Robert Neece wrote:
>
>
>Rich, AG6K, writes:
>
>>The ESR of flash-caps appears to be higher due to use of thinner plate
>>material - which allows the designer to pack in more C - and more Joules
>>per unit of volume.
>
>This is of great interest, Rich. Let us suppose that the plates
>are ultra thin, but that the insulation is of the standard dimension.
>
>If that be true, *voltage* ceases to be the issue. Instead,
>*current* becomes the problem. Does it make any sense to imagine
>the following?
>
> Thin plate material is OK in flash service, even to handle
> the huge (but only occasional) current flow in a discharge,
> because the slow rate of recharge requires tiny current flow,
> and there is a long static interval between discharges. Thus,
> there is no tendency to overheat.
>
> In filter-cap service, though, the cap is charged and
> discharged (albeit only incrementally) all the time,
> so some current ingress and egress goes on constantly.
> Might this create a heat accumulation problem that would
> be absent in flash service?
>
>Just a thought, Rich. Your reaction?
Whatever the cause of failure is, it can be very slow acting. I've had
two of these capacitors last for 2-3 years each, including many high-
duty sessions of contesting.
The final failure is by voltage breakdown, but excess voltage isn't
necessarily the root cause. I strongly agree with Rich's downrating to
60% voltage as a practical rule-of-thumb, but wonder if that isn't also
related to a downrating of the energy stored (to 60%^2 = 36%). Also I
agree with Rich's recommendation to test the capacitor with a low-
voltage variac set to give the required ripple current (or more), and
check for heating. Unfortunately, neither of these is a guarantee
against long-term failure.
Finally, special thanks to Rich's web pages for warning about those
3.5kV capacitors that had been "uprated to 5kV" by selective use of an
eraser. One of them had crossed an ocean into my power supply!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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