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Topic: Current sensing without IC's (Read 238 times) previous topic - next topic

michaelbend

Apr 19, 2017, 08:11 pm Last Edit: Apr 19, 2017, 08:17 pm by michaelbend
Hi guys,

I'm working on a school project. I'm asked to design a power monitor module for an arduino. Sensing voltage is pretty easy using the analog input of the arduino, the problem comes when I want to sense the current.

I am not allowed to use any IC's, so no ASC712 unfortunately. I don't think I'm allowed to use any other hall-effect based method, since making a hall-effect sensor myself is pretty hard, as well as those hall-effect sensors use an internal op amp (which is considdered an IC). So I need another sollution.

The max deviation is 20%, so I don't need it to be dead accurate, but some accuracy is nice :) The expected voltage will be between 3.3 and 20V and the current won't exceed about 2A.

Do you guys have any idea on how I can tackle this project?

Thanks! 8)

jremington

If it is direct current, use a resistor, the Arduino ADC and Ohm's Law.

Wawa

"power monitor" smells like mains power.
If so, an isolated sensor (current transformer of hall) has to be used for safety.

A low value (e.g. <1ohm) shunt resistor can be used for low voltage circuits.
Shunt voltage has to be within the supply range of the Arduino.
That could mean connecting one terminal of the shunt to Arduino ground, and the other side of the shunt via a 10k resistor (for pin protection) to an analogue input.
Only uni-directional currents can be measured this way, because the analogue input can only measure positive voltages.
Example in post#27 here.
http://forum.arduino.cc/index.php?topic=352403.15
Leo..

michaelbend

If it is direct current, use a resistor, the Arduino ADC and Ohm's Law.
Thanks for your reply, I have been thinking way to complicated. I believe this will work fine in my case. Thanks for thinking along :)

michaelbend

"power monitor" smells like mains power.
If so, an isolated sensor (current transformer of hall) has to be used for safety.

A low value (e.g. <1ohm) shunt resistor can be used for low voltage circuits.
Shunt voltage has to be within the supply range of the Arduino.
That could mean connecting one terminal of the shunt to Arduino ground, and the other side of the shunt via a 10k resistor (for pin protection) to an analogue input.
Only uni-directional currents can be measured this way, because the analogue input can only measure positive voltages.
Example in post#27 here.
http://forum.arduino.cc/index.php?topic=352403.15
Leo..
Luckily I'm not using mains :)

Your explanation is really helpful, the shunt resistor is simple yet effective. Great thinking on the current direction, I was thinking a putting the shunt on the negative side of my power supply, good thing you prevented me from messing that up :)

Wawa

I was thinking a putting the shunt on the negative side of my power supply.
Yes, that is possibe.
Look at that diagram again. The shunt is part of the load, not part of the supply.
If Arduino ground is connected to the power supply side of the shunt, then a positive voltage is generated across the shunt.
Leo..

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