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Thread: how to measure amps on multimeter

  1. #1 how to measure amps on multimeter 
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    OK - embarrassing as it is I need to confirm this - so my multimeter is set to 20m to read AMPS and the reading is 0.07 - is this 7mA?

    How do I interpret the meaning of 20m - does that mean it will go to mA or A of 2 decimal places?


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    Brassica oleracea Strange's Avatar
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    Without being familiar with your meter, I would assume 20m means 20mA. And I would further assume that if it was measuring 20mA on that setting, it would display 20. Which suggests that 0.07 means 0.07mA (70 uA). Does it have a 200 uA scale?


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    Quote Originally Posted by Strange View Post
    Without being familiar with your meter, I would assume 20m means 20mA. And I would further assume that if it was measuring 20mA on that setting, it would display 20. Which suggests that 0.07 means 0.07mA (70 uA). Does it have a 200 uA scale?
    Thanks for the reply - yes it does, it has (in order) 200u, 2000u, 20m and 200m...

    How does it work then, surely if it was measuring 20mA then it would max out on 20.00 and 7mA would be 7.0, no? Or even measuring 20mA would mean that 1.0 is one lot of 20mA (can you see the logic in that one?)...?
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    Quote Originally Posted by fatman57 View Post
    Thanks for the reply - yes it does, it has (in order) 200u, 2000u, 20m and 200m...

    How does it work then, surely if it was measuring 20mA then it would max out on 20.00 and 7mA would be 7.0, no? Or even measuring 20mA would mean that 1.0 is one lot of 20mA (can you see the logic in that one?)...?
    I imagine on the 20mA setting, it actually maxes out at 19.99 - this allows them to use a single segment display for the leading digit but still go up to 2 (almost). The ranges will just set the max that can be displayed, but the display will still show the actual current measured (within that range).

    Scaling it to mean that 1.0 was 20mA would be too much mental arithmetic for most engineers!

    So, if you set to to 200u then it should read about 70.0 for the same current.

    One approach I used to take was to set the meter to the least sensitive setting (2A or whatever) and then gradually turn it down until you got the most accurate reading. For example, on 20m it will read 0.07, but on 200u it could display the current more accurately, which could be anywhere between 65.0 and 74.9.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by fatman57 View Post
    Thanks for the reply - yes it does, it has (in order) 200u, 2000u, 20m and 200m...

    How does it work then, surely if it was measuring 20mA then it would max out on 20.00 and 7mA would be 7.0, no? Or even measuring 20mA would mean that 1.0 is one lot of 20mA (can you see the logic in that one?)...?
    I imagine on the 20mA setting, it actually maxes out at 19.99 - this allows them to use a single segment display for the leading digit but still go up to 2 (almost). The ranges will just set the max that can be displayed, but the display will still show the actual current measured (within that range).

    Scaling it to mean that 1.0 was 20mA would be too much mental arithmetic for most engineers!

    So, if you set to to 200u then it should read about 70.0 for the same current.

    One approach I used to take was to set the meter to the least sensitive setting (2A or whatever) and then gradually turn it down until you got the most accurate reading. For example, on 20m it will read 0.07, but on 200u it could display the current more accurately, which could be anywhere between 65.0 and 74.9.
    Thanks! I had to make sure as I just got an arduino and am measuring the consumption of a 12v 120mm fan...@ 5V which is what the arduino puts out it seems to be consuming 7mA at both free spin and stall...I wonder if the microprocessor is doing that...?

    One more thing though (after thought) - if it is set to 20m would not 7mA be 7.0 rather than 0.07? It made sense when you said it was putting out the correct value, so would the max on 20m be 0.2?
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    Quote Originally Posted by fatman57 View Post
    One more thing though (after thought) - if it is set to 20m would not 7mA be 7.0 rather than 0.07?
    That is why I said it was probably 0.07mA (70 uA) (Which doesn't seems unreasonable for standby/leakage current...)

    It made sense when you said it was putting out the correct value, so would the max on 20m be 0.2?
    No, the max on 20m is almost certainly 19.99mA.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by fatman57 View Post
    One more thing though (after thought) - if it is set to 20m would not 7mA be 7.0 rather than 0.07?
    That is why I said it was probably 0.07mA (70 uA) (Which doesn't seems unreasonable for standby/leakage current...)

    It made sense when you said it was putting out the correct value, so would the max on 20m be 0.2?
    No, the max on 20m is almost certainly 19.99mA.
    Thanks - 0.07 was actually running by the way, and same reading at stall...!
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    I'm confused. I am not familiar with the Arduino ... does it not have separate 12V and 5V supplies?

    Maybe you could look up data on the fan and see how much current it is supposed to draw (probably not much). And it might be the roughly same if it is stalled (rather than disconnected) - depends on the type of motor.
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    Quote Originally Posted by Strange View Post
    I'm confused. I am not familiar with the Arduino ... does it not have separate 12V and 5V supplies?

    Maybe you could look up data on the fan and see how much current it is supposed to draw (probably not much). And it might be the roughly same if it is stalled (rather than disconnected) - depends on the type of motor.
    Arduino puts out 5V max but allows 12V as power supply - on the same multimeter using a 12V power supply fan reads 0.3 on 20m which is the same as written on the rear of the fan itself (12V 0.30A)...I thought it strange too...
    Last edited by fatman57; March 1st, 2013 at 03:12 PM.
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    So the 12V supply will be driving the fan. I assume a 5V supply for the electronics is derived from that. So if you measure the 5V supply it will not include the current for the fan (unless I have misunderstood how it works - maybe I should go and look up a schematic online...)
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    Quote Originally Posted by Strange View Post
    So the 12V supply will be driving the fan. I assume a 5V supply for the electronics is derived from that. So if you measure the 5V supply it will not include the current for the fan (unless I have misunderstood how it works - maybe I should go and look up a schematic online...)
    No sorry...the 12V was an independent test using a separate power supply. The 7mA was with the same fan driven directly from the Arduino @ 5V 100% PWM duty cycle.
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