how to simulate ohms for gauges

[smckeown]

I've seen that some people are able to tell what ohms electrical gauges need by simulating the input in terms of varying ohms. How is this done ?

[stevieturbo]

A Variable resistor

[dilbert]

Typically an analogue electrical instrument (with a coil of wire and a magnet) will be designed to measure current. Digital guages will most normall be geared to measure a voltage.

The zero is usually indicated with zero amps, or volts, and the maximum reading (F.S.D. or full scale deflection) is indicated at a set voltage or current. If you do not know the FSD, you can work it out using a variable resistor and a battery.

If the FSD is not aligned with the singal range that you want to indicate then you can use a thing called a shunt. Since most meters are very sensitive, to either voltage or current, a resistor connected in parallel across the meter terminals is usually very effective as a below unity gain component, which both protects the meter, and allows it to indicate accurately.

To establish the value of resistor to use, you need to know the FSD, and the input impedance of the meter.

Some special purpose guages, already have a shunt built into them, where they are designed to work with a specific sensor or transducer. These guages, will require some sort of instrumentation amplifier, to make them work with something they were never intended to be used for.

[smckeown]

cheers, are there testers etc. that you can buy to simulate the ohms

[dilbert]

stevieturbo said:

A Variable resistor

I think he got it!

[stevieturbo]

smckeown said:

cheers, are there testers etc. that you can buy to simulate the ohms

Yes, Gunsons do a Sensor Simulator.

[smckeown]

brilliant, thanks guys. I did google steve's suggestion but got nothing useful product wise

[dilbert]

What sort of guage/sensor is it that you have?
I looked at your profile, and it says IT architect, but you don't seem that clued up. Are we missing something here?

[Zad]

Alas IT people aren't taught electronics any more

I mean - that might involve students getting their hands dirty. It would also imply doing things with your hands which is _so_ working class!

For resistive sensors, something like this should do:

http://www.maplin.co.uk/Module.aspx?ModuleNo=2203&...

Not sure what sort of range you will need, maybe 10k? I imagine you would need linear ('Lin') rather than Logarithmic, but I dunno for sure.

[stevieturbo]

Cant seem to find that Gunsons stuff online. I know I bought my Sensor Sim from Halfrauds a few years ago.
They must have discontinued it or something

[blitzracing]

If you have an ohm meter, why not simply measure the resistance of the sensor your are trying to mimick ? This will at least get you in the right ball park before you choose a variable resistor. Some sensors are not linear (like variable resistance temperature senders) that can make things awkward if you are trying to "mismatch" senders and instruments (with shunts or otherwise). Things like fuel gauges can use a heater and bimetal strip to move the needle, so they need quite a lot of current to drive them, so the sensor resistance will be low.

Mark

[Mr2Mike]

blitzracing said:

If you have an ohm meter, why not simply measure the resistance of the sensor your are trying to mimick ?

I think the OP has a gauge and no sensor, at least that's the way it reads to me.

Temperature/fuel level sensors typicaly have resistances in the ~20 Ohm (i.e. fuel level sensor at max) to 2k range IME. A suitably rated variable resistor (probably wirewound to cope with the power dissipation required) can be used to simulate the sensor.