Martin

Hall Effect Current sensor / transducer

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I have attached am ACS712 Hall Effect current sensor to my T7 via GND / AIN2 powered from the 5 volt VS supply on the T7. 

It is rated as a 30 Amp device (there are 5 Amp and 20 Amp) devices available. It can measure both DC and AC but I am only interested in the DC factor.

It has an output of 2.5V with zero current flowing through the sensor. Its minimum voltage out will be 0 Volts at -30 Amps and its maximum voltage out at 30 Amp will be +5 volts.

How do I map this so that I can read the full range of -30 Amps to +30 Amps into DF.

Kipling can see the 2.5 Volt output from the sensor and I can use DF to pull that through to the screen in DF but I need DF to convert or map the 0 - 5 volt swing  (maximum) through to -30 to +30 Amps.

I should imagine that this would be a vary useful piece of information to many DF / T7 users.

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Assuming you are seeing the proper voltage swing, to convert from Volts to Amps you would use a Conversion.  Click on CONVERSIONS: in the Workspace, then Add a new conversion.  Give it a name, like "HallEffectCurrent" or something.  The Expression in your case would be:

Value / 5 * 60  - 30

Of course you can reduce that if you want, but showing it unreduced makes it more obvious what was done.  So Value/5 takes 0-5 and converts it to 0-1.  * 60 converts it to 0-60 (amps), then the -30 converts it to -30 to +30 amps.

Finally, go to your input channel and select the new Conversion and Apply.

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I took a look at the ACS712 and have 2 comments:

1.  The nominal offset is half the supply voltage.  If the supply voltage is exactly 5.0, then the offset will be 2.5, but if you are using VS to power the sensor you should read back the actual value of VS in real time and use that in your math rather than assuming 5.0.

2.  The nominal sensitivity is 66 mV/A, which means a swing of +/-1.98 volts, not +/-2.5 volts.

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Oh wow... Thanks very much to both comments and posters above.

I've just implemented the suggestions and apart from fiddling with the offset it worked spot-on.

I played with setting the offset via Kipling and within the conversion as suggested. Both worked perfectly.

Next in my list is to get the entire configuration into a start-up routine that is passed to the T7 rather than having dedicated local T7 settings.

Another excellent lesson learned.

Martin

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Hi LabjackSupport...

That bit about the 0.066V / Amp gradient was interesting.

I took your hint and did my own calibration run stepping from 0 - 30 Amps. The hall effect sensor averaged out at 0.0622V / Amp.

So of course the maximum swing actually works out to be 60 x 0.0622 = 3.72 volts not 3.96 volts as the specification sheet suggests.

Once this was plugged into the calculation the actual current monitoring turned out to be pretty tight.

Lesson learned... never trust a spec sheet.

Thanks again.

Regards - Martin

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