-----Original Message-----
From: Carl
Sent: Sunday, November 04, 2012 4:29 AM
To: Jim Thomson ; amps@contesting.com
Subject: Re: [Amps] plate voltage meter shunt
Date: Sat, 3 Nov 2012 14:15:44 -0700 (PDT)
From: Douglas Johnson <w9iix1@yahoo.com>
To: AMPS <amps@contesting.com>
Subject: [Amps] plate voltage meter shunt
I am installing a meter to measure plate voltage 1-5000v@ 1a?dc, the meter
has a 1ma fs rating. ?the serial resistance I have made up in the past has
been 3 watt metal film resistors in series. since they can only rated for
200v per resistor it takes many to do the job. Can some other resistor
type with a higher rating?be used in a safe manner for this
purpose?...tnx, Doug,w9iix
## it’s a meter multiplier string..and not a shunt ! You can buy 1%
tol 3 kv rated resistors and also 1% tol 10 and 15 kv rated resistor’s...
from mouser. 5000/.001 = 5 megohms. Use 10 x 500K ohm resistors in
series.
I mount em on either perf board or drill out some micarta, plexiglass, or
UHMW. Just mount em so they go down one side,across, and back up the
other side. I mount the entire assy directly to the meter studs !
To pull that off, you need to make the board big enough. The usual deal
is to make it rectangular shaped, taller than wider, then whack two holes
for the meter studs towards the top of the board..with the rest of the
board hanging down.
## you can mount the board in a few diff configs. The idea is to keep
the 5 kv end well away from the meter itself. 5 kv enters resistor
#1...... and exits #10..then onto the positive of the meter. NEG of
meter goes DIRECTLY to the
B- end of the HV supply. Take is directly to the B- end of the HV caps
if you can at all possible. Don’t do some arrl fubar and run it to the
chassis.
## On my hb amps, I use a HV fuse in series with the 50 ohm glitch
resistor... in series with the B+. In those cases, I use one HV meter
in the HV supply, and a 2nd HV meter in the RF deck itself. If the HV
fuse blows open,
the HV meter in the HV supply will still read 5kv. The HV meter in the
RF deck will read ZERO. This is done for safety purposes. In some
cases, I will use a 3rd box, full of HC caps.... and that box also gets
it’s own HV neter.
later....... Jim VE7RF
Why the heck would anyone use 10 HV resistors? That would look like the mess
of 1/4W on a Harbach board that I always replace with three 5M Ohmites
anyway.
For 5KV and that meter 5 Ohmite Slim-Mox 1M 7.5KV resistors is all that is
needed. They are 1.25W rated and only $3.90 ea at Mouser. Mount on a
phenolic, and not perf board, and elevated on short phenolic, ceramic, or
other standoffs. The idea is to not have the resistor body on the phenolic
where years of HV will attract a possible conductive path. Perfboard is more
prone to moisture absorbtion but in a dry area its OK.
Carl
KM1H
### Sure, that will work just fine....and less resistor's required. With
5 kv applied, the
diss = 5 watts. 5 x 1.25 watts = 6.25 watts. So you are running em at 80%
of their
diss rating, which may be a concern.
## I used 10 of the Slim-MOX 1 Meg 15 kv types, rated for 2 watts, 1%
tolerance.
With 10 kv applied, they diss a total of 10 watts....using a 0-1ma meter
movement.
They were $5.00 each at the time, and way less in bulk. I bought 50 of em,
enough
to make 5 x HV metering assemblies.
### With 10 in series, they total exactly 9.980 megohms. That is exactly
.2% on low
side. With 7.7 to 8 kv applied, and .77 to .8 ma flowing, they don't run
hot. With .77ma
flowing, each resistor dissipates .5929 watt. Since they are 2 watt rated
devices, they don't
heat up, and they don't drift. The typ 1 ma meter is aprx 50 ohms dc
resistance.
## Until the Ohmite Slim-Mox hv types were used, we had nothing but
problems with
metering resistors. The HV meter's would drift, mof resistors would
internally break down,
etc. Back in the early 90's, you could buy 2 watt MOF resistor's with a
750 vac rating, 1 kv
dc.....and they worked good. But they are all just 450 vdc rated these
days, pita.
## we needed something 100% reliable, bullet proof, drift free, and over
rated...and asap.
I found the ohmite Slim mox types in the mouser on line catalog, so that was
what was used.
Mouser also carries 3 kv rated resistors, but they are not ohmite Slim-Mox
types, forget
who made em, but they were tried and tested on a 4.5 kv supply....and work
well. On that
supply, the meter reads 0-5 kv.
## I also tried Bill, W6WRT's method, and although it works, I wanted an
HV meter that is
totally independent of EQ resistors. In one of the HV supplies, Oil caps
were used. In the
RF decks, I require a stand alone HV meter. With the 50 x slimox resistors
I bought, and
5 identical assy's made, I got identical results on all 5 of em. They are
all .2% on the low side.
## Now the meter movement itself is the limiting factor for accuracy. No
big deal since any 0-1ma
meter can be tested and calibrated beforehand on the bench. Just insert a
fluke DVM in series
with the 0-1 ma meter, and apply... say .8 ma as read on the fluke dvm,
then tweak the set screw
on the 0-1 ma meter till it reads exactly .8 ma.
## The large perf boards I bought from mouser were all made from this
green fiberglass material.
They won't absorb moisture at all, I let one of em sit in water to test for
that. I use the same perf boards
to make FWB assemblies. I also used em to make up series 6A10 diode
assy's for bias for GG amps.
Think the ones I used had .080" holes in em, which will take the .060" leads
from 6A10 diodes etc.
## Never had a problem with em since 1990, but they are not cheap. Phenolic
will absorb moisture, too brittle,
has no strength to it, pita.
later........ Jim VE7RF
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