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Re: HV DC power supply design.....
Where the PRV rating of a single diode is not suffieient for the
application, similar diodes may be used in series. (Two 500-PRV diodes in
series will withstand 1000 PRV, and so on). When this is done, resistor and
a capacitor should be placed across each diode in the string to equalize the
PRV drops and to guard against transient voltage spikes. Even though the
diodes are of the same type and have the same PRV rating, they may have
widely different back resistances when they are cut off. The reverse voltage
divides according to Ohm's Law, and the diode with the higher back
resistance will have the higher voltage developed across it. The diode may
break down.
If we put a swamping resistor across each diode, the resultant resistance
across each diode will be almost the same, and the back voltage will divide
almost equally. A good rule of thumb for resistor size is to Multipy the PRV
rating of the diode by 500 ohms. For example, a 500-PRV diode should be
shunted by 500 x 500, or 250,000. ohms.
The shift from forward condition to high back resistance does not take place
instantly in a silicon diode. Some diodes take longer than others to develop
high back resistance. To protect the "fast" diodes in a series string until
all the diodes are properly cut off, a .01 uf capacitor should be placed
across each diode. The capacitors should be noninductive, ceramic disk, for
example, and should be well matched. Use 10% tolerance capacitors if possible.
I have a circuit drawing on my web page at
http://home.earthlink-dot-net/~gweaver/hv-page.htm
The home page is http://home.earthlink-dot-net/~gweaver/
When I build something like this I like to over engineer it just a little to
give it a safety factor or about 10% to 20%. Put a couple extra diodes in
the circuit for safety. Put a current limiting resistor in the circuit too.
Gary Weaver
At 12:34 PM 1/8/00 -0700, you wrote:
>Original Poster: "Tero Ranta" <tero.ranta-at-bigfoot-dot-com>
>
>Dear List,
>
>as you might have noticed, I'm trying to design a stiff DC power supply
>using two MOTs and a voltage doubler pair. It's not only for TC use. I'm
>looking for general HV DC power supply design guidelines, how to do the
>diode/capacitor strings, how to calculate needed values for filter caps, how
>to connect the voltage/ampere meters etc.
>
>I have a 1996 ARRL (radio amateur) handbook, which has a tiny section on HV
>power supplies, it tells me what I should do in general, like use many
>diodes in series, but that's not enough, because I'd like to know what
>cap/resistor values should I connect in parallel with them etc.
>
>I have a tendency to screw up things, that's why I want to even once do
>something that ends up working well for years. :-)
>
>If you know a good source of information regarding the subject, please let
>me know.
>
>Thanks in advance,
>Tero.
>
>
>
>