GregS ) writes:
In article , "P.Gregory" wrote:
Wise?
I could put just as many links saying the opposite , as in series , Not
parralel and not paraletic , few few ohms aint gunna do what you yanks say
"diddly squat" unless you get perfect diodes , read as matched ..paul



"Bryan" wrote in message
om...
It would be wise to NOT use load-sharing (aka equalizer) resistors &
capacitors. With modern rectifiers, if more than one diode is in a
string,
there will be no problem as long as they are of the same type number. See
http://www.somis.org/D-amplifiers3.html.
73,
Bryan WA7PRC

P.Gregory wrote:
Also do not forget load sharing resistors and bypass caps...
"GregS" wrote in message
...
I am having some difficulty finding a bridge rectifier of 3 Kv and 3
amps.

??

thanks
greg
N6GS

I think I am willing to try using them without caps and resistors.
I allready have 1KV 3 A HER308's. And I have a bunch of thermal
epoxy. I just might purchase some of these to make things easier.

http://www.fairchildsemi.com/ds/FF%2FFFPF04F150S.pdf

greg

The parallel resistors and capacitors in parallel with series diodes
dates from the early days of semiconductor diodes.

The resistors were there to "equalize the reverse voltage drops".
The diodes back then weren't particularly great, and there were worries,
I can't remember if real or imagined, that the diodes couldn't handle
the reverse voltage they'd see. The diodes were there to make sure each
saw the same reverse voltage.

The parallel capacitors were to protect against spikes. Again at a time
when there was real or imagined worry that the precious semiconductor
diodes would be easily damaged.

This was the same period that would always warn about heatsinking the
leads of semiconductors when soldering them, to protect them against
heat.

YOu'll see this talk of parallel resistors and capacitors with the
use of series diodes in older handbooks and magazines, and then it fades
away. I doubt it will hurt anything, but either the diodes got better (in
those early days one often had no choice but to put some diodes in series
just to handle average B+ voltages, let alone outright high voltage), or
the whole thing was an illusion, and wasn't actually needed.

Series resistors limit the current, to protect the diodes against current
surges. The 1971 ARRL Handbook says you need them if the winding resistance
of the transformer are too low to do the limiting itself.

Michael VE2BVW

I'll chime in, too, on the opinion that "no caps, and no equalizing
resistors is OK." I've built a lot of full wave center tap diode strings
and full wave full bridge diode strings and with a lot of diodes. I
_would_ recommend, however, that you put in lots of diodes (eg. all 1 kv).
I put in enough in each leg so that my PRV sum for the diodes is *at
least* two times the peak AC volts. So, for a 1 kv DC output, I would have
at least a sum of 3 kv of PRV diodes (or, three 1 kv diodes in series,
preferably four, for each leg). If you get spikes going up that high, they
are usually going to be very transient and the diode will act like a zener
diode and start drawing zener current in the reverse direction. As soon as
that happens, it puts a _load_ on the transient and, unless there is a ton of
energy in the transient and it is quite long, this will keep the voltage
from going higher. I've seen lots of gear with lots of diodes in the HV
legs without balancing resistors or capacitors, and I've been using big
diode strings in 2-3 KV DC supplies (kw level) for decades, and no
problems. Also, the inductance in the primary of big transformers should
tend to limit current surges anyway. You might get bigger voltage surges
when you turn the power switch off.

===== no change to below, included for reference and context =====

On Thu, 12 Apr 2007, Michael Black wrote:

GregS ) writes:
In article , "P.Gregory" wrote:
Wise?
I could put just as many links saying the opposite , as in series , Not
parralel and not paraletic , few few ohms aint gunna do what you yanks say
"diddly squat" unless you get perfect diodes , read as matched ..paul



"Bryan" wrote in message
...
It would be wise to NOT use load-sharing (aka equalizer) resistors &
capacitors. With modern rectifiers, if more than one diode is in a
string,
there will be no problem as long as they are of the same type number. See
http://www.somis.org/D-amplifiers3.html.
73,
Bryan WA7PRC

P.Gregory wrote:
Also do not forget load sharing resistors and bypass caps...
"GregS" wrote in message
...
I am having some difficulty finding a bridge rectifier of 3 Kv and 3
amps.

??

thanks
greg
N6GS

I think I am willing to try using them without caps and resistors.
I allready have 1KV 3 A HER308's. And I have a bunch of thermal
epoxy. I just might purchase some of these to make things easier.

http://www.fairchildsemi.com/ds/FF%2FFFPF04F150S.pdf

greg

The parallel resistors and capacitors in parallel with series diodes
dates from the early days of semiconductor diodes.

The resistors were there to "equalize the reverse voltage drops".
The diodes back then weren't particularly great, and there were worries,
I can't remember if real or imagined, that the diodes couldn't handle
the reverse voltage they'd see. The diodes were there to make sure each
saw the same reverse voltage.

The parallel capacitors were to protect against spikes. Again at a time
when there was real or imagined worry that the precious semiconductor
diodes would be easily damaged.

This was the same period that would always warn about heatsinking the
leads of semiconductors when soldering them, to protect them against
heat.

YOu'll see this talk of parallel resistors and capacitors with the
use of series diodes in older handbooks and magazines, and then it fades
away. I doubt it will hurt anything, but either the diodes got better (in
those early days one often had no choice but to put some diodes in series
just to handle average B+ voltages, let alone outright high voltage), or
the whole thing was an illusion, and wasn't actually needed.

Series resistors limit the current, to protect the diodes against current
surges. The 1971 ARRL Handbook says you need them if the winding resistance
of the transformer are too low to do the limiting itself.

Michael VE2BVW