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
type
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
typical
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!
73
Bob Groh, WA2CKY
________________________________
From: Ed Heimbach <supermechanic18360@yahoo.com>
To: Amps@contesting.com
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
(2.5
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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