[Boatanchors] Panel Meter Circuit?

[J. Forster]

With the meter Gene has, a series combination of roughly a 14.6 kOhm
resistor and 24.5 volt Zener should be fairly close to making it direct
reading. The values may need a bit of trimming.

-John

===========



>
>
> 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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