# Choosing the ideal current shunt value

Joined Apr 15, 2009
133
Good day all.

I am looking at doing some voltage and current monitoring with my Arduino Uno.

The voltage range is 0-15vdc, and 0-5amp.

I know I can monitor the voltage by just using simple voltage divider into the Arduino ADC port.

Regarding monitoring the current I know I can buy current/voltage monitoring ic's that I can interface with the Arduino, but I would like to keep away from that.

So what I would like to know is how do I go about sizing the right current shunt resistor vavlue for a 0-5amp load, so that I can use the Arduino adc port.

Looking in my used parts bin, I have found a bunch of "DALE LVR-3 0R01 1% 3W" Resistors.

Would this be suitable?

Best Regards.

#### mcgyvr

Joined Oct 15, 2009
5,394
Typical current shunts give 0-50mv or 0-100mv full scale output.

a .01 ohm 3W
.01 x 5^2 =.25W
That will work just fine

5 x .01 = 50mV

#### crutschow

Joined Mar 14, 2008
26,053
You likely will want to amplify the 50mV to a higher voltage to get reasonable resolution in the Arduino.

And if you can't use the shunt in the ground line then you will need a differential amp that can work at the supply rail to do the amplification.

Joined Apr 15, 2009
133

I am indeed looking to use the current shunt on the ground side.

As I have not used op-amps before, can someone recommended me a suitable op-amp to convert 0-50mv to 0-5v.

Or a link to where I can find the required information.

Thanks again.

Best Regards.

#### crutschow

Joined Mar 14, 2008
26,053
You can use the non-inverting op amp circuit shown here. If you only have a single power supply then you will need to use a single-supply op amp such as an LM358.

What is the supply voltage(s) you have to power the op amp?

Edit: The LM358 and many common op amps have a typical input offset of several mV which would be a significant portion of your signal. If that's a problem for you, then you would need a low-offset rail-to-rail op amp.

Last edited:

Joined Apr 15, 2009
133

My circuit has a +12vdc input, then a 7805 to supply +5vdc for the microcontroller.

I am just reading the Operational Amplifiers section of the allaboutcircuits website.

#### crutschow

Joined Mar 14, 2008
26,053

My circuit has a +12vdc input, then a 7805 to supply +5vdc for the microcontroller.

...................
Is the 12V regulated or from a rectifier and filter?

Joined Apr 15, 2009
133
I am using a mains power adaptor with a fixed 12vdc regulated output.

#### crutschow

Joined Mar 14, 2008
26,053
I am using a mains power adaptor with a fixed 12vdc regulated output.
So you could use that or the 5V to power the op amp, depending upon the amp limits.