During the early 1960s I worked at a BC station where we had a 5 kW FM
transmitter. It had a full-wave bridge rectifier consisting of 4 boards,
each of which had some 16 400-piv diodes on it. I think (but don't
remember for sure) that DC plate voltage was about 3200. I think the
boards had a resistor across each diode.
Every once in a while there would be a sound like an M16 on full
automatic as the diodes on one of the boards would let go. Replacement
boards came with capacitors, which I believe solved the problem.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 14/12/2022 9:55, sm0aom--- via Amps wrote:
The large characteristic spread of silicon rectifiers of "yesteryear" often
resulted in reliability problems.
One early task I got as a systems engineer in 1981 was to investigate why the 6
kV six-pulse bridge rectifiers in a total of 15 5 kW MF/HF SSB transmitters
failed randomly. The reasons were found to be transients on the mains supplies,
caused by switching of inductive loads.
The safety factors in the expensive rectifier "sticks" originally used were not
that large, and a contact with an applications engineer at German semiconductor
manufacturer Semikron netted a booklet about the benefits of controlled avalanche
characteristics, as they handle transients much better. No parallel equalising networks
also were needed.
The rated PIV of the originally supplied rectifier sticks was only 8 kV, so a
design goal of 12 kV was chosen. 2 A DC load was used as a rated current into
the choke-input filter.
Semikron had a controlled avalanche diode, SKa 1/17 which permitted an average
current of 1.2 A at 70 C ambient rating, and 1700 V PIV. 8 diodes in each leg
provided somewhat additional safety factors.
It turned out that the rectifier reliability problems vanished entirely after replacing
the old "sticks" for the whole remaining service lives of the transmitters,
which finally were scrapped in the mid-90s for other reasons.
73/Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : 4cx250b@miamioh.edu
Datum : 2022-12-13 - 18:21 ()
Till : g8gsq@gmx.com
Kopia : amps@contesting.com
Ämne : Re: [Amps] .01uf caps across diodes ?
An interesting point, Steve. If I understand you, you’re saying that
if a modern diode in a series string approaches reverse breakdown, it
will start to conduct, thus shifting its overload voltage to other
diodes in the string. Before the advent of controlled avalanche
diodes, however, an overloaded diode would just short circuit, thus
permanently shifting its voltage to the remaining diodes in the
string. This process, once started, would likely lead to the
destruction of the entire string. To me, the takeaway message is that
there’s little to be gained these days by adding .01uF caps to each
diode. This is a helpful explanation, Steve. Thanks!
73,
Jim W8ZR
Sent from my iPhone
On Dec 13, 2022, at 1:28 AM, Steve Thompson <g8gsq@gmx.com> wrote:
It's not only uniformity, also the 'controlled avalanche' of modern diodes. As
you reach breakdown they don't fail instantly from too much reverse voltage, they
start to conduct slightly. In a string of diodes this results in voltage sharing
happening automatically.
Steve G8GSQ
On 13/12/2022 02:11, MU 4CX250B wrote:
Hi Bill, I remember that argument,too, but It doesn’t carry much
water with today’s uniform components. The back resistance of a series
string of diodes is so high that I doubt an uneven capacitive voltage
divider would cause problems. Normally one designs an HV diode bank
with at least a breakdown voltage safety factor of 2, and preferably
more.
73,
Jim W8ZR
Sent from my iPhone
On Dec 12, 2022, at 10:19 AM, hzp_electronics@juno.com wrote:
Hi Jim
The best argument that I have heard about capacitors across diodes is that
older diodes had uneven capacitance. Under certain conditions the capacitance
would cause the voltage to divide unevenly between the diodes. The added
capacitors were made large enough to swamp out the capacitance difference and
keep the voltage division more equally.
\
Whether the caps are needed with modern diodes, depends!
73
.... Bill K3HZP
---------- Original Message ----------
From: MU 4CX250B <4cx250b@miamioh.edu>
To: Tim Duffy <k3lr@k3lr.com>
Cc: jim.thom@telus.net, amps@contesting.com
Subject: Re: [Amps] .01uf caps across diodes ?
Date: Sun, 11 Dec 2022 22:32:29 -0800
Hi Tim,
I’ve followed this debate for years and frankly I don’t think adding
.01uF caps across each diode in a diode string makes much difference
one way or the other. It’s true that there will be some suppression of
fast current and voltage transients, but these aren’t likely to cause
damage to modern components. To me the downside of the practice is
that adding 0.01uF caps results in additional opportunities for
capacitor failures, loose solder connections, unsightly wiring, and a
bunch of extra components that provide minimal benefits. I’m inclined
to leave them out. (BTW, one commenter suggested that sharp voltage or
current spikes wouldn’t make it through the filter capacitors. That’s
untrue because the filter caps have a self inductance that keeps them
from filtering spikes with high frequency components. )
73,
Jim W8ZR
Sent from my iPhone
On Dec 11, 2022, at 7:03 PM, Tim Duffy <k3lr@k3lr.com> wrote:
W8ZR Jim
Can you give advice on this topic?
73
Tim K3LR
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of jim.thom
jim.thom@telus.net
Sent: Tuesday, November 29, 2022 2:28 PM
To: amps@contesting.com
Subject: [Amps] .01uf caps across diodes ?
What's the consensus on wiring .01uf disc caps across diodes in a HV supply
?
W8JI sez they are required, and used in ameritron amps....and used to
eliminate switching transients.
Some say the caps are required to meet emc specs on commercial gear.
I can't see how anything can get past the HV fitter caps.
KM1H also uses caps across diodes. He also used another method, with a
.01uf cap across an entire series string of diodes, like a .01uf @ 5 kv
type.
Orr's books recommend caps across diodes... to eliminate noise on TX on
ssb. .
Are they required...or not ?
Jim VE7RF
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