What type of voltmeter movement is this?

Thread Starter

EB255GTX

Joined Apr 30, 2011
62
So... I got a number of automotive style voltmeters from the FleaBay, for a project.

I intended to rescale them to 5V FSD so i can drive them with PWM from a microcontroller, but upon opening one I find a mechanism that i've never seen before.

I don;t know what to search on to find out more - can anyone suggest the name/type to look up? There's also no scaling resistor(s) on the small PCB, so if the number or ratio of turns on the coils is what scales it, I'm just going to buy some more conventional ones!

Interestingly the movement is very damped.IMG_0352.jpgIMG_0353.jpgIMG_0354.jpg
 

Tonyr1084

Joined Sep 24, 2015
4,198
Meters that move a needle are called D'arsenval Movement (DM). Not sure exactly what sort that meter is. Possibly the needle is moved by magnetic forces such as DM. Many panel meters I've seen lately are stepper type movement such as speedometers in cars. Tachometers, gas gauges, etc.
 

Thread Starter

EB255GTX

Joined Apr 30, 2011
62
Yes, this one is definitley not D'arsonval, and not a stepper. It appears to be two coils at 90 deg to each other - none of the usual tiny springs and so on for a moving coil or moving iron. It has polarity, i..e indicates DC only. The response is very slow/dmaped i.e. a step application of 15V takes about 2 sec to get the needle to the 15V position, and removing power it takes about the same to decay. Reversing power drives the needle the opposite direction.

Never seen one like it!
 

Thread Starter

EB255GTX

Joined Apr 30, 2011
62
I would guess that it is some kind of ad hoc stepper.

Bob
There's no control IC visible on the PCB, and the decay of the pointer back to zero when the voltage is removed is controlled and slow in the same way as on the way up when voltage is applied. if it was a stepper it should stop where it is when the power is removed.....

I think it's some kind of magnetic engine where a voltage causes a flux build up and removing that voltage causes a flux drop, but it seems like somehow the motion of the pointer opposes said flux such that it moves slowly - and when power is removed the motion of the pointer falling causes a generated voltage that opposes it falling quickly.

I'm reminded of a magnet falling down a copper tube slowly, showing Lenz's law.
 

Thread Starter

EB255GTX

Joined Apr 30, 2011
62
Where does the needle attach?
Oh fair question, of course i've been looking at the thing all day but not shown it clearly in the photos - here's a vid to answer your question plus show the movement - damped on the way up (step application of 12V) and also on the way down (total removal of power) plus polarity sensitivity (quicker reverse direction if polarity swapped):
 

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debe

Joined Sep 21, 2010
1,156
Its a cross field gauge, volt meter uses 2 conections. They are very common also as fuel & temp gauges in vehicles. But use 3 terminals.4.JPG
 

Thread Starter

EB255GTX

Joined Apr 30, 2011
62
alec_t, debe, kubeek - THANK YOU!

Very interesting indeed, and not something that seems to be commonly mentioned - you always see moving coil and iron vane in textbooks and websites but not this.
 
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