Current comparator circuit

Discussion in 'The Projects Forum' started by jswilson, Apr 19, 2010.

  1. jswilson

    Thread Starter New Member

    Mar 4, 2009
    6
    0
    Hi,
    I am needing to build a circuit to perform a test on a part. It is from an automobile. The power source will come from right off the battery in the car. It will be a comparator circuit where a green LED will light if the current on this part is between 6 to 60 amps. If the current is below or above this value, a red LED will light. Does anyone have a circuit that will work for this test? I did electronics years ago and I am very rusty on my electronics now and I don't have time to go back and pull the books back out.

    Thanks You!
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi,

    I have a similar circuit on hand, which I have tweaked to suit your needs. If you’re interested we can pursue further. The circuit is attached. The current sensing device can be anything that will output a voltage for current in. I have used a 0.0833, 300W resistor for now.

    If you are too "rusty" you may want to buy something off-the-shelf. Let us know what you think.

    Regards,
    Ifixit
     
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  3. jswilson

    Thread Starter New Member

    Mar 4, 2009
    6
    0
    Thank you for the schematic ifixit. To cut costs, I am going to use an anolog panel ammeter. Here is how the tester is to work: there are 4 resistors to test. .2-2.0 ohms. You hold the tester, connect it via clips to 12 volt supply. push a main test button. The tester will test resistor 1 for 5 seconds then shut off automatically. Then you turn a selector switch to resistor 2 push the button and so on. My question is though in order to perform this test, the circuit will have to handle up to 60 amps of current. Anything more than that and I need the tester to shutoff. What would be a circuit to do all this? when you get up into this high of a current value, I am not sure on compenonts or he design of the circuit.
    Thanks!
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi,


    Please answer a few questions before we proceed.
    1. Are you testing the resistors? If so what is the pass and fail criteria?
    2. Do all the resistors go across the 12V battery, one at a time, for the test?
    3. Only the .2Ω resistor will draw 60 Amps at 12 V and dissipate 7200 Watts. Is it a huge device, otherwise it will burn up?
    4. The 2Ω will dissipate 72 Watts. What are the other two values?
    5. Do you have a 60A selector switch in mind?
    6. Do you have a 100A analog current meter? What type?
    7. To shut off automatically, you will need a 100A automotive type relay. A starter solenoid would do.
    8. Starter solenoids could be used in conjunction with a smaller, less expensive, selector switch.
    9. Do you have the skill to build and test a circuit like the one attached earlier?
    Regards,
    Ifixit
     
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