Date: Sun, 19 Nov 2017 00:26:36 +0000
From: Alan Ibbetson <alan@g3xaq.net>
To: amps@contesting.com
Subject: Re: [Amps] Glitch resistor + fuse?
<Jim: how did you measure the 2ms fuse opening time? I can't see it is all
<that easy with a multi-kilovolt supply dumping many tens of amps through
the fuse and glitch resistor!
I wonder if any US readers use a crowbar circuit as described in the Eimac
paper to really cut down on the energy a valve has to endure during an arc?
There is the outline of a circuit using a string of thyristors (SCRs) in
figure 2 here
http://www.qsl.net/oe5jfl/flashover.htm
although the circuit does not offer protection during the first half
second, until the gate trigger reservoir capacitor has charged up. It also
may not keep the thyristors triggered for 20ms or more, until after the
primary-side relay/breaker/contactor has dropped out. A better approach
might be to use transformer coupled gate triggering, driven from the low
voltage DC supply that is already provided for the control logic. Some kind
of pulse train circuit (NE555?) would be needed to keep the thyristors
triggered during the "follow on" phase.
73, Alan G3XAQ
## use a string of HV multiplier resistors, like u would for a HB meter, like
say,
10 x 1 megohm in series.... with a 1 ma meter, so it reads 0- 10 kv. Forget
the
0-1 ma, just add some lower valued resistors, say 2-3 of em, in series with the
string
of higher valued resistors. Wire this between B+ and B- of HV supply.
For this
exercise, you can temp directly bond the B- of HV supply, to the chassis.
## Put the scope across the lower value resistor. I used a DPST hv
contactor, open frame type,
with one pole to apply the B+ to chassis dead short. 2nd pole does the
trigger on the scope.
## It can also be done by extrapolating the Buss Fuse melt curves, using
the calculated fault current.
114 to 154 A of fault current will open off a 3 A rated HV fuse extremely
fast.
## Ok if the fault current is from B+ to chassis or metal, no big deal.
However if the B+ arced
inside the tube, from anode to grid..... and of course the grid is bonded to
the chassis, now you have
a major issue. Why would the anode arc to the grnded grid. IF the tube
does not high pot test high enough,
could be one cause, but the typ hi pot test on a given tube is typ in excess
of double its B+ rating.
## A 3-500Z will typ hi pot test to well over 8 kv. With 4 kv under load on
the anode, and driven, the RF AC
component in the PI tank circuit..or Pi-L..will back feed through the plate
block cap... back onto the tube.
End result is less than 8 kv, like 6.4 kv.
## On tubes like a 3CX-3000A7 and a 3CX-6000A7, with their big internal 225
watt rated grids, bonded to the
cold chassis, the grids are very robust. The issues start up when using tubes
like a 3CX-800A7 with its tiny 4 watt
max grid dissipation. Any internal arc from anode to grid..which is grnd to
chassis through the socket..... and probably
an energy diverter would be required.
## That Oe5 circuit has been used, and modified for use up to 7-8 kv.... but
the energy storage in the filter caps was very low,
typ using low values of filter C. I looked at it, and a few variations of
it, and came to the conclusion that the complexity was not
worth the effort for the 3x3 and 3x6 tube I was using. Touch wood, no
internal tube arcing so far. Even on a 8877, if an anode
to grid arc occurred, the tube would be destroyed.
## the 120 vac HV 2 pole contactor I have, are not easily sped up. The vac
relays I have, even the spst, NC types, with moly contacts,
and sped up, I would not trust to open off fault current. Its beyond the
ratings of the vac relay in question. The real large vac contactors,
the big ones a foot long, are too slow top operate, and open off fault current.
After going around in circles for 2 years, threw in the towel,
and went back to the buss sand filled fuse + glitch R. The sand, or whatever
the white granular material is inside them, does a good job at
extinguishing any arc..and any plasma. The Buss HV fuses can be obtained
with or without the granular material.
## The Buss, 5 inch long, granular filled HVU series HV fuses I use, have a
short circuit rating of 2500 KVA. The longer version
is the Buss HVX, and is 10 inches long, also granular filled, and has a SC
rating of 12,000 KVA. Both are .81 diam and use .75
inch fuse clips. I use the type with the end retainers, so there is no way the
HV fuse can migrate out of the pair of retaining clips.
The silver plated clips are mounted on appropriate rated ceramic stand offs.
You can also wire 2 x HVU fuses in series for greater
B+ and also higher interrupt capacity. A 3 kv supply + 50 ohms glitch, with
the resulting 60A of fault current, will open off a 1 or
2 A rated hv fuse extremely fast. I have not come up with a reliable
alternative. You could also use a larger ohmic value than 50
ohms to limit fault current, but that all depends on max current the tube or
tubes in question draws. If its only 1 or 1.5 A, a bigger
glitch like a 75 ohm 100 ohm, could be used, provided the sizing and wattage
was done correctly, and the heat and diss and also V drop
across the glitch assy is acceptable. I have mounted the HV fuses in both
the vert and also horz plane, both work.
## 3 kv divided by 75 ohms = 40A fault current. 3 kv divided by 100 ohms =
30 A fault current. You have to factor in what is it you are
trying to protect. Trying to open off the 240 vac mains, under a fault
condition is an extremely tough act, most mechanical contactors are
not fast enough. A pair of hd, vac relays, with molly contacts would be hard
pressed to open fast enough, and also handle the 240 vac fault
current. I took the easy way out, and opted for a 2nd HV fuse, wired between
xfmr sec, and input to FWB diode board, in one leg only. It will
save your diode board...provided its sized correctly.
## On this latest supply project, since I had two complete FWB diode board
assys, I used both of them, and wired them in parallel, with equal length
hv wires from xfmr sec, leg 1..and also the output of the single HV AC fuse,
in xfmr sec, leg 2. Ditto with the outputs of both FWB boards..to filter
cap assy. Also a mag-hyd breaker in the 240 line, inside the HV supply. Then
a standard thermal breaker in the main 200A panel. So far, so good.
Crow bar anything, anywhere, and all that happens is one or both HV fuses open
up, event over. But I did use 4 x 200 ohm, 1.5 inch diam x 18 inch long,
Type AS globars...all in parallel, to make the 50 ohm glitch assy. Globar pn
1038AS.
Jim VE7RF
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