How does this gauge work?

Discussion in 'General Electronics Chat' started by Gdrumm, Jan 2, 2015.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    It's a friend's Harbor Freight Battery 100 amp Load Tester.
    The needle moves freely when you blow on it, the load heats up as it should, but the thing doesn't work.
    The needle doesn't move when connected to a good battery.

    I re-soldered a broken resistor wire, thinking that was the problem, but that hasn' fixed it.
    I checked the resistor and it reads 12.5 K oms, as it should.
    I cleaned up the face, and the zero adjust screw, and made sure it was properly aligned during re-assembly.
    I removed the guts of it, and did not see any disconnected or broken wires.

    What else might the problem be?

    Thanks for any insights.
    Gary
     
  2. bwilliams60

    Active Member

    Nov 18, 2012
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    This being a load tester, I assume you are referring to the voltmeter not working as opposed to the ammeter. If it is the voltmeter that is not working:
    1) With a DVOM, check the voltage going into the voltmeter circuit board on the back of the meter. There should be a smaller gauge wire coming from large positive and negative cables once it enters the enclosure. They will go directly to the circuit board and then to the meter.
    2) If that voltage is okay (battery voltage), the problem is either on the circuit board or the gauge itself is defective.

    Post pics of the rest of the unit please with the model number.
     
  3. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I'm under the impression that this sort of ammeter is basically a galvanometer... most likely the problem is that the winding's blown out or there's a defective contact
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The meter usually acts as a voltmeter across the load shunt, as cmartinez said it could have blown, measure the meter with an ohm meter and see if you get a reading.
    It could have blown open if 1/ the shunt has opened or too great an input voltage.
    Max.
     
  5. bwilliams60

    Active Member

    Nov 18, 2012
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    He is showing the voltmeter in his picture. A lot of those cheapy laod testers had a small circuit board on the back of the meter. Not sure what he has, that's why I asked for more pictures.
     
  6. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Here is what a good one looks like, and the bad one is shown disassembled.
    There is no "board", just the gauge as seen in the pictures.

    I know these deflection needle devices can be touchy, and I'm guessing electrical magnatism deflects the needle.
    Assuming that, and the fact that the needle moves easily, and I don't see any broken wires, what else might be wrong?

    I'm not sre where the shunt is?
    It's basicall just this gauge that doesn't deflect when connected to a good car battery.
     
  7. bwilliams60

    Active Member

    Nov 18, 2012
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    What is the resistance of the coil on the gauge? You say you have a good one there. Can you check the resistance of the good one and compare the two. The unit is just using magnetism to read voltage. There should be a small permanent magnet and when you apply current to the coil, it causes the needle to deflect accordingly. I suspect Max may have hit it with an open coil due to excessive voltage on the coil.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is the shunt I mentioned, the (volt) meter just reads the volt drop across it when you place the shunt across the battery, the rate of decay on the meter shows the ability of the battery to hold a charge.
    Max.
     
  9. bwilliams60

    Active Member

    Nov 18, 2012
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    I'm confused because he doesn't mention or show a shunt anywhere. Generally a shunt will be used in the ammeter portion of these testers. I am wondering if the resistor he has mentioned is being used as a "shunt"or base value for meter movement?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    DSC00149.jpg appears to be the load shunts I am familiar with in these battery testers.
    You can see two terminals that appear to be for the meter in parallel.
    Max.
     
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  11. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I'll see if I can check the coil.
    I think I understand it better now.
    Thanks,

    Gary
     
  12. #12

    Expert

    Nov 30, 2010
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    That zig-zag piece of wire is the shunt. The only thing left is the 1 ma meter movement with its 12k resistor. As I see it, both halves are independent from each other, so which ever half doesn't work must be broken.

    12k resistor is good? Try the meter movement coil.
     
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  13. bwilliams60

    Active Member

    Nov 18, 2012
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    Gdrumm, have you found the problem with this unit? Just curious.
     
  14. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I tested the coil on the meter, and it read 1.6 ohms.I'm not sure what it is supposed to read.
    Everything looks good, and checks out good as far as I know what and how to test.

    So, the short answer is no.
    Thanks
     
  15. bwilliams60

    Active Member

    Nov 18, 2012
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    Not sure what yours should be but the one I have here is measuring 155 ohms. I have an Ultra Pro unit and the ammeter is not working. I just found a broken wire going to a potentiometer or trim pot shaped like a tin can with three legs. Never seen one like that before. Learn something new every day. Do you have another ammeter you can try?
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Your OP says the shunt heats up so we can assume that is OK, using a regular voltmeter across the shunt when applied to a known good battery, read the voltage you get it should be 12v and decay from there, you should be able to rig up a regular moving coil Ma meter and calculate a series resistor to sub for the meter.
    Does your meter deflect when you apply an ohmmeter across it?
    Or try passing a small current using a series resister.
    Max.
     
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  17. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Max,
    That puts me in over my head.
    I'll have to read your post when I'm rested, and see if I can do it.
    It's rocket science to me (or was it rocket surgery?) (pun intended).
    Thanks,
    Gary
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Per GWB. o_O
    Max.
     
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