For my replica oldtimer I purchased a set of replica Smiths gauges https://www.ebay.co.uk/itm/115504666144

The appearance is fine, given the price, and I can probably live with inaccurate readings so long as I know what they are, but the voltmeter is important. To be any use at all I think it should be reasonably accurate over the range 10V to 15V so I can see the battery voltage when I switch on the ignition, check that it's charging when the engine is running, but not going over about 14.5V maximum.

As delivered, it reads roughly okay up to about 12V but hardly moves when I increase the voltage.
Input/Reading
9V / 9.0
10V / 10.8
12V / 12.2
14V / 13.1
15V / 13.2
19V / 14.0
Shorting out the internal zener diode - around 5.6V - it now reads 8V with 2V input, 12V with 4.5V input and 15V with 15V input. So I need to find a way to reduce the voltage across the coil at low input voltages (similar to the zener) but progessively "short out" the zener as the voltage rises - ideally with a simple circuit which can be squeezed into the gauge enclosure. The resistance across the coils is a miserable 150 Ohms so it needs 100mA to show 15V!

Grateful for any ideas...

 

Gauge in question:

Do you have a current source that you can use to measure the full-scale deflection current?

 

Gauge in question:
View attachment 281226
Do you have a current source that you can use to measure the full-scale deflection current?

I'm probably being a bit dim here, but I did a quick check with a 47 Ohm resistor in series, wacked the power supply up to 31V which is the most it can do and the meter still reads just over 15V and the voltage across the resistor was 6.3V so a current of 134mA. If the coil resistance is 150 Ohms I'd expect the current at 15V (without the resistor) to be 100mA? The meter simply runs out of steam with minimum change in deflection as the current rises above 80mA. I don't think it'll move much further even with a higher current?

 

Two possibilities then; either there's something restricting the movement mechanically, or the magnetics aren't strong enough to overcome the return spring. What do the magnetics look like?

 

Is there an adjustment screw on the back?

 

Here's the simulation of a circuit that may work for you.
It uses a TL431 adjustable shunt reference to generates a near ideal, variable Zener function that can substitute for the present Zener.
Q1 is a buffer to provide more output current capability.

It's shown adjusted to about 7.5V equivalent Zener voltage, which gives 4.5V across the gauge with 12V applied (yellow trace) since you stated that 4.5V directly across the movement gives an indicated 12V.
You can tweak the trimpot U2 to adjust the emulated Zener voltage and calibrate the reading.
The gauge readout accuracy will depend upon how linear it is versus applied voltage.

 

I'm probably being a bit dim here, but I did a quick check with a 47 Ohm resistor in series, wacked the power supply up to 31V which is the most it can do and the meter still reads just over 15V

It would be better if you made a current source and measured the current required for full scale deflection. Then we can subtract the zener voltage and figure out what's wrong with the meter. Could be a bad meter movement, shunt, or something else.

 

Is the voltmeter a moving coil type or a heated bimetal type?

 

Two possibilities then; either there's something restricting the movement mechanically, or the magnetics aren't strong enough to overcome the return spring. What do the magnetics look like?

Good points. The design is truly weird with two small ring magnets (about 4mm in diameter) on either side and the coil is wound around a former twice, at right angles - with no return return spring, so somehow it is aligning itself within the magnetic field. Taking out the two little magnets it goes to full scale! I may have to take it apart again to photograph it. In summary, maybe the magnets are too strong. I can't see anything restricting the movement mechanically.

Is there an adjustment screw on the back?

Is the voltmeter a moving coil type or a heated bimetal type?

No, sadly not. And it's a moving coil type "but not as we know it" (with apologies to Spock)

It would be better if you made a current source and measured the current required for full scale deflection.

Thanks for the idea. The truth is, the pointer hardly moves above 15V with increasing current, I'm reluctant to keep going

Here's the simulation of a circuit that may work for you.

It uses a TL431 adjustable shunt reference to generates a near ideal, variable Zener function that can substitute for the present Zener.

Q1 is a buffer to provide more output current capability.

This is great, I had in mind somehow shorting across the zener by turning a transistor on and I think you've nailed it! I confess, it's a workaround, rather than fixing the meter but I like it.

 

Quick update. Took it apart again. Tried to prise off the pointer but don't want to break it, so can't see the innards from the front. Note the two little magnets which were not attached to anything, just held in place when re-assembled. I've now discovered that rotating them changes the position of the pointer - need to get my head around this and find a way to stop them moving when I re-assemble. The poles are across the diameter of the magnets. I can vaguely understand how the coil(s) must be trying to rotate to align with the field due to the magnets but it's not obvious to me. Must be a nightmare to assemble in the factory...

