[Boatanchors] Old Electrolytics Any Good?

[wa4tuk-rf at comcast.net]

There's nothing to disagree with in the post below but one word of caution.  In addition to the usual high voltage precautions, 1000mf at 450 volts represents a LOT of power that will make a pretty good fireball and bang if it is allowed to discharge all at once.  I would be sure to discharge with a resistor and monitor the charge with the voltmeter to know when its dissipated.  I would also store these after forming with the terminals shorted for good measure even if they are electrolytics.

Pat
wa4tuk

-------------- Original message -------------- 

> 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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