[AMRadio] Power factor / Capacitor engineering conciderations

Thu Apr 4 22:56:38 EST 2002

Thank You !

WAØSTX


----- Original Message -----
From: "George Pritchard" <gpritchard at comtechpst.com>
To: <amradio at mailman.qth.net>
Sent: Thursday, April 04, 2002 10:16 AM
Subject: RE: [AMRadio] Power factor / Capacitor engineering conciderations


> 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
>
> _________________________________________________________________
> Join the world's largest e-mail service with MSN Hotmail.
> http://www.hotmail.com
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio
>