 

Considering that there is nothing to mechanically align the magnets, I don’t think they are part of the design. Perhaps they were added by someone for some reason.

 

This is great, I had in mind somehow shorting across the zener by turning a transistor on

Note that my circuit replaces the Zener, not shorts across it.

 

Considering that there is nothing to mechanically align the magnets, I don’t think they are part of the design. Perhaps they were added by someone for some reason.

With them in place the pointer goes fully to the left, take them out and it swings all the way to the right! So I think they have a purpose but I simply can't get my head around how it's supposed to work... Thanks all the same, I'm stumped by the design so any and all thoughts are welcome. The assembly sits in a can which is also magnetic and for re-assembly I put them in the can and they naturally align with the poles facing the side wall so twisting them and getting them to stay where they are put will not be easy. So frustrating....

 

I think it works like a stepper motor - the ratio of currents in the two coils determines the direction of the pointer.
I’ve seen fuel gauges done that way. Fuel gauges are made with a lot of mechanical damping.
Does the zener regulate the voltage across one coil (and hence the current through it), and the difference between the zener and battery voltage set the current through the second coil, so that the magnetic field depends on the ratio of a fixed current to a variable one.

 

The assembly sits in a can which is also magnetic

Do you mean the can is permanently magnetised with distinct poles, or just made of magnetic material?

 

I think it works like a stepper motor - the ratio of currents in the two coils determines the direction of the pointer.
I’ve seen fuel gauges done that way. Fuel gauges are made with a lot of mechanical damping.
Does the zener regulate the voltage across one coil (and hence the current through it), and the difference between the zener and battery voltage set the current through the second coil, so that the magnetic field depends on the ratio of a fixed current to a variable one.

You may be recalling the thread on fuel gauges... the second coil is to regulate against battery voltage changes by providing a voltage referenced field. A voltmeter doesn't need, nor does it want, that.

With them in place the pointer goes fully to the left, take them out and it swings all the way to the right!

I have a feeling this quote is significant. Assuming this effect is with no volts applied this suggests some form of spring is present or the moving elements are pre-magnetised. Maybe the small magnets are for calibration, but it seems very random if that's the case?

 

I think it works like a stepper motor - the ratio of currents in the two coils determines the direction of the pointer.

I’ve seen fuel gauges done that way. Fuel gauges are made with a lot of mechanical damping.

Does the zener regulate the voltage across one coil (and hence the current through it), and the difference between the zener and battery voltage set the current through the second coil, so that the magnetic field depends on the ratio of a fixed current to a variable one.

Good thought. I was wondering if it's something like an air core meter but I can't get to see inside to see if the coils are separate. There are only two tags where the coil wire is soldered and the resistance between them is 150 Ohms, measuring the same in both directions. When I short the zener (as shown in photos) it simply shifts the reading by around 6V but the gauge still works (without the 6V offset) - regardless of whether it's in the can or not, so I'm pretty sure that the zener is just being used to stop the pointer moving until the voltage is above 6V. But I do like the idea of the direction of the field varying with the current so maybe there is an internal zener which I can't see that is doing this

I have a feeling this quote is significant. Assuming this effect is with no volts applied this suggests some form of spring is present or the moving elements are pre-magnetised. Maybe the small magnets are for calibration, but it seems very random if that's the case?

Without the magnets, I can't detect anything magnetic when I dangle a paper clip near the coils. And it's easy to move the pointer back to zero (marked 8V) and watch it slowly return to full scale as if it's damped. There is no visible spring. I believe the adjustment of the magnets may give me a result but I agree it's not an elegant way to calibrate if that is the intention.

Do you mean the can is permanently magnetised with distinct poles, or just made of magnetic material?

The can is just made of magnetic material - weirdly it doesn't seem to make any difference if the movement is sitting in it or not - although presumably protecting the movement from external magnetic fields

 

I can vaguely understand how the coil(s) must be trying to rotate to align with the field due to the magnets

My guess would be that the coils are wound on a former including soft magnetic material, which aligns with the magnets when the pointer is on 8V. This provides a magnetic alternative to the usual hair-springs.

 

My guess would be that the coils are wound on a former including soft magnetic material, which aligns with the magnets when the pointer is on 8V. This provides a magnetic alternative to the usual hair-springs.

Except that the pointer goes to full scale when the magnets are taken away so there must be some kind of mechanical device to twist it, possibly taut wire or hair spring?

 

Hmm. That's a puzzle.

With them in place the pointer goes fully to the left, take them out and it swings all the way to the right!

Is the swing to the left or right with the coils energised or not?