How to properly measure currents with hall sensors 1

[SolarTrap]

Hello All,

I am using a hall sensors (ACS756, 24VDC, 50A bi-directional, link) to build a battery gauge. The problem is the 'zero point' of the sensor. The sensor is fed by 5V and by the specs sheet it should deliver 2.5V if no current is flowing. I am using the analog input of an ATmeaga2560 to measure that voltage. This is my code:

float currBattAmps(float raw)
{
    return ((V_ARDUINO*(raw -zeroAbOffset)/1023.)/ -.04);  // 0.04 for 50A
}

V_ARDUINO = 5.0 and the zeroAbOffset is a value that I have measured when I 'calibrate' the sensor. I take 10000 measurements when no current is flowing and average them.
This comes out between 511-509 and seems to fluctuate for unknown reasons. It also seems to be temperature dependent. Since the minimal resolution of the sensor (in combination with 10 bits A/D) is +/- 0.122A this means quite a big difference and could throw SOC by a lot.

Any experience with this?

Markus

 

[roydm]

Browsing around the web I came across a Flux Gate transducer for DC Current.
It seems to me that with several turns through the primary it would be quite sensitive.

 

[SolarTrap]

It has been almost 6 months now and I ended up using a shunt to measure current instead of the hall sensor. I re-designed the entire board and have attached a pic of the current PCB layout for reference.
I am having a Kickstarter campaign running since Monday now so wish me luck to get this project off the ground

Markus

 

[itsmoked]

Thanks for the update!
You have what looks like a well thought out layout there. I see you went with a Maxim shunt amp.
I assume it did the the trick for you.
Good luck on your KStarter project.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[SolarTrap]

Might look like it but it's actually a TI:

http://www.digikey.com/product-detail/en/INA286AID/296-38960-5-ND/3687712

It is more temperature stable and less noisy

 

[itsmoked]

Well.. I believe Maxim was the first, Ti essentially copied it but I take your point.

Keith Cress
kcress -

http://www.flaminsystems.com