[AMRadio] Power factor / Capacitor engineering conciderations

[George Pritchard]

A short capacitor tutorial:
The "power factor" for capacitors is mostly an issue with "higher frequency"
due to the "ESR" (equivalent series resistance)of the capacitor at the
operating frequency, and to some extent the choice of dielectric. At 15,750
kHz for horizontal sweep circuits and HiFi audio, the inherent inductance of
the capacitor degrades the expected nominal capacitance value to some
degree. The "ESR" of a capacitor is a very interesting science across the
frequency spectrum. If the frequency is high enough, the capacitor
eventually becomes and inductor for two mechanical reasons. First... the
length of the wrap, or plates, and the diameter or width of the wrap of the
foils and dielectric is an inductance by design. This inductance is
partially in series and to a lesser extent, in parellel. The lead length and
thickness of the leads also adds inductance in series with the capacitor. So
far... none of the above is a problem at 60 or 120 Hz for DC filtering of
rectified AC. Even the dielectric heating at these low frequencies is
generally insignificant (assuming low leakage), and also depends on the peak
to peak AC amplitude apapplied to the cap and what the RMS AC current is.
Leakage current is the main culprit and destructive. As the frequency goes
up... all of the above play a significant roll on the effectivness of the
capacitor and can lead to heating. For this thread concerning power supply
filtering at 60, 120 or even 400 Hz, use what ever you have and it will
work, as long as the dielectric / electrolyte is is not dried out or
distorted. Standard electrolytics are fine for DC - 400 Hz. For frequencies
400 - 1000 Hz use mylar, paper (not the best), metalized polyester. For
"audio" 1000- 20,000+, use higher "quality" types, polyester, polycarbonate,
tantalum (especially in switching power supplies). These types will also
have less leakage currents. For "RF" above 20,000+... use ceramic, Teflon,
glass, saphire and AIR dielectrics. The ESR of capacitors finally "plays
into" another condition at VHF and higher RF frequencies: The resonant
frequency of the capacitor at some elevated frequency! It will be a series
and parellel (has both) resonances at two different frequencis. It should
also be noted that a voltage DE-RATING of 100% is typical for Military
engineering, but for power supplies in our ham stuff... 25% over rating is
fine (50% if you can fit it in!). Now for temperature conciderations:
Aluminum electrolytics are the worst. They loose 50% of the capacity at cold
temperatures (below 0C). Tantalums are much better. Ceramics are the best.
All the trade-off are physical size and dielectric when it comes to
temperature issues. I hope this is not to boring and is informative.
George (project Engineer for Militay avionics and RF microwave). Thanks for
the chance to pass on info on this most interesting subject... CAPACITORS

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of John Coleman
Sent: Thursday, April 04, 2002 10:01 AM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] Power factor


	The power factor of capacitors is never quite perfect that is if the
waveform across the capcitor is compaired to the wave form across the
resistive only load they will not be exactly 90deg difference.  This is due
to internal series resistance (not reactance) of the capacitor and the
internal shunt resistance of the capacitor (accumulative leakage and
dialectric loss). Electrolitics or notorios for poor power factor especialy
as the frequency increases.  Special capacitors are often used in coupling
the defection power to the coils of a deflection yoke on a CRT because some
capacitors will get hot and consume the power and also change the shape of
the defection current passing through the yoke causing nonlinear sweep.
This is also true of HIFI amplifiers where the speakers or connected in
series with a coupling capacitors.  It's been a long time since I fooled
with the math on this but servicing this stuff I have found that you can't
just replace a bad electrolitic with any thing that matches the value.  I
have also found that a lot of manufactures made monitors with a poor choice
of coupling caps to the deflection yokes.  Sometimes in order to get the
repair out and not have to worry about it I would replace a 1uf/400v
electrolitic with a 1uf/600v mylar.  Much larger and hard to get in but no
more trouble.

73, John, WA5BXO



-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of Donald Chester
Sent: Thursday, April 04, 2002 8:24 AM
To: billsmith at ispwest.com
Cc: amradio at mailman.qth.net
Subject: [AMRadio] Power factor






>From: "Bill Smith" <billsmith at ispwest.com>

>If the cap shows high leakage, or a high power factor, it is probably
> >spent, but if it acts as a new capacitor you have saved a good part.

What exactly do you mean by a capacitor's power factor?  How do you measure
it and exactly what are you measuring?  I am familiar with the concept when
talking about transformers, but what about capacitors?  I would assume the
power factor of a good capacitor standing alone would be zero, since the
(ac) current would lead the voltage by 90 degrees.

Don K4KYV

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