# Measuring small currents...

Discussion in 'Embedded Systems and Microcontrollers' started by umer27, Mar 10, 2009.

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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. ### davebee Well-Known Member

Oct 22, 2008
539
46
10 bits resolves one part in 1024, so if you amplify your sensing voltage to where the maximum signal reaches the top of the ADC range, you're right on the borderline of being able to resolve 1 milliamp on a 1 amp signal, and even that 1 milliAmp measurement is likely to contain some ADC conversion noise.

You could amplify the signal and get more resolution but then you could only get that over part of the full range of signal current. What you really need is more ADC resolution.

Have you considered an alternate way to do it?

You could use an external ADC like the TC3405. It converts to 16 bits, has 4 channels, one channel is differential, which is just what you could use across the shunt resistor, has a serial interface so it doesn't take too many pins, and it only costs about \$5.

It should easily get you the precision you're looking for over the entire range of current, without needing an op-amp and its supporting power supplies and circuitry.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
This is duplicated in the Projects Forum, so I am closing this thread.