Inaccurate automotive voltmeter

debe

Joined Sep 21, 2010
1,320
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.VDO VOLT METER.jpg1.JPG2.JPG3.JPG4.JPG
 

Ian0

Joined Aug 7, 2020
6,315
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
@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.
 

Thread Starter

Jerry-Hat-Trick

Joined Aug 31, 2022
168
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 281303View attachment 281304View attachment 281305View attachment 281306View attachment 281307
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 off
 

debe

Joined Sep 21, 2010
1,320
I just set mine up so it was sort of accurate @ 12V & 14V, was only interested in 12V with Ign on then expect 14V with engine running in my vehicle.
 

BobTPH

Joined Jun 5, 2013
5,794
Notice no additional ring magnets in the description or pictures posted by debe. I don’t think those belong there.
 

debe

Joined Sep 21, 2010
1,320
I would suggest Smiths instruments do things just a little different to VDO but simmilar. If you purchased the meter new then the magnets are supposed to be there.
 

Ian0

Joined Aug 7, 2020
6,315
Perhaps they designed a fancy new meter, and when they built it, they found that it worked backwards, then some old boy figured out that if they put a couple of magnets inside the case. . . . .
 

BobTPH

Joined Jun 5, 2013
5,794
The no voltage position of the pointer can be adjusted. The question is what direction it moves when a voltage is applied. I still think someone messed with this meter and it may be fixable.
 

MrAl

Joined Jun 17, 2014
9,552
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 problem is you bought it from ebay :)

With most DC (and even some AC) voltmeters you can either add a resistor in series or lower the existing resistor in series in order to change the full scale deflection (and thus all the readings) to the right value. Some meters even come with a face plate that is calibrated for a higher voltage and the basic movement is a lower voltage and you have to use an external resistor to get the needle deflection right.
I have such a meter i use in my car and it was fairly inexpensive and it reads very close to exact.

My advice would be to buy a new meter with a wider face. The increments will be larger then and inherently more accurate with the right resistor.

When you can change the resistor yourself you can even create more than one range. If the meter face reads 0 to 15v for example you can make it 0 to 1.5v or 0 to 150v just by changing one resistor, which of course means if you use a selector switch you can have a variable range volt meter. All i needed was 0 to 15v for the car so i just use one external resistor.
The meter is about 3 inches wide so the deflection angles are decent. I can tell the difference between 13.6 and 13.8 and 14.0 volts without having to stare at it for an hour. The response is fast too so if i connect it to the OBDII connector i can watch it as the car starts and that gives a very strong indication about the health of the battery as it ages. If it dips too low while cranking it's time to replace the battery.

I like the zener diode idea though. That can expand the range between two more interesting voltages such as 8v and 15v rather than 0 to 15v and that can increase the resolution between 8 and 15. I'd be tempted to do that with mine but i'd have to recalibrate the face plate or just mentally remember how the new readings correlate to the actual voltage. I probably dont need to increase the resolution though it seems good enough as is.
 
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