Moving Coil Voltmeter Query

I modified an old voltmeter (dial panel meter) to change it's range from 15 - 30V to a range of 2.5 - 5V. On the face it mentions it has 1kOhm/V. There was a 30K resistor inside it, but I replaced that with a 4.7K resister.

All seems to work well with it reading ~1mA at full deflection at ~5V (near enough).

The bit I want to learn about is what is it about this voltmeter that causes it to have a starting deflection of half the full deflection voltage? Is there something in how the coil was constructed that does this? I googled few things and haven't been able to help my learning on this matter.

Hi,

The bit I want to learn about is what is it about this voltmeter that causes it to have a starting deflection of half the full deflection voltage? Is there something in how the coil was constructed that does this? I googled few things and haven't been able to help my learning on this matter.

Great that you have successfully got the meter range changed.

There must be another component in the circuit, or the movement has been setup with a bias on the needle return spring so you need 2.5V to lift it off the stop.

What was the meter from?

Tom... :slight_smile:

TomGeorge:
There must be another component in the circuit, or the movement has been setup with a bias on the needle return spring so you need 2.5V to lift it off the stop.

What was the meter from?

I can't find anything that might be in the circuit. I've poked and prodded and all I've found is that the negative terminal goes to a connection to the core and the positive terminal goes through my replacement resistor and then connects to the large 'case' that is around the core separated by an air gap. (the silver base you see under the face label)

Is there a way that the core might be wound in some way so it only moves with half the voltage? I say that out aloud and now I figure with KISS, it would have to be just a movement restriction as it'd be an easier answer.

Not sure where they are from. There are date 1961 and have nFe / Fe. on the face. They are just something that came in a bundle from ebay. Had to break the glass on one to work out how to get inside. There is a sprung ring that sits in a groove between the cap and the housing. If I can get that to close the cap should just push off.

There may be a diode rectifier in line, either to make it an AC voltmeter, or to protect against reverse voltages. The diode voltage drop makes it non-linear at low voltages.

Oh, now I see that the range doesn't start at zero... well that greatly increases the chances that there is a junction or zener diode in series with the coil. You could detect that with a DMM.

Usually there's a screw to zero the meter but I don't know if you'd have enough adjustment to make it behave like a regular meter. (I think that adjusts the spring rotation.)

I've seen meters that are zeroed in the center for positive or negative readings but I don't remember seeing a meter like that before.

Yes the hairspring mount lever rotates in a friction clutch arrangement so you can adjust the rest position, for many meter movements this would be the zero point, for one like this you have to calibrate this with a test voltage.

These meter movements have two hairsprings, one at the front and one at the back, and use them to connect
electrically to the coil. This means that the adjustment angle for the meter is quite limited as the needle
rotates half the angle of the front adjuster (there may be a back adjustment that can be set at manufacture
time but its not usually accessible).

I have seen this before - a line voltage monitoring meter that covers 100-130VAC. It had a zener circuit to restrict the range to over 100V.

Hi,
These could have come from truck dashboards, 24V system, 15 to 30 would be enough to monitor battery condition without a constricted range.

Tom... :slight_smile:

aarg:
There may be a diode rectifier in line, either to make it an AC voltmeter, or to protect against reverse voltages. The diode voltage drop makes it non-linear at low voltages.

I tried checking diodes and the meter reads the same resistance in both polarities.

MarkT:
These meter movements have two hairsprings, one at the front and one at the back, and use them to connect electrically to the coil.

you’re correct about the two hairsprings, but other than the big resin sealed flathead screw on top the coil axis, I can’t see anything that looks adjustable. I guess that would be the ‘adjustment’ bit, but making that change is probably beyond my skill to do and not damage it completely.

Thanks for the suggestion though. And thanks to all that gave the information. I’ll just have to live with the half range scale for the project.

Here’s some closer pics of the insides.

This is a good well illustrated write up on PMMC (Permanent Magnet Moving Coil) meter movements. If you look at the images on page two note the hairsprings and pivots. I can set my limits however I wish to mechanically. Old automotive dashboard current meters would read negative and positive currents with a center zero.

Meter sensitivity such as Ohms/Volt is the meter movement full scale current. For example a cheap meter may have a sensitivity of 1,000 Ohm/Volt. The full scale current will be 1/1000 = 0.001 Amp or 1.0 mA. The old Simpson 260 and 270 series meters had a sensitivity of 20,000 Ohms/Volt and the meter movement is 50 uA full scale. Ohms/Volt and FS Current are reciprocals.

Current meters have a shunt resistance in parallel with the movement for current ranges while volt meters use a series resistance with the meter movement. By carefully choosing a resistance and winding my coil I can have a meter which does not start noticeably deflecting until a certain voltage is met. I make my face scale accordingly. Keep in mind these meters were in use long before most solid state electronics were available.

Ron