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From: email@example.com (Ben Bradley)
Subject: Re: linear power supply noise questions
Date: 12 Nov 2002 20:40:27 -0800
NNTP-Posting-Date: 13 Nov 2002 04:40:27 GMT
In sci.electronics.design, "Walter Harley"
>"Spehro Pefhany" wrote in message
>> How about this circuit?
>> [polarized cap in parallel with diode]
>What you and Frank B. are saying, in effect, is "the only issue with
>electrolytic cap polarization is that you need to avoid the
>substantial reverse DC voltage." But, I know I've heard (not
>from reliable sources) that it's important to keep a so-called
>voltage on a cap in order to maintain the dielectric. That failure
to do so
>results in noise and premature failure of the cap. (Sort of the
>problem from a potentiometer, where you want to avoid DC current flow
>through the wiper terminal.)
>Is this a myth, or maybe just a large overstatement? Should I just
>polarized cap and get on with life?
I've heard this too, so it must be true. :)
Actually I'm pretty sure there's some real experience that says
electrolytics without significant voltage (such as at least half their
rated voltage) on them will eventually lose their 'forming' and lose
most of their capacitance.
>How much reverse polarization is too much?
So much that it makes it substantially worse than operating all the
time with no voltage (such as a few millivolts offset from an opamp)?
I'll take a SWAG at perhaps one or two volts. I think a few millivolts
of either polarity is effectively zero volts as far as the effect on
an electrolytic's life is concerned. But if this were for something
important, I'd look into it more.
>Is it a one-time catastrophic
>failure mode, or a gradual degradation thing?
Definitely gradual, though some caps in some circuits under some
conditions can have catastrophic failures - specifically, big ones in
power supplies, where the device hasn't been power up in several years
- the cap has gradually deformed, and suddenly applying full voltage
shorts the capacitor, which then causes fireworks (protecting the fuse
in the circuit). If a small-signal cap shorts out, perhaps the worst
that happens is an opamp output gets stuck at a rail.
>Thanks for all your help, everyone!
The real way to solve your original problem is NOT to 'fix' the
noise on the negative power rail, but rather to use two caps and a
resistor like this:
Gnd o (to opamp circuit)
| (+) | (+)
= C1 = C2
| (-) | (-)
This gives you the AC reference to ground (and NOT V-) that you
want, while putting substantial DC voltage on both electrolytic
capacitors to keep them formed. The value of R is driven by two
factors I can think of. First, you want it high enough that it doesn't
affect the operation of this 'composite capacitor'. But you also don't
want it so high that it takes forever to charge the caps. The caps
might have a few (tens or even hundreds of) microamps of leakage
current, and so resistor of 1 Meg would never charge them. I'd say 5k
to 50k should work okay. That brings up another problem, your circuit
better not be bothered by the leakage current of C2. If it is, a
tantalum will have much lower leakage (at much higher cost).
Credit for this circuit apparently goes to Walt Jung (if not
someone before him) who has used it in high quality audio circuits.
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