[Boatanchors] Old Electrolytics Any Good?

[Mark Foltarz]

Jim,

  Electrolytics that have not been used for a while must be "reformed." This is
the process of applying ectricity to the capacitor while limiting current so as
to electrolytically reform the dielectric.

   The setup can be as simple as a variac, a transformer to supply the high
voltage,  a rectifier,  a voltmeter to measure voltage across the cap and a
milliammeter in series with the cap and power supply to measure leakage
current.

  Hookup the variac, transformer and diode to supply variable dc. Add the
volmeter across the output and the milliammeter in series with the output.

  Connect the capacitor. Make sure the variac is in the off position and apply
power to the variac. 


  Slowly bring up power on the variac while watching the voltmeter and
milliammeter.  You will notice each time you increase the voltage you will get
a spike in the series current. The current will diminish slowly to some leakage
value. It should happen fairly quickly. Keep the series current down to a
couple of MA.

  I personally  take the applied voltage beyond the working voltage to perhaps
500 vdc. There is always a surge voltage value that the manufacturer expects
the capacitor to cope with. However, depending on you application you may not
have to go this high. I just do this so that a capacitor meant for service at
450v will not having any more forming to do in circuit. 

   BE CAREFUL TO DISCHARGE THE CAP AFTER FORMING. It now has full voltage on
it.

   After discharge, see how quickly the cap takes full voltage without drawing
a great deal of leakage current.

  I am guessing you picked up some of the larger computer grade caps with the
screw terminal tops.  You m,ay be surprised at how much life may be left in
them.  

  de KA4JVY

  Mark

  
   
--- Jamesnf at aol.com wrote:

> I stumbled across a box of 1000Mfd @ 450v electrolytics on a shelf in the 
> basement.  I think I brought these home from a hamfest 20 years ago, maybe 
> longer.  Is there any possibility these could still be usable after all this
> time?  
> Or should I just deposit them in the trash?
> 
> Jim W9TM
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