[AMPS] HV Power Supply Capacitors

[km1h@juno.com]

--------- Begin forwarded message ----------
From: "Tom Rauch (W8JI)" <W8JItom@worldnet.att.net>
To: amps@contesting.com
Subject: Re: [AMPS] HV Power Supply Capacitors
Date: Mon, 31 Mar 1997 08:17:26 -0500
Message-ID: <19970331135536.AAA3078@fred>
References: <19970331054727.AAC3226@[205.231.11.159]>

>Common sense tells us the larger an object, the more heat it can
>dissipate.

That is a completely misleading statement.

>If you have two capacitors with equal ESR, and one has a much larger
>surface area, the larger one will be able to dissipate more power with
>less temperature rise.

Now your improving.


>That is why a larger heatsink runs cooler. Sometimes people forget that
>in their zest to be "helpful".

Again very misleading.

>The equalizing resistors should be selected to equalize the voltage, so
>you really have to know the leakage resistance tolerance of the
>capacitors to pick the correct resistor.
>The resistor should "swamp out" leakage tolerances. In general as low a
>value as pratical is best because it insures more equal voltage division
>even if one capacitor is more or less leaky than the rest.

Maybe a low value equalizing resistor was the norm back in ancient
history but technology has advanced a lot since your days at Heathkit. 
The CDE 381LX series has a lower ESR  and ripple current rating at 120Hz
and is half the size of the CGS style used in Ameritron amps. But yet
your bosses seem to enjoy taking cheap shots at their competitors and try
to flim-flam the ham that doesn't understand the issue. BTW, I have
absolutely zero financial interest in Command or any other amplifier
manufacturer. 

The true issue in equalizing voltages in a series string requires
matching leakages AND capacitance values. In the CGS series (standard
"computer grade") I have measured  220MF/450V  with as much as a 30 % 
capacitance difference within the same date code batch. This has been
verified by Allen Harbach of Harbach Electronics who does all of those
Heathkit PS upgrades.  Leakage also varies. Allen uses capacitors that
are matched to within 1% or so and they work absolutely fine with 100K
equalizing resistors. In a SB-220 the voltage drop across all 8
capacitors varies about 10V top. Try that in an Ameritron or any other
amp that just grabs a handful from the stock room. I measure a 30-50V
differential here even with a 50K resistor. 

The CDE 381LX appears to have a very good production tolerances. Out of a
batch of 500 they are all within 5%. The low ESR plus the high ripple
current rating (over 1 Amp  in the 270MF version) makes them ideal in
even the more demanding doubler circuits used in many amps. As
replacements I have yet to have a failure. Their major shortcoming is the
small physical size which requires some creativity.

The 30+ year old "computer grade" cap is rapidly being replaced by newer
technology Tom; maybe you need to revisit the issue or hire a consultant.
Or at the very least advise your bosses that their ad is offensive to
intelligent people. 

73....Carl   KM1H    

>73 Tom




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