>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.
Good engineering practice says use good quality capacitors, high
reliability resistors, both within their ratings, as well as a suitable
glitch resistor to limit peak capacitor discharge current during a fault.
>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 have seen 'em explode, so one needs shrapnel shields.
>> 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>
>
This was at Raytheon's Point Mugu facility.
>>>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'.
>
RR
>>>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?
>
All of the failures I've seen are during operation.
>>>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).
>
Or a too-old/sulfated battery, or ...
>>>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,
No resistors is way better than feculent 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.
Electrolytic caps have a several times weight, cost, and size advantage
over oil/paper dielectric caps.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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