>
>Peter Chadwick wrote:
>>Bot not identical voltages across them.
>>
>>If you have branded diodes from the same date code, and/or guaranteed
>>controlled avalanche characteristics, then probably it's OK. If not, I still
>>maintain voltage equalising resistors are a good idea. But not the cheap any
>>source resistors - a voltage equalising resistor across a 1kV diode should
>>be rated as a 1kV resistor. (I think Rich has made this point in the past,
>>saying that Mouser do suitable resistors at low price). Disc ceramic
>>capacitors across each diode do need matching in capacity - the +80/-20%
>>tolerance typical on a 1000pF 1 kV disc could make things worse, rather than
>>better. Do such disc caps drift the same way with time and temperature, does
>>anyone know? If not, they should be avoided.
>>
>> > The 1995 to 2000 Handbooks advise to not use "equalization".
>>
>This is another of those recurring FUQs (Frequently Unanswered Questions
>- what else?).
>
>When it came around about two years ago, I checked direct with the
>author of the revised Handbook chapter (a power supply engineer to MIL
>standards). He said that "equalization" became necessary because the
>first rectifier diodes had relatively low PIV, were too expensive to
>throw in lots more diodes in series, and failed catastrophically by
>arcing over or through the silicon die. All of that has changed. Modern
>rectifier diodes have higher PIV, lower cost and also have avalanche
>breakdown characteristics which are not necessarily fatal.
>Therefore in his professional view and that of the diode manufacturers
>he consulted, it is not necessary to use equalization components - so
>that's what the Handbooks now say.
hardly
- page 11-9 of the Handbook:
"In fact shunt resistors produce a low impedance source of damaging
current to any diode that may have reached avalanche potential."
>
>
>For me, the whole question about how close you can come to breaking down
>a string is not relevant. If in doubt, use more diodes - they're cheap
>enough! We amateurs cannot insist on diodes that have been specially
>high-temperature-reverse-bias tested, but so long as the diodes are
>reasonably well matched, eg full-spec brand-name diodes from the same
>bandolier, we should be pretty safe. It *has* to be worth the extra few
>cents.
>
>However, someone also pointed out that the economics are different for
>large amplifier manufacturers, who are very cost-conscious but can get
>well matched ceramic disc caps much more cheaply than we amateurs can.
>Then the economics may tip back towards using fewer diodes with
>equalization capacitors.
>
>There are also EMC considerations where capacitors can helpful in
>reducing reverse breakdown spikes going back through the mains, so
>perhaps capacitors may be making a comeback in these more EMC-conscious
>days.
>
>>They don't advise nichrome parasitic suppressors either. Just because it's
>>the ARRL Handbook, it's not guaranteed to be correct.
>>
>Depends who's writing it... and also who's reading it :-)
>
It seems a bit curious that you missed the statement about damaging
current. cheers, Ian.
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