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### Topic: Problem Interfacing with shunt resistor current measurement circuit (Read 963 times)previous topic - next topic

#### Tokis

##### Sep 05, 2017, 10:46 amLast Edit: Sep 05, 2017, 11:56 am by Tokis Reason: Edit: added simple diagram of actual circuit
Good day all

I am trying to implement current measurement using the shunt resistor method. I have a device that's powered with 3V and draws 30mA while in normal operation. I'm trying to monitor that current with an Arduino Nano. I'm using a low-side setup like the left hand circuit in the diagram attached and a 1 Ohm resistor to generate a 30mV drop across it.

When I measure the voltage across the resistor with my meter I get the expected 30mV. But as soon as I couple the ground from the circuit to the arduino ground, the measurement drops to 6mV.

I'm not sure whats causing this. Could it be that the arduino ground is somehow offsetting the circuit?
I've not been able to find much info on this and am quite stumped at the moment. I may try using a voltage follower to provide isolation.

I appreciate it if anyone can provide an explanation or point out some obvious thing I may be missing.

Thanks

#### jackrae

#1
##### Sep 05, 2017, 10:55 am
Provide a sketch of how you actually have the components connected - otherwise we'd just be guessing

#### Tokis

#2
##### Sep 05, 2017, 11:57 am
Provide a sketch of how you actually have the components connected - otherwise we'd just be guessing
I'v added a simple diagram of how it's connected. The arduino is powered via USB

#### MarkT

#3
##### Sep 05, 2017, 12:08 pm
Good day all

I am trying to implement current measurement using the shunt resistor method. I have a device that's powered with 3V and draws 30mA while in normal operation. I'm trying to monitor that current with an Arduino Nano. I'm using a low-side setup like the left hand circuit in the diagram attached and a 1 Ohm resistor to generate a 30mV drop across it.

When I measure the voltage across the resistor with my meter I get the expected 30mV. But as soon as I couple the ground from the circuit to the arduino ground, the measurement drops to 6mV.

I'm not sure whats causing this. Could it be that the arduino ground is somehow offsetting the circuit?
I've not been able to find much info on this and am quite stumped at the moment. I may try using a voltage follower to provide isolation.
You need to measure only the voltage across the shunt.  That means using a 4-terminal method - the two
sense wires from the shunt carry no current (ie you cannot use the ground lead from the Arduino as it
carries current, and thus drops voltage unrelated to your measurement).

In this case one way is to run ground from power supply to Arduino, then from Arduino to the shunt.  Run a
second wire from the low side of the shunt to another analog input.  Measure the difference between the
two analog pins.

The ground is routed this way to ensure the shunt voltages are not below the Arduino ground voltage, since

Another approach is to use an opamp in a differential amp configuration - that will measure difference and
amplify as well, which is useful.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### Tokis

#4
##### Sep 05, 2017, 03:36 pm
Appreciate the response. Thanks for clarifying that. I'll hook up a single supply for both the arduino and the device and try your first suggestion. I'm not too concerned with accuracy but if needed I'll go the op-amp route.

#### Wawa

#5
##### Sep 06, 2017, 01:47 am
30mA through a 1ohm shunt is 30mV across the shunt.
Default resolution of the A/D of a 5volt Arduino is ~5mV (5volt / 1024).
5mV steps can be lowered to 1mV steps by enabling 1.1volt Aref.
Leo..

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