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Re: [Amps] What's wrong here- dropping resistor for DC meter?

To: Ed Heimbach <>,
Subject: Re: [Amps] What's wrong here- dropping resistor for DC meter?
From: Robert Groh <>
Date: Sat, 9 Jul 2011 07:28:39 -0700 (PDT)
List-post: <">>
As other's have noted, you should NOT have a non-linear response (assuming the 
meter itself is linear).  Couple of thoughts occur to me:

1.  Note that resistors have a breakdown voltage themselves (depends on the 
of resistor).  Usually you can find the working voltage maximum on the 
manufacturer's data sheet (or a data sheet of a comparable resistor).  A 
number that sticks in my head for a 1/2 watt resistor is 500 WVDC.  On that 
basis you should have at least 10 resistors in series.

2.  Obviously you need to know, and factor in, the value of the meter 
resistance.  You can measure that on the bench with a bit of care.  

3.  Knowing all of this, you can figure out a way to adjust the string for 
calibration. Details TBD - a variable resistor across the meter (say, with a 
nominal mid-point value of 4x the meter resistance).

4.  Watch your power dissipation in the resistors!  Be very conservative - I do 
not like to run my resistors at over 1/4 of their rating.

5.  Obviously watch the resistor tolerance.

6.  And, finally (bet you never thought this would end!), make sure your basic 
meter is linear!  

Have fun!

Bob Groh, WA2CKY

From: Ed Heimbach <>
Sent: Sat, July 9, 2011 7:06:05 AM
Subject: [Amps] What's wrong here- dropping resistor for DC meter?

 Want to meter HV out. Power supply variable from 0 through 4.5 kv dc
 Have 1 through 5 v DC movement.
 I built dropping resistor string to multiply meter by 1000X. 
 Meter now has a non-linear response.
 I.E., 1 volt position corresponds to 1000 volts, 3.25 volt on meter scale  
equals 4.5kv.
It would be nice if my max. volts (4.5 k) corresponded with the 4.5 mark on the 
scale card.

 Easy way out would be to  jiggle resistor values to get mid point on scale 
volts) to correspond with 2.5 kv, and then make new meter scale to agree with 
indicating needle.
 Any better solutions?

73 ab3ht

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