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...
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...