Integrating ohm meter into tester

Thread Starter

Circuits123

Joined Dec 7, 2012
69
I'm thinking of building a tester for antique automobile gauges that would allow me to to adjust the input resistance with a 500 ohm potentiometer. To do this I'd like to have an integrated ohm meter but I haven't been able to find any meters except those built into multimeters. Does such a thing exist? Any suggestions for how to find one?
 

MaxHeadRoom

Joined Jul 18, 2013
21,639
There are many digital displays on ebay for $2.00 that should be able to be modified/used.
Give a little more info of what you are doing exactly.
Max.
 

Thread Starter

Circuits123

Joined Dec 7, 2012
69
I'm new to electronics and learning a lot as I go along. The idea would be to have a series circuit with the gauge being tested (attached with alligator clips on the terminals), a power source (10 or 12v), a potentiometer, and the meter. I'd like to be able to adjust the resistance and see how it affects the reading on the gauge being tested. Usually I can tell whether the gauge is functioning properly by comparing the resistance with the gauge reading.
 

MaxHeadRoom

Joined Jul 18, 2013
21,639
I assume these are moving coil meters, they are often marked in small letters with the resistance of the meter for calibration/application purposes.
Max.
 

Thread Starter

Circuits123

Joined Dec 7, 2012
69
I assume these are moving coil meters, they are often marked in small letters with the resistance of the meter for calibration/application purposes.
Max.
Yes, they're moving coil. I'm just looking for a simple, repeatable way to adjust the resistance [the potentiometer] and to know what resistance it is. So if I know the gauge should read 1/2 at 50 ohms, I can try it out and see if that's what it actually does. I know I could just keep disconnecting everything and using my multimeter. But I was wondering if there was a way to include a permanent meter in the circuit.
 

Reloadron

Joined Jan 15, 2015
5,667
An ohmmeter is actually a voltmeter in that we pass a known current through a resistance and measure the voltage drop across the resistance. For example an ohmmeter may have a known resistance, as Max stated and at a given current will display a full scale current, for example 500 Ohm 1.0 mA FS (Full Scale). So in reality an old classic car fuel gauge was just a resistor used as a current limiting device allowing the fuel gauge to travel from F at full scale to E on the bottom of the scale. A Google of basic ohmmeter circuits should yield a dozen or more basic circuits. Temperature gauges worked on the same principal. Oil pressure gauges were generally an actual bourdon tube type pressure gauge.

Ron
 
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