Multimeter current draw question

Discussion in 'General Electronics Chat' started by CrazyAl, Nov 25, 2010.

  1. CrazyAl

    Thread Starter New Member

    Aug 27, 2010
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    Hi, whilst testing the current draw of a small circuit I have put together with my multimeter I have come across something that is confusing me.

    My circuit is powered by 2 AA battery's so obviously there is not that much current available to begin with but just to be safe I firstly connected up my multimeter using the 10amp jack and it read about 0.480A.

    Then I gave it a try on the mA jack but this time it read out about 340mA.

    So my question is which reading is more likely to be the correct one? and does anyone know why my multimeter is doing this. I have not been measuring anything over the meters limits btw.

    Thanks!:D
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    AA batteries can dump several amps when short circuited with an ammeter.

    It appears your fuse didn't blow, so the batteries must be alkalines with a low charge.

    Had they been fully charged, your 400mA fuse would have blown.

    As far as the readings, higher ranges are for, well, higher ranges. Their accuracy on some meters isn't as good as it could be. Usually you should start with the highest anticipated range and work down to the one that is about 50%-75% max scale.
     
  4. CrazyAl

    Thread Starter New Member

    Aug 27, 2010
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    Thanks guys sorry I have not replied as I have been busy...again.

    @bertus Thanks for the simple and clear information, sounds about right.
    Which reading would you take as the "right" one?

    @thatoneguy correct, The batteries I am using are alkaline's out of some old disposable cameras as I tend to get through allot of batteries in my projects it would get expensive constantly buying new batteries, so I have an arrangement with a local supermarket where they give me old disposable cameras so I can use the batteries (I guess that counts as doing by bit for the environment too lol).

    Thanks again!:D
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
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    They are all "right" values, as long as you understand that the ammeter's shunt resistance is part of the circuit.

    There are two common ways of measuring current: 1) measuring the voltage drop across a resistor and 2) measuring the magnetic field caused by the current.

    The method using the magnetic field can be very convenient as in clamp-on ammeters that let you measure the current without breaking a wire in the circuit. But you can expect to pay $100 to $200 and on up for an instrument that measures both AC and DC currents. I like the old HP 428B for this, but few people have those. Here's a $170 device that can measure currents down to around 1 mA (I trust it to around 5 mA typically). I believe it was Electrician who posted a link to a nice $100 unit that plugs into your DMM; search some of the old posts to find it.

    The other method is to insert a known (low) resistance in the circuit and measure the voltage across this shunt resistor. The design factors in tension are to lower the resistance to decrease the burden on the circuit, but keep the resistance high enough so that you can make a reasonable voltage reading with your DMM. You can measure the resistance of your shunt resistors using a 1 A or 10 A DC current from a DC power supply; just measure the voltage drop across the resistor (make sure the voltmeter's contacts are closer to the shunt than the current connections). A typical DMM that resolves voltage to 0.1 mV lets you measure to 0.1 mΩ with a constant 1 A DC current. If you search on the web, you can find a number of 1 A current sources that will work. The LM317 datasheet shows a simple one that only needs a couple of resistors. You can design shunts with the Open Office spreadsheet in resistor.zip here. 14 gauge and 12 gauge solid copper wire (e.g., from a chunk of Romex) can make useful shunts.

    If you also want to use the shunt for line-frequency AC current measurements, fold the required length of wire in half and wind the pair of wires around a circular form. This folding/winding will reduce the inductance by about a factor of 10. A single coil of 48 turns of 12 gauge wire around a piece of 3/4" PVC pipe 6 inches long will have a 60 Hz inductive reactance of about 10% of the DC resistance unless you use this doubled-winding technique (I just went through an exercise of building such a coiled shunt with some 16 gauge wire and confirmed the inductance formula predictions with actual measurements, which is why the numbers are on the tip of my tongue... :p).
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you are simply testing to see if a battery is "good", put a 100 ohm resistor across the terminals and measure the voltage. If it is below 1.4V, the battery is weak/dead.

    Short circuiting to get max current actually sucks a lot of life out. This is why Alkaline batteries only last for around 20 pictures in some digital cameras, while NiCd battereis in the same camera last for 100. NiCd/NiMH can supply a much higher current than the alkaline chemistry, and recharging the flash is a big current to voltage exercise.
     
  7. CrazyAl

    Thread Starter New Member

    Aug 27, 2010
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    @someonesdad Thanks for that info and the link, I will keep it all in mind.

    @thatoneguy That is what I have basically been doing except with a multimeter and a battery tester . I guess I will just have to buy a good set of NiCd/NiMH rechargeable batteries.

    Thanks again:D
     
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