[Boatanchors] Panel Meter Circuit?

[Jim Wiley]

Here is a way to build an expanded scale voltmeter, which may work with 
the meter movement you have.  In fact, if the meter you have is actually 
just a DC milliammeter, regardless of what the dial scale says, this is 
exactly what you need.

First, decide what scale you want.  Lets assume you want a meter that 
reads from 24 to 30 volts, as shown on the illustration you provided.  
This is a range of 6 volts.  ( 30 - 6 = 24)   I was faced with this 
issue when working up meters for battery charger, and wanted an analog 
meter that had good resolution around the "float" point for a 24-volt 
battery plant.  That is about 27 volts.

I first built a small regulated "source" that delivered 24 volts DC.  I 
was able to source a 36-volt point in the battery charger to work with a 
3-pin regulator (LM7824), which provided the necessary 24-volt 
reference.  I installed a 1000 ohm resistor from the regulator output to 
ground to provide a minimal fixed load for the regular, just to keep to 
happy.

The meter I had was a 0 to 1 mA DC meter.   Since I wanted to measure a 
6-volt "delta" (24 to 30 volts) I basically wanted a 6 volt meter that 
would ride "on top" of the 24-volts from the regulator.

Using basic Ohms law, I calculated that 6 volts at 1 mA would require a 
6000 ohm series resistor between the battery plant and the meter to 
allow 1 mA to flow when 30 volts was present.  because the meter had 
some series resistance, I used a combination of a 5600 ohm resistor and 
a 1000-ohm variable resistor (rheostat) which allowed me to calibrate 
the new 24 to 30 volt meter exactly, using my DVM as a reference

The "hot" side of the meter goes via the (approximately) 6000 ohm 
resistor to the battery plant,  and the "cold" side of the meter goes to 
the junction of the 7824 output pin and the 1000-ohm bleeder resistor.

I carefully opened the milliammeter, erased the numeric marks using a 
drafting eraser guide and an an ink eraser, and applied new digits using 
decals, and reassembled the meter.  Presto - a meter calibrated from 24 
to 30 volts, with 27.0 volts at top center of scale, and no non-linear 
readings.

I have used this technique many times now, and it works like a charm, 
for all sorts of expanded scale meters.  The basic idea is to determine 
the "delta" to be measured, and then build up a meter that measures only 
the delta, ignoring anything outside the range of interest.

To do this trick with negative voltages, simply substitute a negative 
regulator, (e.g. a 7924) and of course a negative polarity supply.

You can use different regulators to obtain various ranges, and even some 
sleight of hand to work this into the kilovolt range, by using a voltage 
divider from the source to be measured, for example 100:1 to divide 3000 
volts to 30 volts, and measuring that.  Some calculations will be 
required to determine the expected current flows and to adjust the 
calibrating resistors required.


- Jim, KL7CC



On 10/22/2012 2:01 PM, W2HX wrote:
> Hi all,
>
> I am doing a battery project, installing 28V battery system in a 12V vehicle with DC-DC converters and charging circuits, etc.  I want to use some traditional round analog meters to monitor voltage and current.  So I picked up a nice expanded scale (24V to 30V) panel meter which can be seen here:
>
> http://www.w2hx.com/x/1967LRS2/V-Meter.JPG
>
> I guess in my naiveté I assumed that this would read volts directly. Well, no such luck!  Here's what I know about the meter. First, its resistance reads about 230 ohms. Second, full scale (30V reading) appears to be about 360 mA (more precisely, 0.36 A - my ammeter does not have any more resolution than hundredths of an amp). It is very hard to measure the low end of the scale, perhaps 0 mA.  To the best I could, I measured the current that corresponded to each voltage marking on the meter.  Here is the result:  http://www.w2hx.com/x/1967LRS2/MeterRange.JPG  It looks pretty linear.
>
> First Question is. How do I build a circuit that when attached to my battery will show the right voltage?
> Second Question is. Are there expanded scale meters that will read direct volts? Or do they all behave in this way?
>
> Thanks!
>   
> 73 Eugene W2HX
>   
>
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