Melt time does not equal stoppage of current flow at these voltages. The
fuse element is replaced with a low impedance arc that keeps on pumping
energy into the fault for several AC cycles until the fault clears, mains
fuse blows or the voltage drops below the level to maintain the arc which is
much lower than the voltage that started it. The fastest fuses are very
long and filled with sand or other compounds, also pricey. I've seen the
binding post HB fuse and they have the same problem of slow time. If
someone thinks these methods are protecting a #40 grid wire in the Eimac
tubes most of us are using it's a dream. If the target is compliance with
Eimac's bulletin 17 then this doesn't do it. If better than nothing is the
goal then it's fine. If there really was a fuse that would stop current
flow in 2ms at these voltages every amp manufacturer on the planet would be
using it.
I pulled a 3cx3000a7 out of an 8K ultra yesterday with a dangling grid wire
causing an intermittent grid to cathode short, ouch. I can buy a lot of
protection for the $1500 the tube will cost but not my money.
BTW this topic was beat to death on this reflector several years ago, more
reading in the archives.
Vry 73
Paul
Paul Hewitt
WD7S Productions
QRO Homebrew components
http://home.earthlink.net/~wd7s/contents.htm
-----Original Message-----
From: Jim Thomson [mailto:jim.thom@telus.net]
Sent: Monday, April 07, 2014 1:34 AM
To: Paul Hewitt; amps@contesting.com
Subject: Re: [Amps] HF2500 Power Supply Caps
## Check out the Buss HVU series of HV fuses...then extrapolate the curves.
Then test on the bench
with a scope to verify that data. BTW, those fuses can easily be rebuilt.
I have also tried hb hv fuses,
encased in either teflon spagetti tubing or heat shrink.....suspended
between standoffs. Then crank up the
screws tight, in a CW fashion, so there is lots of tension in the fuse wire.
## For folks that want to use a 25 ohm glitch and say a 3 kv supply.. fault
current is 120 A. Stuff 120 A through
a 1 A rated fuse...and you will soon see it blows open pretty damn quick.
Even with a 50 ohm glitch and same 3 kv, the 60A fault current will still
open up a 1 A rated fuse very fast.
## You could put the scope across the cold end of the last resistor in a
HV multiplier string.
http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resou
rces/product-datasheets-b/Bus_Ele_DS_6003_HV_Series.pdf
You can see that with just 4A flowing through a 1A rated HV fuse..it will
open up in 10 msecs. Now stuff 50-60-120 amps through the same
1A rated fuse..and you will measure a LOT less than 10 msecs.
Jim VE7RF
-----Original Message-----
From: Paul Hewitt
Sent: Sunday, April 06, 2014 2:21 PM
To: 'Jim Thomson' ; amps@contesting.com
Subject: RE: [Amps] HF2500 Power Supply Caps
Hi Jim
I see your posts about high voltage fuses disrupting current in 2ms so how
about a part number/manufacturer. I would like to see the data sheets I2T
curves. Most of these fuses open times are measured in seconds and are very
large.
Thanks &73,
Paul
Paul Hewitt
WD7S Productions
QRO Homebrew components
http://home.earthlink.net/~wd7s/contents.htm
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Saturday, April 05, 2014 1:29 PM
To: amps@contesting.com
Subject: [Amps] HF2500 Power Supply Caps
Date: Sat, 05 Apr 2014 08:41:38 -0700
From: Bill Turner <dezrat1242@wildblue.net>
To: amps@contesting.com
Subject: Re: [Amps] HF2500 Power Supply Caps
On 4/5/2014 8:06 AM, Carl wrote:
> but more C doesnt buy anything useful.
REPLY:
More C can actually be harmful. In case of an arc, more C just dumps more
energy into the arc with potentially more damage.
73, Bill W6WRT
## With a 50 ohm glitch Resistor installed, the peak fault current remains
the same, regardless
of C used. However, the 50 ohm glitch + total C will form a RC time
constant, whose duration
will be longer when more C is used. With 2500 vdc + a 50 ohm glitch
Resistor used, peak fault
current is 50 Amps..... which will easily blow open a 1-2 Amp rated HV
fuse...and under 2 msecs.
## The 50 ohm glitch resistor LIMITS the peak fault current. The HV fuse,
just prior, and in series with the
50 ohm glitch resistor, INTERRUPTS the peak fault current. You can cro-bar
the supply all day long if you want.
All that happens is the HV fuse blows open.
## use whatever size cap you want. The 440-600 uf types work just fine.
The bigger the total C used, the better the
dynamic regulation of the supply. The ripple is also lower, and in
proportion to C used. A 600 uf cap will have one third the
ripple of a 200 uf cap..and way better dynamic regulation on ssb and cw.
Your load line is not all over the map.
Now this all works with the proviso that a 50 ohm glitch R and HV fuse is
used.
## As far as regulation goes, you require a plate xfmr that doesnt sag to
begin with....and big enough ga wire from HV supply back to main 200A panel.
You also require large enough rated relays and
contactors. Any Z between main panel and plate xfmr
will create V drops..which is bad news with any high C filter.
Jim VE7RF
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