Re: Diode Strings

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

 >
 >
 > Yes, I've considered that after seeing them on Ebay. At 10Kva, my target,
a
 > pig can push almost 3/4" amp which doesn't leave much safety margin. Well,
 > not as much as I'd like. I could have gone with 1N5408s (1000V 3A) but I
got
 > the 6A10s at a good price IMO. I'll be using this bridge not only for the
 > pig but a MOT stack and NST bank depending on the project so I'd like
 > something a bit on the hefty side to *help* eliminate surge current
 > situations.
 > It's just that I'm having trouble understanding the need for resistors,
and
 > in some cases advice to even using caps across each diode. And there
always
 > seems to be talk about recovery time. If those are issues I needn't worry
 > about than all the better. :)


More of an issue in the past when devices weren't as consistent.  There is
good reason NOT to put capacitors in (see later (say, last 10-15 years) of
ARRL handbook for an explanation).

Resistors equalize the reverse voltage drops if reverse leakage currents are
different from piece to piece.  This is an issue if you're using mystery
surplus parts that are 20 years old in a big bin, but not really an issue if
you're using all new parts from the same lot.

Back when diodes were expensive, it was a good trade to use fewer diodes and
add resistors (when diodes were $0.50 and resistors $0.03...).  Now, you can
just add a bunch more diodes to give yourself some margin and be done with
it.  If you follow conservative practice (i.e. expected reverse voltage <
0.5 PIV rating on diode), then you don't much care if some turn off faster
or slower than others, or if the reverse voltages don't distribute perfectly
evenly.


On the other hand, if your diodes are from random lots OR are arranged in a
way that might lead to inconsistent temperatures/voltages, then equalizing
resistors might be useful.



 >
 > It'll be a while before I actually build the bridge and building with
 > confidence is much more fun than building with doubt. I just want to be
sure
 > I'm building something I that isn't going to give the wife support for her
 > argument that I'm a nutcase. :D
 >
 > Rick W.
 >
 >