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Re: DC Supply



Hi Jim,

At 12:36 22/09/98 -0600, you wrote:

>In a classic three phase full wave bridge, you only need 6 diodes
>because the transformers would be connected in delta. I'd have to think
>about the implications of the grounded secondary centertap on this,
>since it sort of makes your system connected in Wye.

No-one else commented on this & I lack the background theory (as you
probably guessed !).  I assume I still connect all of the centre-taps to
ground & this will not be a problem.

>Therefore, I'd go with the semiconductor rectifiers. Your NST's are
>probably a nominal voltage of something like 15 kV (peak voltage of 1.4
>times that (21 kV), however, because of the grounded centertap, you
>really are going only need half that. You can get replacement microwave
>oven rectifiers are around 14 PIV at 200 mA, but they are a bit pricey
>($14 each).

Greg Leyh suggested stringing 1N4007s (I will use the 3A variety 1N5408) as
these are cheap & readily available in  Oz.

Was thinking I will need to cover ( 15 kV x 1.4 + fudge safety factor ),
say 25 kV but from what you say I should be able to get away with ( 7.5 kV
x 1.4 x 2) = 20 kV.

This will be 6 lots of 20-strings !

>You can also string a bunch of the 6kV HV diodes available from a number
>of mail order sources (like All-Electronics (www.allcorp-dot-com)), but
>you'll need to pot it in silicone or oil. I wouldn't bother with the
>parallel resistors and capacitors commonly seen in a string of these.
>Just put enough diodes to get double the voltage you want.  If the
>diodes all come from the same manufacturing lot, their switching
>characteristics will be well matched enough to avoid problems.

Most of the HV diodes available here are << 0.5 A rating, which should be
OK, but anything used in a TC seems to need to be over-rated by some
margin, so I was leaning to the 1A or 3A variety :-(

Both Greg & Malcolm Watts suggested parallel 'RC snubbers'.   I am not
familiar with these so may well follow your suggestion of omission ;-)
(I assume this still holds for a long string as proposed?)

Would it be advisable to house these in a narrow PVC tube & fill with oil ?

thanks,

Mark

http://www.cobweb-dot-com.au/~dkfinnis