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Differential Current Acquisition

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Hello,

I'm working on a project where I must measure the AC current of a low consumption lamp (~120mA) and send it via RF using a MCU. The point is:

I have found different methods to measure AC current and there's one of them which I do not understand. It is the method used by the kill-a-watt units:

https://reindeerflotilla.files.wordpress.com/2011/08/kill-a-watt.png

Could anyone explain how it is working? Please shed light on the matter:)

Thank you in advance! :thumbsup:

P.S: Before this method I tried the well-known ACS723 but the values obtained were wrong compared to an ammeter:(
 

Hi,
I tried the well-known ACS723 but the values obtained were wrong
Click to expand...
What do you mean with "wrong values"? Do you think they are off specification?

Maybe they are not useful for your low current...

****
your circuit:
* there is a shunt. Voltage drop is proportional to current.
* and there is a difference amplifier. It amplifies the voltage across the shunt and adds an offset voltage. That´s all.
***

But maybe you should start to specify your needs.

Klaus
 

The offset of the ACS723 is 20mV with a sensitivity of 400mV/A giving an offset current error of 50mA.
So it's not surprising that it was not accurate for your 120mA measurement.

You need a sensor with more sensitivity.
 

Thank you for your support:)

The fact is that I have to figure out a way to measure low currents for AC and I do not find any solution except the allegro series...

Any idea? Could you please support me on how to design such a kind of circuit with a shunt?

Thank you in advance.
 

If the lamp is powered from the AC mains you need isolation.
For that you could use a current transformer.
The current shunt is placed on the isolated output of the transformer and one end can be grounded, so it doesn't need differential amplification.
A Google of "small current transformer" will give you many hits.
 
Last edited:

Could you please describe how would you measure with the current transformer + shunt without differential amplification? I've never tried to measure negative voltages with the ADC of a MCU so I do not know what would happen....

Assuming in that case the ADC returns 0 (Vref-) I guess the solution could be monitor for instance...500 readings and then calculate the average and check it with the current transformer datasheet to calculate the current? What do you think...Am I right?

Again, thank you for your valuable support.
 

Two ways to avoid negative voltage from a current transformer output (which is generated across a burden resistor):

1) Rectify the AC into DC with an op amp precision rectifier circuit.
Measure the DC voltage with the ADC

2) AC couple the voltage through a capacitor with a DC offset at the output equal to 1/2 the ADC input range.
Measure the offset AC voltage with the ADC.
 

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