Variable earths

[torblednamris]

Howdy all,

The vast majority of the threads in here are well beyond my comprehension, so please excuse my naive, and somewhat simplistic questioning.

My query relates to old automotive gauges such as Fuel and Temperature gauges, that utilise variable resistors at the earthing point, such as the temp-dependant sensor in the cyclinder head, and the moving armature in the fuel tank.

These gauges are effectively 'heated' by a coil of wire around a spring-loaded arm that moves the display needle proportionaly as the earth is increased. I know these type of gauges have a certain name, I just cant remember it.

Mapping the circuits, I've found they are fed 10vdc from a regulator IC at the back of the instrument cluster, through the coil and into the wiring harnesses to the variable earths. No matter what the reading on the gauge, there is always the same voltage across the two sides of the actual gauge. Does this mean that these particular circuits are actually varying the amperage?

I wanted to somehow attain a voltage reference from each gauge to feed into a visual LED bargraph using the LM3914 display driver. But if the voltage never changes this is obviously not possible, is it?

Sorry for the lengthy post, I find it hard enough to understand this stuff, let alone write it out!

Thanks for any help,

Rob Smith
Canberra, AUSTRALIA

 

[BrianG]

Hello Rob,

The type of gauge you refer to is, in my automotive experience, a "hot wire" ammeter and is thus current operated. It usually consists of a thin wire which is under tension and operates a linkage to the dial pointer. As the current increases in the wire, it heats up and progressively goes slack due to expansion. The tensioner system converts this into the required pointer movement.

Whilst fuel gauges are variable resistances operated by a float, temperature gauge sensors are usually a zener diode, not a resistor.

With regard to the "voltage regulator IC" giving 10V, if this is on an old vehicle are you sure it is an IC and not a mechanical / thermal voltage regulator? These devices give strange readings with electronic meters and DVMs because they maintain a mean voltage by pulsing the battery power on and off via a thermally heated bi-metallic relay contact. This pulsing is not visible on the hot-wire gauges because of their slow response times.

If you want to get electronic readout but still want to keep the original gauges, you should be able tap off a sample signal by putting a small resistor in series with the particular circuit. This can then be filtered to remove the pulses, and amplified as necessary, by a simple op-amp circuit before going to the bar-graph device.

 

[torblednamris]

Hi,

Thanks for the reply BrianG. Your description of the type of gauges I have was exact.

And yes, it is an old vehicle, an Aussie built 1978 Holden UC Torana. And I'm pretty sure the voltage regulation is done by an IC, I can't read a model number, but it's definately in a TO-220 3-pin IC Package, but now you've got me thinking 'were they around then'? I know we only got the wheel in '76.

You've definately cleared things up for me, in effect these gauges are ammeters. In that case, time to get the Multimeter out again!

Thanks again for the help.

Rob Smith
Canberra, AUSTRALIA

 

[47gandalf]

Perhaps the name you're trying to remember for meter movements is d'Arsonval.

These movements suspend an armature within low-friction bearings and use the magnetic field generated by the current passed through their windings to deflect the needle.

We used to make meters for many different signals by scaling this current appropriately and making custom scales which are attached to the front (viewing side) of the meter with screws.