# measuring very small current

#### umer27

Joined Feb 28, 2009
25
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 ?

#### KL7AJ

Joined Nov 4, 2008
2,229
Use a teeny-tiny meter.

#### KL7AJ

Joined Nov 4, 2008
2,229
In all seriousness, that can be done with a simple passive analog meter. In fact, microammeters are readily available.

eric

#### mik3

Joined Feb 4, 2008
4,843
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.

#### umer27

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

#### umer27

Joined Feb 28, 2009
25
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..

#### umer27

Joined Feb 28, 2009
25
and If I do this, can I measure current differences as small as 1 mA ?

#### umer27

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

#### mik3

Joined Feb 4, 2008
4,843
If you set the reference voltage of the ADC properly you will be able to detect voltages of less than 1mV.

#### umer27

Joined Feb 28, 2009
25
set the voltage reference properly ?

#### thingmaker3

Joined May 16, 2005
5,083
and If I do this, can I measure current differences as small as 1 mA ?
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.

#### umer27

Joined Feb 28, 2009
25
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) ...

#### umer27

Joined Feb 28, 2009
25
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.

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..