Date: Tue, 14 Nov 2017 17:25:14 -0700
From: Jim Garland <4cx250b@miamioh.edu>
To: Jim Thomson <jim.thom@telus.net>
Cc: Amps group <amps@contesting.com>
Subject: Re: [Amps] Glitch resistor + fuse?
<Great comments, Jim.
<You're right, of course, about a 50ohm/10W glitch resistor being
inadequate. I panicked when I read your comment and checked the
schematic of my latest two power supplies. It turns out I used 10 ohms@
50W, not 50 ohms@10W. I got things backwards in my comment. Whew!? I
realize that 10 ohms may be a bit too low for most power supplies, but
these particular supplies can source a little over two amps, so 10 ohms
isn't unreasonably small in worst case conditions. In addition to the
glitch resistor and HV fuses (two or three in parallel, depending on the
RF deck), I also have have an overcurrent trip circuit and, like you, a
double pole magnetic circuit breaker in the primary line.
Re vacuum relays, I agree that that most any moderately sized vacuum
relay in the HV line can stick if subjected to a DC short to ground.
I've tried to minimize that problem with Gigavac relays that are
designed to hot switch DC voltages of 10kV at 10A, if I"m remembering
right. I had an internal arc in a new GU74B, which tripped everything,
but the Gigavacs worked fine. (The Gigavac relays aren't intended as
protection devices, but just to route the HV around to different RF
decks.) Years ago, I learned the hard way that ordinary vacuum relays
designed for RF switching should NEVER be used to interrupt DC currents.
Here's a tip, I learned at the same time. Sticking vacuum relays can
often be fixed by holding them near a powerful magnet. Once freed, they
seem to work normally, but I imagine they don't meet full specs for
current capacity.
BTW, are you sure that the Drake .82 ohm fuse resistors are 1/2W?? I
looked in my resistor bin and found 2W resistors. Probably doesn't make
any difference. Whatever, they sure make a lot of noise when they blow!
73,
Jim W8ZR
## One note on the magnetic hydraulic type breakers in the 240 vac line....
they are not heat activated, like a thermal breaker, so ambient temp has no
effect
on them. One day at the telco I worked at, we lost all the air conditioning
due to a fire
caused by a faulty programmable contactor. The ambient temp inside the
building soon rose from
70 F to 95+ F. Funny thing was, the row of 125 A three pole breakers for the
3 phase rectifiers
were all blazing warm, normally they run at ambient temp. The biggest issue
with thermal breakers
is false or nuisance tripping, due to elevated high temps, like in summer.
Flip side is in really cold WX, the current trip point ends up on the high
side by quite a bit.
## You can also order the magnetic hydraulic types with aux contacts, which
come in handy for monitoring
the status of the breaker. You can also get them with a mid trip point
option. That offers the advantage to
readily distinguish between a breaker that has actually tripped, vs one that
has been manually shut off. With any
mid trip type, you know the breaker tripped because of a current overload.
## All 4 of my L4Bs have a .82 at .5 watt CC resistor for the HV fuse...
tiny thing, standard .5 watt CC size. Its
also in all 4 x manuals. One I bought new in 1977, the other three were
obtained used, in the 90s and early 2000’s.
The last 3 had hardly any hours on them, like 20 –25 hrs max.. with oem
packaging.
## I just found a kilovac catalog beneath my coffee table..and had forgotten
about the vac relays designed to hot switch
DC ! Yrs ago, I bought some nib, surplus kilovac KC-32 ceramic vac relays.
These have 12 vdc coils, and are SPST,
NC types. Rated at 25 kv and 45 A. Tungsten contacts. The tech notes
indicate that although the contacts are rated at 45 A,
dont try and hot switch 45 A with them. The NO version of the same relay is
a KC-22.. rated at 65 A. NO contacts have
lower contact resistance, since they rely on magnetism. NC contacts rely on
spring tension, and have higher contact resistance.
## I would not trust the KC-32 to open off a hard fault current with much
over 2250 vdc. 2250 divided by 50 ohms = 45 A
fault current, with a hard fault. With 3 – 8 kv, fault current is sky high
with 50 ohms. And any glitch that is more than 50 ohms, combined with
high B+ levels usually means excess glitch dissipation during normal
operation. Where they might work is for a soft fault, wired in
series with the B+ line, prior to the HV fuse and glitch Resistor. Or perhaps
between the xfmr sec..and input to diode board.
## I went through this several years ago, trying every config you can think
of. Even a sped up vac relay isnt any faster than the fastest HV
fuse. The KC-32 is speced at 18 msec operate time. Sped up, you can reduce
that to 2.7 to 3.2 msec. I have not figured a way to speed
up those HV open frame relays, since they operate on 120 vac. A HV fuse is
well under 2 msec, and will easily handle the hard fault.
I bought a surplus box of Buss HVU-3 series fuses, which are 5 inches long,
sand filled. Will interrupt up to 2500 KVA.
## also lucked out and bought surplus, 4 x 200 ohm type AS globars, meant
for energy discharge. Regular globars, like type SP, like
used in dummy loads, will also work. So will wire wounds. But they all have
to be sized correctly to handle normal CCS currents.
## One issue I forgot..when using GG triodes is when you lose B+, like from an
open HV fiuse, and amp is still being driven, the grid current
will skyrocket. The grid now thinks its the anode. A low B+ thereshold circuit
will take care of that, it looks similar to a string of B+ meter
multiplier resistors, with an extra 1-2 lower value resistors tacked onto the
end. V drop across the low valued resistors... or more precisely,
lack of v drop across them, is what triggers the low B+ circuit, shutting off
the xcvr by a few methods, like opening off the TX inhibit lead, or
applying some negative vdc to the xcvrs alc buss, or opening off the key line.
A grid over current circuit will do the same thing.
Jim VE7RF
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