12V Aux Keeps Dying While Off

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by ep3eddie, Nov 23, 2021.

  1. pasadena_commut

    pasadena_commut Active Member

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    As long as the meter is zeroed right next to the wire and in exactly the same orientation before it is clamped on it will be reasonably accurate. Sure, you wouldn't bet your life that 15 mA is not really 30 mA, but that reading is definitely different from 500 mA. The one absolute exception to this is if there are other wires which change currents during the measurement. That would render the zero value moot and the reading entirely suspect. That isn't the case though on the ground lead from the 12V battery, as that is all by itself in a corner.
     
  2. ChapmanF

    ChapmanF Senior Member

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    Sure, if the clamp meter is the only tool you have, you'll find ways to get the best readings you can using it, and those tricks can help, as can flipping it around and averaging the (absolute values of the) two readings. But all of those 15 mA, 30 mA, and 500 mA figures are going to be well below the 3 amp accuracy limit in the manufacturer specs, which is a limit they don't have much incentive to exaggerate.

    Of course they give worst-case specs, and if you've had a particular meter for a while and checked it against some other references, you can gain some confidence that it is giving you better results than the worst-case specs imply.

    Still, if it's milliamps of current you've got questions about, putting a conventional ammeter in-line is how you'll know you've got the reading with no funny business.
     
  3. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Too bad the clamp meters can't achieve better accuracy, and come in at lower prices too. Maybe some outfit could design one, cheaper, and more suitable for this sort of check.
     
  4. ChapmanF

    ChapmanF Senior Member

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    AC clamp meters are easy. The current is constantly changing, so Faraday's law just does its thing, and you can hardly clamp a wire around another wire without having made an AC clamp meter that works.

    DC clamp meters are a different story. The time derivative of the current is more or less zero, so they can't rely on that, and have to depend on Hall effect sensors. I still think of it more as "pretty nifty to be able to buy those at all and have them work as well as they do."

    ... though like many other things, Hall effect IC's are off-the-shelf stuff now, and the car is full of them (built into the shift lever, accelerator pedal, and so on). But they don't need super accuracy to detect where R and N and D and B are.
     
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  5. rjparker

    rjparker Senior Member

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    Having a dc clamp meter that costs $35 and can get you within 500-1000 ma is amazing and is a good starting point on a parasitic draw test. If you see 1250 ma when the computers should be in sleep mode, you move to the fuse mv method (better) or straight to the series meter measurement (best).

    The dc clamp meter may not help much if the parasitic draw is 150 ma when it should be less than 35 ma. So it can tell you when the draw is really bad but is not conclusive if its slightly high. The inaccuracy can also make you believe the draw is within specs when its not. For other uses such as measuring battery charging current its great.



    Attached Pic for Mendel.
     

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    #25 rjparker, Nov 29, 2021
    Last edited: Nov 29, 2021
  6. pasadena_commut

    pasadena_commut Active Member

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    The UT210E DC ammeter (clamp) accuracy specs are not as bad as the 3A ChapmanF quotes. They say:

    Range Resolution Accuracy
    2.000A 1mA +-(2% + 8)
    20.00A 10mA +-(2% + 3)
    100.0A 100mA +-(2% + 3)

    So for a real 35 mA it will read in whole mA and be +- (.02*35 + 8). Let's assume it rounds .7 to 1, so that is +- 9 mA and would be 26mA to 44mA. Again though, this is only if no wire nearby has a change in DC current or starts producing an overt magnetic field (like near a solenoid or electromagnet) since the device was zeroed.

    Oddly the AC ammeter accuracy is slightly lower than this. It has the same Range/Resolution pairs but the accuracy is +-(4% + 10) for all ranges.
     
  7. rjparker

    rjparker Senior Member

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    As someone who owns one, I assure you its a waste of time at low ma. Does not work at all.
     
  8. ChapmanF

    ChapmanF Senior Member

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    The PDF that I linked in #9 is directly from www.uni-trend.com and does not seem to say that. But it was just a one-page spec sheet. Are these specs from a different doc I didn't find?
     
  9. pasadena_commut

    pasadena_commut Active Member

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    They are from the instruction sheet which came with the meter.

    When I first received the meter it was tested with just a battery, some resistors, and some wire to hook it together. The values measured were always "as expected" given that they were only10% and 20% resistors. In my hands this meter works acceptably. But, as I said above, it can only be used where there will not be stray magnetic fields which change once the device is zeroed, whether from current flowing through a nearby wire or a magnetic tool which has moved nearer to it. For those reasons I wouldn't trust its readings inside a computer case, or even most locations under a car's dash. It is useful though on the wires running to a car's door close switch or the 12V battery ground, because there is nothing else near those locations.

    Here is a lengthy review, with some measurements shown:

    https://www.eevblog.com/forum/testgear/a-look-at-the-uni-t-ut210e/
     
  10. mr_guy_mann

    mr_guy_mann Senior Member

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    Also have that model of Uni-T at my shop. I can zero it and get reading on a car, then move the meter a few inches along the same cable and the reading can change by 100mA. I use it to "triage" key off draw- is it over or under 1/2 an amp? (ie, do I trace the draw immediately or get my DVOM for a direct in-line measurement. I haven't found the Uni-T to be consistent at very low currents.

    Even my "good" clamp (Amprobe LH41A) gives me a 40-50mA spread.

    Posted via the PriusChat mobile app.
     
  11. pasadena_commut

    pasadena_commut Active Member

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    Moving the meter away from the location where it was zeroed is a no no. So is rotating it, except possibly around the axis through the center of the clamp loop. Setting the meter to zero nulls out all the magnetic fields, from all sources, but only in that one position and orientation. Ideally after setting it to zero one would move the test wire into the clamp region rather than moving the meter over to the wire. If the meter must move it should only be by the width of the clamp arm where it opens. That is, zero it with the test wire almost touching the place where the clamp opens, then open, move the meter the 5 mm or so to bring the wire in, close and measure.

    It is apparently possible to magnetize (and demagnetize) these meters, but so long as the zero button is used properly it normally does not matter. See:

    https://lygte-info.dk/info/DMMClampDemag%20UK.html
     
  12. pasadena_commut

    pasadena_commut Active Member

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    That could be within the accuracy range for that meter at the measured current.

    Use this web page:

    Magnetic fields of currents

    to see that a 10mA (0.01A) current measured at a radius of 5 cm (.05 m) is only .0004 Gauss, where the Earth's magnetic field is 0.5 Gauss. Iron and steel near the measurement site will modify the Earth's magnetic field, and these devices are sensitive enough to see that.
     
    #32 pasadena_commut, Nov 29, 2021
    Last edited: Nov 30, 2021
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