@Irving It seems like the zener regulates the voltage across one coil (and hence the current through it), and the difference between the zener and battery voltage sets the current through the second coil, so that the magnetic field depends on the ratio of a fixed current to a variable one.Have a read of this on that type of instrument. Also i have never come a cross one of these style voltmeters thats accurate. This Volt meter was in a vehicle dash board. They were very common some years ago, Fuel & temp gauges were the same syle.View attachment 281303
Hey, this looks right! Many thanks. I'm thinking that the Zener shown in the dotted line box is probably internal whilst the Zener on the outside of my movement is simply to shift the voltage by around 6V so it reads from 6 to 16 instead of 0 to 10. I'll try checking with an external resistor in series to see if I can confirm the existence of an internal Zener but if the internal resistor is large compared to the coil resistances this may be difficult if not impossible to see. Maybe I'm just expecting too much from this gauge which frankly isn't much better than the ignition light going on and offHave a read of this on that type of instrument. Also i have never come a cross one of these style voltmeters thats accurate. This Volt meter was in a vehicle dash board. They were very common some years ago, Fuel & temp gauges were the same syle.View attachment 281303View attachment 281304View attachment 281305View attachment 281306View attachment 281307
Except that without them the pointer simply goes to full scale....Notice no additional ring magnets in the description or pictures posted by debe. I don’t think those belong there.
The problem is you bought it from ebayFor 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...
True!The problem is you bought it from ebay
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