~~~~~~~~~~ this one never got sent out ~~~~~~~~~~~~~~~~
In <200206092218.g59MIn9F019957@contesting.com>, on 06/09/2002
at 03:20 PM, Richard <2@mail.vcnet.com> said:
>>>Richard 2@mail.vcnet.com
>>>Fri, 26 Apr 2002 13:12:30 -0700
>>
>>> . . . Digi-Key sells 500v-rated
>>>100k-ohm, 3w resistors, also mfg by Panasonic, which make suitable V
>>>equalizers.
>>
>>A Mallory engineer back in the 1960's put a non-equalized electrolytic
>>string in a published project with the admonition that such R/eq were
>>folly since during failure-mode they actually worked against the best
>>interests of the individual failing electrolytic cap.
>A failed electroytic V-equalizer resistor dominoes the entire string of
>caps.
So maybe that is why he did not use them. <g>
> A failed-shorted electrolytic can do the same thing if the other
> caps can't take the extra potential burden caused by a failing cap that
>got too much V because equalizer resistors were not used.
But we are not discussing the fool who leaves no margin, are we? Let's
always have 3 extra electrolytics in our string, beyond what a normal,
prudent rating would require. Thus we are setup to accommodate 3 separate
failures without going beyond norms. Failures that most likely will ocurr
as separate events. All that is left to do here is to sample a few of the
caps for V on a regular basis in order to see if any have 'fused'
(shorted).
> A failing
>electrolytic is history anyhow, so his argument is codswallop.
Remember, he is not trying to protect the electrolytics at all. If one
fails, so be it, since there is no consequence beyond that single failure.
> I worked at Raytheon when an engineer stated that transistor
>V-regulators can not work floating at kilovolts above ground. Management
> bought it and the decision cost Raytheon much money. His idea is
> cockamamie.
You certainly won't see me going out of my way to tout Lincoln, MA as a
town where 'everyone is above average'. <g>
>>And even hastened
>>the avalanching of the entire series (but don't hold me to this part of
>>it). I just remember this as sage advice from someone who should really
>>know.
I meant to place a qualifier before 'sage' like 'seemingly'.
>>He described failure-mode of, say, one of the electrolytics as offering
>>'zero ohms resistance' at turn-on and continuing as such over the ensuing
>>seconds when normally it would begin developing a Vb, ie, 'taking a
>>charge'. As it persists in this obstinant state it presents a 'short' to
>>the string, thus a resistor across it will be of no significance
>>whatsoever. And the presence of resistors across the others likewise is of
>>no value to them either, good cap or bad.
>I prefer to replace the whole string since, when one cap fails, the
>writing is on the wall.
Agree, however if adequate warning of subsequent failures could be
devised, I'd be willing to toss the first bad one and wait for the
bell-curve to get closer. But are most failures 'etch punctures' at
startup, or are they likely to occur during heavy exercise as well?
>>If our obstinant cap suddenly pops 'good' it does so on its own.
>The fairy godmother twanged it with her magic wand ?
As I said (read on) I was just covering all possibilities with that one.
And I was referencing early failure like right at startup, which frankly
is a mystery to me. I was thinking that a cap may show a short at startup,
but the short will fail above a certain current level and the cap will
live for another cycle. Sort of like a car whose dash lights up fine when
you turn ignition ON but then all power is lost as you try to crank it
(bad battery contact).
>>The presence of that resistor does not assist. If it remains 'bad' the rest of
>>the string simply remains under the new parameters giving the good caps a
>>bit more voltage each. Note, I'm not saying a failed cap will ever 'pop
>>good' again- just trying to cover all possibilities.
>>
>>I noticed in the Gi7b transmitters that the 6m rig HAD R/eq in the ps, but
>>that the hf rig did NOT. Odd. Then a bit later I noticed Richard's comment
>>above.
Ever since filling my neighbor's parlor with smoke when I was a kid (I
turned on their floor-model shortwave) I've been wary of electrolytic
failure. In small rigs <1000v I just put them in series with no resistors,
but always recent, matched stock. And HV I use oils. Never a failure to
date. But I do have matched sets of 'computer grade' electrolytics waiting
to be seriesed-up in a HB amp.
>cheerz
>- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
>www.vcnet.com/measures.
73,
Bill
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