage current is too high and the cap is bad.

If the cap settles down to a low leakage at rated voltage (0.5ma or
preferably much less), I'll next push the cap... by raising supply V until V
hitting cap is 10% OVER the rated voltage... so 33volts and 16.5 volts for your
examples. All the while keeping an eye on the leakage current as before.

Depending on how long since use, I'll let them sit at that voltage and low
leakage rate for at least 30 mins... up to several hours... Then to finish, I
slowly discharge the cap with a 1k resistor.

Since electrolytics are chemical devices, they can explode from overpressure
caused from overheating from too high a *leakage current* (not the same as the
charge current!) and otherwise be dangerous to work with if you don't know what
you're doing. Always wear goggles.

JS

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