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

[Dr. David Kirkby]

On 07/ 9/11 01:06 PM, Ed Heimbach wrote:
>   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

I'd be tempted to open the meter and see if there is a non-linear device in 
there - most probably a diode.

One thing that's very important is that you stop the meter voltage ever rising 
too high if the coil fails. Consider what happens if the coil goes open-circuit. 
Then the + terminal of the meter will go to 4.5 kV and that would be dangerous, 
as it will not be rated for such use.

If you really can't get this to be linear, I'd just scrap the meter. To me at 
least, having a lookup table for a volt meter is not sensible. It should not be 
necessary to have a correction for non-linearity.

Perhaps the movement has a fault, but I'm guessing there's a diode there.

It would be interesting to know if you could plot a graph of input voltage vs 
input current. That should be a straight line and would give you the resistance 
of the meter. You might find it's not, and the apparent resistance changes

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