[Collins] Capacitors

[Dr. Gerald N. Johnson, electrical engineer]

Oil has been used with paper for a century. The paper is just a
container for the oil dielectric. The infamous black beauties are just
that oil impregnated brown kraft paper in a molded black plastic case.
The quality of the capacitor depends on the quality of the paper, the
quality of the oil, and the lack of moisture in the oil. Kraft paper is
notorious for having conductive inclusions.

Some of the metal cased oil filled capacitors are quite good, some are
as bad as the black beauties. The only way to check is to measure
leakage current. I use a 400 volt power supply and a VTVM. I hook the
meter and power supply negatives together. I hook the capacitor between
the power supply positive and the VTVM probe tip. I turn on the power
supply. Initially the charging current makes the VTVM read several
hundred volts, but as the capacitor charges through the 11 Megohms of
the meter, the meter voltage drops. A good capacitor will have so little
leakage that the meter will stay on scale on its 1.5 volt range. There
will be variation as the supply voltage wanders with line voltage
changes but its usually under 1 volt with a good capacitor. This
represents a leakage current under 0.1 microamp, practical for Orange
Drops.

.001 mf is too small for audio coupling and bypass, and on the small
side for IF bypassing.

73, Jerry, K0CQ, Technical Advisor to the CRA.
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.