measuring very small current

Discussion in 'The Projects Forum' started by umer27, Mar 10, 2009.

  1. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    Hay all , Im using a 0.1 Ohms @ 3 watts (wire wound resistor) as a shunt for measuring current of a solar panel...

    Its a 10W solar panel.. , Isc=0.7 A and I hope to accurately determine the current as accurately as possible..

    Im using an ATmega8 AVR controller , with a 10 bit ADC on it..


    As, V=IR,
    in my case...

    Imax= 1 A (its ~700mA but lets say for simplicity)

    Voltage = 0.1 x 0.7 =0.07 V

    So, using an op-amp with a gain of 71, gives me .... 0.07 x 71 = 4.97V

    I think I would have problems with differentiating such a small signal as lets say I wanted to measure a difference in current of say 5 mA ... I will use an instrumentation amp for better results ... but ..

    the question is should I use a larger resistor, so I can measure the current more accurately .. (I know losses would increase but I am willing to trade that for higher current accuracy..)

    has anyone ever measured differences in current as small as 1-5 mA ?
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Use a teeny-tiny meter. :D
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    In all seriousness, that can be done with a simple passive analog meter. In fact, microammeters are readily available.

    eric
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A larger value resistor will give more accurate results because its resistance its not close to the wires resistance and the wires won't affect the measurement significantly. Also, a bigger voltage will be dropped and it will be less affected by noise.
     
  5. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    how large are we talking here... Im thinking 1 or 2 Ohms should be enough..
     
  6. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    Loss at 1 Ohms..

    P= square(0.7) * 1 = 0.7 Watts

    @ 2 Ohms.. its 1.4 Watts.. not bad, since I have to measure the current for a very small amount of time..
     
  7. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    and If I do this, can I measure current differences as small as 1 mA ?
     
  8. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    PS, im using the current value in my microcontroller.. so any teeny tiny meters wont do..
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If you set the reference voltage of the ADC properly you will be able to detect voltages of less than 1mV.
     
  10. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    set the voltage reference properly ?
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Stick with the original resistor.

    1mA = 0.001A

    0.001A*.7Ω = .0007V

    .0007V x 71 = .049V = 49milliVolts

    If your ADC is only 8 bit, and your reference voltage is 5v, you can pick out steps of 19 or 20 mV. That's a little more than twice the resolution you are asking for. If your ADC has 10 bit resolution, all the better.
     
  12. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0
    No, I want to be able to measure currents from 0 to 1 Amp with a resolution of 1-2mA ...

    My initial resistor is/was 0.1 Ohm.. 3 Watts (wire wound) ...
     
  13. umer27

    Thread Starter Member

    Feb 28, 2009
    25
    0


    so i guess... initially I had thought.. of a 0.1 ohm resistor in my 1st post..
    1mA=0.001 A
    0.001A * 0.1 Ohm = 0.0001V
    0.0001V x 71 = 0.0071V or 7.1mV ... thats inviting noise to destroy my readings..

    I think the resistor should be in the range of 0.8 to 1 Ohm..
     
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