[Boatanchors] RE: Capacitor Help (High Voltage Electrolytics)

[Barry L. Ornitz]

Ron Freeman, K5MM, asked about getting 100 uf, 500 volt 
electrolytic capacitors.

A few companies such as Nichicon and Vishay (both Asian), 
and Cornell-Dubilier still make 500 volt electrolytics but 
these are extremely difficult to find.  Most manufacturers 
make electrolytics with working voltage ratings no higher 
than 450 volts.

The problem is that it is extremely difficult to make such 
capacitors.  To form the anodic aluminum oxide film on a 
typical 450 volt electrolytic, a forming voltage in excess 
of 600 volts may be used.  The anodic film formed, the 
actual dielectric of the capacitor, will be about 900 
nanometers thick (about 35 millionths of an inch for those 
thinking in English units).  This is less than one 
hundredth of the thickness of a human hair.

At voltages above 600 to 700 volts, scintillation effects 
occur in the anodic film.  This is where microsparks occur 
at surface irregularities.  Unless the current is very 
limited, this leads to rapid puncture of the insulating 
film and a shorted capacitor.  With great care, 500 volt 
electrolytics can be built, but most manufacturers settle 
for 450 volts as a safe upper limit. [Similarly, tantalum 
electrolytics have a practical upper voltage of around 50 
volts with 35 volt units being the highest voltage units 
commonly available.]

You may occasionally find older electrolytic capacitors 
with ratings up to 800 volts.  But if you open these up, 
you will find they are just two lower voltage units in 
series.

This should give you a hint on how to get the capacitance 
you need.  Just use two lower voltage, higher 
capacitance units in series.  If you use randomly picked 
electrolytics, I would go with two 300 volt units in 
series, but if you purchase new units from the same 
manufacturer, two 250 volt units should suffice.  The reason 
for applying a larger safety factor with different units is 
due to variations in leakage current.

Balancing resistors, to equalize the voltage across the two 
capacitors, are a good idea.  Typically you would size the 
balancing resistors to draw at least 10 times the 
capacitor's rated leakage current.  Remember that resistors 
have a maximum voltage rating too.  Most resistors made 
today in the 1/2 watt size are rated to only 250 volts (and 
sometimes 350 volts).  This is independent of the power 
rating of the resistor.


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