From: Jim Thomson
Sent: Saturday, September 21, 2019 10:53 AM
To: amps@contesting.com
Subject: HV Fuse
Date: Sat, 21 Sep 2019 09:50:11 -0400
From: "Paul Christensen" <w9ac@arrl.net>
To: "'amps'" <amps@contesting.com>
Subject: Re: [Amps] HV Fuse
<The bigger issue is the use of a HV fuse between the transformer secondary and
C-input filter in an attempt to save the rectifier stacks during a fault event
that occurs after the filter cap and before the glitch resistor. I've not seen
it implemented anywhere but was <mentioned by VE7RF in this post:
<http://lists.contesting.com/_amps/2017-11/msg00149.html
<Even with step-start, I'm skeptical of its effectiveness due to the high
initial surge current at start-up. So, perhaps just relying on the 20A ABC
mains fuses is still best.
<The Alpha 70V uses four Semtech SCH5000 HV rectifier blocks. They're rated by
Semtech at 5KV PIV but only 0.5A at 55 degs C, or 1.0 A with continuous forced
air at 55 degs C. This is a 1960s design and barely meets the margins for this
amp. I have <spares so I used them in the repair. But, if it occurs again,
I'll either use modern equivalents with better specs or a string of 6A10s on a
new, custom PC board.
<Paul, W9AC
## 6A10 diodes. Rated at 1kv at 6A CCS, 400A half cycle surge. IE,
8.3 msec. 6A10 typ will HI pot test to 1.4 kv.
## 10A10 diodes. 1 kv at 10A CCS, 400A surge rated. They too
hi-pot test to 1.3 to 1.4 kv.
## 10A07 diodes. 1 kv at 10A CCS, but 600A surge rated. hi-pot to
1.3 to 1.4 kv.
## Mouser sells all 3 types above. I typ buy either 100 / 250 at
a time.
## Both the 6A and 10A diodes above can easily be paralleled. The
current split is typ 50-50.... and rare worse case is 49-51.... or
51-49.
## I also recently tried paralleling 2 x complete FWB assys.
Each assy uses 24 x 6A10 diodes in each of the 4 x legs. Equal
lengths of HV wire used from
xfmr to each FWB assy. Again, the current split was 50-50.
## years ago, I built a 6.8 kv supply with 60 x 1N5408s. 15 per
leg... in a FWB. 10 kva, 4.8 kv hypersil pole pig used. One day, I
had the diode board on a swivel
mount, positioned wrong, at an angle and the B+ arced to chassis.... on
the output of the diode board assy. Took out all 60 x diodes in a
split second.... followed by the
240 vac – 50A rated mag- hydraulic breaker..... which was the.... instant
trip version, with the oil removed.... tripped off. Replaced with 60
x 6A10s. But this time, a 2nd HV
fuse installed... between xfmr sec.... and FWB. 2nd HV fuse can go into
either xfmr sec leg. I tested the config, several times using a HD
open frame, 30 kv, 60 A HV relay. Short the B+
anywhere inboard of the 1st HV fuse.... IE: B+ HV fuse + 50 ohm
glitch R.......and all that happens is an open HV fuse #2..... between
xfmr and FWB assy. Diodes
survive just fine. Test repeated times.
## step start consisted of 2 x paralleled 50 ohm, 100 watt wire
wounds, and a 0-15 sec adjustable time delay + contactor. 150uf filter
cap used on the B+. Runs smooth.
Sold B+ supply to a buddy, who uses it daily with his YC-156 amp
for the last 5 yrs.
## The only time that HV AC fuse, between xfmr and FWB, caused
issues, was on another buddys 8 kv supply. Everything ran fine, till
he increased filter C to 163 uf.
Once the step start completed..after 15 secs.... the HV AC fuse opened
up. The secondary surge was too much for the BUSS HVU-3 fuse.
The fix was to replace the 20 ohm
step start resistor... with a 10 ohm unit. Problem was the 1.5 A of
xfmr pri magnetizing current flows through the step start resistor....
with a V drop across it. Thats why the B+ would
only increase to 7 kv.... then jump to 8 kv with step start R
shunted.
## In Pauls case..... use some 6A10s, etc... and call it a day. The
HV fuse between xfmr and diode board is just extra protection for
the diode board.
Jim VE7RF
-
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