[Hammarlund] Re: SP-400 power supply question and HV electrolytics

[Barry L. Ornitz]

John Heck, KC8ETS, asked about a 2 uF 1000 v capacitor in 
his SP-400 power supply.  

As others have noted, this is not an electrolytic capacitor 
but rather a film or far more likely an oil filled paper 
capacitor.  Generally these have a much lower failure rate 
than do electrolytics so there is a good chance the 
capacitor may still be good.  However as the input 
capacitor, it can see power line spikes so it is still 
worth checking.  Oil-filled capacitors are generally very 
reliable.  With an input capacitor like this one, a 
varistor transient voltage limiter on the power input to 
the supply is a good idea for protection.  These will clip 
the spikes before they reach the transformer.  Oil filled 
paper capacitors tend to be self-healing, but their leakage 
goes up with each puncture event.

As for high voltage electrolytics, the maximum practical 
voltage rating of an electrolytic capacitor is around 500 
volts.  Above this voltage, effects such as scintillation 
and localized excessive gradients are prone.  You may find 
a few old electrolytic capacitors rated for higher voltages 
(I have seen 800 volt units), but internally they are two 
lower voltage units in series.  This should tell the reader 
that placing two ordinary electrolytics in series is the 
best way to replace these units.  The placement of 
equalizing resistors in parallel with the electrolytics is 
best.  These resistors should draw at least ten times the 
rated leakage current of the electrolytics.  This will 
insure approximately equal voltage division.  It is best to 
be conservative so using two 450 volt electrolytics in 
series is the way to replace an 800 volt unit.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net