Recently I have replaced the HV capacitor and diode strings in my L-4B. In
addition to replacing the caps, the original equalizing resistors across the
caps were each replaced with 100K, 2 watt ceramic composition resistors.

The original HV diode string along with its equalizing bridge resistors were
removed. The original diodes were replaced with 1N5408 diodes, but following
a source (which I can no longer locate) I did not replace the equalizing
resistors. The source had stated that modern diodes do not require the
diodes to be bridged with equalizing resistors.

The work was performed in August, 2005, and the amplifier has been working
fine ever since. But I have this nagging feeling that the diodes do need to
be bridged. Should I add equalizing resistors, or are my fears groundless?

John, N9JG

No need to use the equalizing resistors. Modern diodes will "zener"
when reverse biased beyond their rated PIV. This will distribute the
inverse voltage across all the diodes. Just be sure to use enough
diodes in the string.

Best Regards,

Ed

"Ed Engelken" wrote in message
oups.com...
No need to use the equalizing resistors. Modern diodes will "zener"
when reverse biased beyond their rated PIV. This will distribute the
inverse voltage across all the diodes. Just be sure to use enough
diodes in the string.

Best Regards,

Ed


I agree with Ed. Older diodes would breakdown, modern diodes
will "Zener" and basically self protect in a series string.

Pete

Well, I was hoping the answer would be something like this. I expect a good
nights sleep tonight, unless I think of something else to worry about!

" Uncle Peter" wrote in message
news:vMsFf.37636$bF.23635@dukeread07...

"Ed Engelken" wrote in message
oups.com...
No need to use the equalizing resistors. Modern diodes will "zener"
when reverse biased beyond their rated PIV. This will distribute the
inverse voltage across all the diodes. Just be sure to use enough
diodes in the string.

Best Regards,

Ed


I agree with Ed. Older diodes would breakdown, modern diodes
will "Zener" and basically self protect in a series string.

Pete

Ed Engelken wrote:
No need to use the equalizing resistors. Modern diodes will "zener"
when reverse biased beyond their rated PIV. This will distribute the
inverse voltage across all the diodes. Just be sure to use enough
diodes in the string.

Yes, and equalizing resistors can actually be a very bad thing, as they
drift with age, heating and high voltage. They can end up dividing the
voltage unevenly, which is exactly the worst thing to do.

73,
Mike, KK6GM

In my day it was not only common to use equalizing resistors across
each diode, but a .01 disc ceramic as well.

12 or 15 years ago, I built a 1KV .5 amp supply and as I recall, used
560K 1/2 watt resistors and .01 uf 1KV caps across each 1N4007. And a
"soft-start" relay in the primary of the HV transformer.

John

Litzendraht wrote:
In my day it was not only common to use equalizing resistors across
each diode, but a .01 disc ceramic as well.

In the olden days, silicon diodes were very expensive, and had process
variations that made individual diodes in a production run quite
different from each other. The original 1N400X series was a grading
process. The manufacturer attempted to make nothing but 1N4007's, but
that didn't always work out. The really bad diodes became 1N4001's,
and the really good diodes became 1N4007's.

[Just for grins, I just took a modern manufacture 1N4001, a diode rated
for 50 PIV use, and hooked it up to my 577/177 curve tracer, and I saw no
significant reverse leakage current all the way out to 1600V.]

The equalization resistors were used with the thought that they would
swamp the leakage current of the diodes in the reverse biased condition,
and make sure that each diode saw only its equal share of voltage.
The truth of the matter is the reverse leakage current will start to
rise rapidly when the diode starts entering its avalanche region, and
the next leakiest diode in the string will start to take over when the
leakiest one starts thinking about avalanching. The string balances,
and protects, itself.

-Chuck

I guess I should have done a bit more research before I posted my question.
According to the 2005 ARRL Handbook (page 17.9) "shunting resistors are
generally not needed across diodes in series rectifier strings. In fact,
shunt resistors can actually create problems because they can produce a
low-impedance source of damaging current to any diode that may have reached
avalanche potential."

It appears as if the Handbook agrees with you.
John, N9JG
"Chuck Harris" wrote in message
...
[stuff]
The equalization resistors were used with the thought that they would
swamp the leakage current of the diodes in the reverse biased condition,
and make sure that each diode saw only its equal share of voltage.
The truth of the matter is the reverse leakage current will start to
rise rapidly when the diode starts entering its avalanche region, and
the next leakiest diode in the string will start to take over when the
leakiest one starts thinking about avalanching. The string balances,
and protects, itself.

-Chuck

Please be aware of that fact that high voltage multi-junction diodes for
microwave ovens are available for next to nothing and can be a good
substitute for long diode strings with a single element.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Please be aware of that fact that high voltage multi-junction diodes for
microwave ovens are available for next to nothing and can be a good
substitute for long diode strings with a single element.

Now THAT's a good idea! Any idea how to test USED ones?

On my morning school-bus route, I see about one discarded microwave oven
per month, and if they're not too far from the bus-barn, I return later,
"liberate" them from the "sidewalk store" and tear them apart for their
super-strong magnets which my grandkids really enjoy. But I've also been
saving the transformers, diodes, fans, and capacitors, with the ultimate
goal of making a high-voltage power supply for a homebrew amplifier. So,
without any high-tech equipment, how might those diodes be tested?
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)