INA226 - Power meter with low-side current sensing

Other Hardware Sensors
1

Hi,
I am working on power meter for battery powered application. Currents shouldn't exceed 20A. I was planning to use INA226 ready module with R004 shunt resistor. Unfortunately it can't handle more then 36V.

I read some threads on this forum and find out that for current sensing I would need to move INA226 to low side. What I am not sure about is: can I still use Vbus input to measure battery voltage, as long as I scale it down to acceptable level? I was thinking something like this:

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The voltage divider could probably by adjusted to scale down to 36 V, but regardless would that thing work? What should I take under consideration?

2

Hi,
I don't see a reason why this circuit won't work. Just make sure that you will multiply your current by 2 in your software. The most important thing for you to take into consideration is that you will place the shunt as close to the ground as possible. That way you get the most accurate measurment.

3

Thank you for speedy reply.

Cytat
Just make sure that you will multiply your current by 2 in your software.

You meant power (or battery voltage), not current? The voltage drop on shunt is measured directly.

4

A shunt measures a voltage drop over a certain wire with a fixed resistance. Because you drop the voltage by half, you will need to multiply the measured voltage by 2 to make sure that voltage divider is accounted for. You can also multiply the current by 2 ( Ix2 = Vx2xR).

5

Wait, I'm stupid. You only have a voltage divider at the Vbus of the INA226. You only have to multiply the voltage by two to get the correct voltage.

1 Like
6

Here you got me lost.

Voltage drop on shunt doesn't go thru voltage divider. As shown above both ends of shunt are hooked up directly to INA226 module. So the current reading shouldn't require any software adjustments. It's only Vbus that needs to bee multiplied by 2, to calculate power.

Am I missing something?

7

The problem with low side is that your load is no longer connected to system ground.

So the "ground" potential of your 20A load can now be up 80mV higher than other devices connected to the GND in your schematic.
This may or may not be a problem but should be taken into consideration when designing the rest of your circuit.

3 Likes
8

That is an important issue that you've rised.

I could place shunt at the lowest point just before battery minus (-) terminal, and then for everything else use shunt positive side as "apparent" ground. Would that be best practice?

I do have other stuff in the system, like MCU. MCU and INA226 will both be powered by Vbat -> 5VDC buck converter. I think this buck should be 5V relative to "apparent GND", and then only INA will be grounded below shunt, thus getting 5V+80mV.

9

For future reference: I did hooked it up like in schematic (post #1). Haven't tested it yet with target load, but for small currents it seems to work flawlessly.

10

I wonder why that was recommended. The INA226 is designed for both high and low side current sensing.

Capture

Certainly the high side voltage cannot exceed 36V.

11

I may have not been clear enough in first post. My application requires maximum 42V on bus thus my interest in low side sensing.

12

Hi Guys. Which library do you use with the INA226?

13

The library manager of the IDE has a selection. Type INA226 in the top field to see them.
Click on "more info" to see the examples and how recent the library was updated.
Leo..

14

I use INA226_WE.h by Wolfgang Ewald with success

15

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