[Hallicrafters] Re: Replacement Capacitors

[Barry L. Ornitz]

Mark Bell, K3ZX, wrote:

> and don't forget those resistors across each of 'em !!

This is an important point when using electrolytic capacitors 
in series.  Also note the plural on resistors.

Sizing a bleeder resistor across each capacitor is generally 
done by having the resistor draw at least 10 times the rated 
leakage current for the capacitor when two or more units are 
wired in series to achieve a higher voltage rating.  With 
modern electrolytic capacitors, the leakage current is much 
smaller than that of years ago.  Thus the bleeder resistors 
can be of a higher value and a lower power dissipation.

But an important point that many hams (and even Heath at one 
time) forget is that even if a resistor is operating within 
its dissipation rating, there is still a maximum voltage 
rating per resistor.  This is typically 250 to 350 volts.

For example, if the rated leakage current is 100 microamps and 
each capacitor in the string is rated for 450 volts, you would 
need to shunt each capacitor by a resistance of 450K ohms.  
This will dissipate 0.45 watts at full voltage.  For a safety 
factor, use 1 watt.  But you cannot use a single resistor here 
as the voltage is above 250 to 350 volts.  Instead use two 
220K half watt units in series.  At worst case with 450 volts 
across the capacitor (and assuming no leakage current), each 
resistor would dissipate 0.23 watts and have only 225 volts 
across it.

Heath had to replace a large number of CRT's in their color TV 
sets because their engineers forgot about a resistor's maximum 
voltage rating.  One of my friends went through three tubes at 
Heath's expense because someone forget this lesson.  Heath was 
using ordinary 1/2 watt carbon resistors in the focus circuit 
with as much as 1500 volts across each resistor!

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