Multimeter with a low resistance when measuring amps

Discussion in 'General Electronics Chat' started by jarl, Feb 18, 2012.

  1. jarl

    Thread Starter Member

    May 16, 2009
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    My current DMM (a $10 ebay special) has a too high internal resistance when measuring in the 10A DC setting. I do a lot of work with battery powered LED lights, so even a small internal resistance will have a BIG effect on current draw. (today I built a very bright light and wanted to check how much current it was drawing- rigged up my multimeter and it stopped being very bright and drew 1.5A- I suspect without multimeter it drew over 3A but have no way to check)

    Can anyone recommend a <$50 multimeter that will accurately measure high currents on low voltage sources without affecting the system being measured?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You do not need a special ammeter. Build it yourself using a low-resistance shunt.
    10A into a 0.01-ohm shunt will give you 100mV which you can read on a decent voltmeter.
     
  3. jarl

    Thread Starter Member

    May 16, 2009
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    What a fantastically simple idea! Thanks :)
     
  4. Lundwall_Paul

    Member

    Oct 18, 2011
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    If your meter cannot measure amps I would suggest checking the fuse. The meter specs of 10meg/volt or 20k/volt have nothing to do with measuring current. Mr. Chips is right you can use a shunt but only if you trust the meter. I would try a different meter to see if you get the same results. Sears has sells Fluke meters at a reasonable price.
     
  5. jarl

    Thread Starter Member

    May 16, 2009
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    It does measure amps, but has a relatively high internal resistance, so when I'm measuring the current draw of an ultra high power LED the current drawn by the LED drops significantly during measurement (which i can see by decreasing brightness)
     
  6. CraigHB

    Member

    Aug 12, 2011
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    All meters have some amount of "burden voltage" which results from the added impedance of putting the meter in-line. I have a high end meter (Extech MM570A) and it has a burden voltage of 45mV/A at its highest range. A measurement at 10A would result in at least a .45V drop in voltage, probably more with the added impedance of the test leads.

    I've used a shunt in the past as mentioned. With a good voltmeter, you'll get an accurate current measurement. Calibrate your shunt by first measuring its voltage drop with a known current. For example, my DC power supply has a fairly accurate current indicator. I can use that to get a precise resistance measurement. I just take a voltage reading across the shunt at a current set by my power supply.
     
  7. studiot

    AAC Fanatic!

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  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I just bought an MS2102 clamp-on AC-DC ammeter, but the low range is 40A, and the resolution is 10mA. Accuracy on the 40A range is ±2.5% ±5 digits, so on a 3 Amp measurement, it could be off by as much as ±80mA.
    I bought it primarily for automotive applications, so this will work fine for me. I chose a clamp-on because (1) it has no burden voltage, and (2) you don't have to break a circuit to make a measurement if a free wire is available.

    It also has voltage and resistance measuring capabilities, using the included probes.
    EDIT: The one I bought is from Sinometer. I believe Mastech is a subsidiary, or vice-versa.
     
  9. MrChips

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  10. CraigHB

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    Aug 12, 2011
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    The clamp-on ammeters certainly have their place and they can be much easier to use, but access can go the other way too. Sometimes you can put a meter in-line where you can't isolate a wire for a clamp-on. It can also be a matter accuracy. At the 10A range, my meter has .5% plus or minus 20 counts. Reslolution in the 10A range is .1mA. That would be an error of 17mA at 3A.
     
  11. jarl

    Thread Starter Member

    May 16, 2009
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    That's assuming current doesn't change with circuit resistance. In this case, with a 3.7V battery and I'm guessing 3+A of current, the resistance of a multimeter will have a huge affect.

    I've got a 5 pack of 3W 0.01 ohm resistors on the way- one should give a voltage drop of 0.05v at 5A. Power drop over the resistor at 10A should be 1W unless I've got my sums very wrong ;)

    Ron: I would consider a clamp on meter, but they're expensive and 0.01 ohm resistors are very cheap! ;)

    Craig: Thanks a lot for those numbers- that tells me that the shunt is probably the best way to achieve what I want as I should only be dropping 10mV/A with the shunt resistor, with the option to put another in series or parallel if required.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    If your circuit is that sensitive to the shunt resistance of the meter, then it likely is also sensitive to battery voltage, and will thus dim noticeably as the battery becomes discharged. Perhaps you need a design less sensitive to battery voltage.;)
     
  13. jarl

    Thread Starter Member

    May 16, 2009
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    It actually works out quite well- lithium ion batteries have a fairly flat discharge curve, so when it starts to dim I know it's time to change the battery as I'm at about 20% capacity :)

    [​IMG]
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    To the OP, I would suggest that your problem is not your meter but your meter leads. The meters have internal shunts which are basically just a short length of heavy wire. By comparison, your leads are long lengths of very small wire. Make yourself some heavier gauge test leads for measuring current and your problem will probably be solved.
     
  15. Ron H

    AAC Fanatic!

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    Good idea. Also make them as short as you can tolerate.
     
  16. jarl

    Thread Starter Member

    May 16, 2009
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    Parts arrived today. 5.5A with shunt, 1.9A with DMM.

    at 0.05A, the shunt agrees with the DMM, so I'm happy enough with the accuracy.

    Thanks all :)
     
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