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Ammeter Tester

Discussion in 'General Electronics Discussion' started by D5627, Apr 6, 2017.

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  1. D5627


    Apr 6, 2017

    My Problem:
    I have a number of railway ammeters that need to be tested for correct calibration (these are non-mainline so official calibration is not required). These ammeters are used to show the driver the current amperage being produced by the Generator, via a shunt in the circuit.

    For one type of ammeter we have, I know that an indicated 2000 amps is 75mv on the input.

    My initial Solution:
    My initial thought was to use an Arduino based solution but I don't think this is a workable solution with my limited knowledge of the code used and its make up and the need to vary the output voltage. What I was thinking was to use the Arduino control an output voltage between 0 and 95mv in steps thus giving me a "test mode" through the full range of indications. I would still like to pursue this if at all practical.

    Another Idea:
    Another idea would be to use a small voltage suitable stepped down to provide no more than 95mv with a suitable potentiometer in the circuit to control the actual voltage although a method of reading this voltage will need to be included for correct calibration. Unfortunately this is now outstripping my knowledge in electronic circles.

    Without wanting to ask someone to do this for me, can anyone suggest a better way that the two ideas I have or even if there is a simple way of doing this? I will post photos as i move through this little project to show what it is I achieve.


  2. shrtrnd


    Jan 15, 2010
    I'm not into Arduino, hopefully somebody will help you with this in a following post.
    What prompts my response is my experience with ammeters in your application.
    MANY times I've found the shunt itself takes the brunt of abuse. It heats-up, and changes resistance values.
    I speculate that you want these ammeters tested because someone is unsure of their accuracy.
    When you get a solution to your 'calibration' issue that you're comfortable with, I strongly recommend that
    you consider replacing the shunt resistors when you reinstall the ammeters, to ensure the accuracy of
    your ammeter readings.
    My discovery of this potential problem, was the result of sometimes having the shunt resistors burn so hotly that they
    opened. A troubleshooting lesson that I have not forgotten.
    Just something I wanted to make you aware of. Good luck with the 'calibration' of the ammeters.
  3. D5627


    Apr 6, 2017
    Thanks shrtnd.

    I was discussing this very subject on Friday with one of my fellow volunteers and once we have a calibrated ammeter the next step is to look at the shunt. This is all part of the general TLC we are giving this particular locomotive.
  4. TedA


    Sep 26, 2011

    It would be helpful to know the internal resistance of the meter, or the full scale current. Sometimes this is marked on the meter, else one or the other value can be measured. Care must be exercised with any measurements.

    If you have just a few meters to check, any computer control is really window dressing.

    Without using any special calibration equipment, a simple solution would be a stable, adjustable DC source, one or more fixed resistors, plus an accurate voltmeter.

    As already suggested, the shunt is suspect, as well as the meter. To check the shunt, you will need a known good shunt, and a source of lots of amps. You would like to test the shunt near full scale.

    What's your budget for this project? If you are starting from scratch and have to buy everything, I can see a couple of hundred dollars being in the ballpark.

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