Monitor amperage and Display

Discussion in 'The Projects Forum' started by Rickg5106, Apr 28, 2010.

  1. Rickg5106

    Thread Starter New Member

    Mar 1, 2010
    Hello All,
    I am just a guy with an idea so point me in the right direction for research and a base for more intelligent questions.
    I would like to run a test that monitors 12VDC motor amperage. The test will start with the motor at a near stall but it will break free. The highest amperage before it unloads is the number I want. I would like to display that number and stop the test. I want to find the force or work required TO break free.
    Now the fun part, the amperage will be different every time and I want the numbers to be comparable between tests. I think it would need to be accurate to tenths and maybe hundreds of an amp (but maybe not the last). I don't know at this time what peak amps will be but I will test the motor to get some kind of number.
    So the questions. Is this some kind of Micro Processor / logic circuit voodoo that will do what I want? In the end I would like to have this package be hand carry so I would like to start small if I could.
    Anybody want to take a stab at this?
    Thanks all for your time, Rick
  2. SgtWookie


    Jul 17, 2007
    You need a peak detector circuit measuring the voltage across a resistance from the motor negative terminal to ground.

    The resistance could even be a piece of wire, as copper wire has a known resistance for a given diameter and length.

    Here's a start:

    Having some idea of a starting current would be a big help.
  3. Bychon


    Mar 12, 2010
    I have a meter that can be set on, "Max, hold". It only goes to 10 amps, but using wookie's idea, if you put a known resistance between the motor and ground, you could set the meter to max, hold on a voltage scale and calculate the amps. Are you willing to spend money to buy a meter?
  4. someonesdad

    Senior Member

    Jul 7, 2009
    To elaborate on Bychon's comments, there are clamp-on current meters that will read what you want. Most of them have a peak function to store the maximum value. Of course, you want to make sure you get a meter or adapter than can read DC currents.

    I got a B&K 316 clamp on meter in a trade that does this task. I like it because it will measure DC currents from 1 mA to 80 A and is a fairly small package (I got it to keep in our RV for troubleshooting, but it's useful enough that now I keep it at my workbench). If you search posts in the last few weeks for this model, another poster mentioned a clamp-on unit that needs to be plugged into a DMM and costs around $100. That also looked like a good general purpose solution for this current measurement problem.

    Realize the meter's sampling rate may miss the true peak current. The only way to know for sure is use a scope with a shunt or current probe.
  5. Rickg5106

    Thread Starter New Member

    Mar 1, 2010
    Yes, I would be able to spend some money. The motor I have is an auto window motor and is geared down rather well. I can't see the peaks at more than 10 amps but that is a guess. I can adjust that with physical weight as long as it doesn't get too heavy to carry. As an aside, I would like the event to also end the test. If it doesn't, I'll get a spike or sawtooth effect until manually turning it off. After the initial surge, the conditions change and although I don't think it will require as much amperage the 2nd, 3rd and however many times it spikes, I don't want to go there because it becomes an unnecessary complication and always leaves a question. Thank you all for your consideration and thought. I will follow the link and see where it takes me.