measuring very small current

Thread Starter

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
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.
 

Thread Starter

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..
 

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.
 

thingmaker3

Joined May 16, 2005
5,084
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.
 

Thread Starter

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) ...
 

Thread Starter

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..
 
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