[Amps] plate voltage meter shunt

[Jim Thomson]

-----Original Message----- 
From: Carl
Sent: Sunday, November 04, 2012 4:29 AM
To: Jim Thomson ; amps at contesting.com
Subject: Re: [Amps] plate voltage meter shunt




> Date: Sat, 3 Nov 2012 14:15:44 -0700 (PDT)
> From: Douglas Johnson <w9iix1 at yahoo.com>
> To: AMPS <amps at 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