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Re: [Amps] plate voltage meter shunt

To: "Jim Thomson" <jim.thom@telus.net>, <amps@contesting.com>
Subject: Re: [Amps] plate voltage meter shunt
From: "Carl" <km1h@jeremy.mv.com>
Date: Sun, 4 Nov 2012 07:29:27 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>


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



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