EMON Current Only Incorrect Power Readings at Idle

Good Morning All!

Firstly thank you to OEM team for supplying this project, its been great working through it.

I have built the currently only circuit using an Arduino Nano and transmitting the data using XBEE (although the wirless medium is not important). Within the circuit i am using two 10K resistors as a potential devider and a 33 burden resistor and i am using this current sensor https://www.sparkfun.com/products/11005.

It seems to be responding correctly when appliances are plugged it, however at idle it is throwing out 40-50 watts constantly which is obviously incorrect. I appreciate that its not taking a reference voltage but that surely wont cause such bad accuracy.

Any help on the matter would be really appreciated as im sure its something simple!


Robert Wall's picture

Re: EMON Current Only Incorrect Power Readings at Idle

There are two possible sources of your unwanted current. One is digital noise coming from the processor itself, the other is voltage pick-up from the cable the c.t. is clipped around. The first you can probably do little about, the second can be reduced if not removed completely by clipping onto the neutral rather than the line cable.

The problem is, the software rectifies the voltage from the burden resistor and any input at whatever frequency appears as a current. I've no idea how you have calibrated your input, so it's hard to judge whether you have a lot of pick-up or a normal amount of pick-up but a lot of amplification in the software. At the risk of overloading your c.t. (and hence introducing distortion and different inaccuracies), you could try increasing the value of the burden resistor and compensating for the increased voltage in the software. As it stands, you will only get fractionally more than 1/4 of the full input range if your analogue reference is 5 V.

It has been suggested that a 2n2 capacitor from ground to the analogue input pin will 'cure' the noise, but I have not tried this.

Dav3's picture

Re: EMON Current Only Incorrect Power Readings at Idle


Thank you for your response.

I have just changed the CT from the live to the neutral line, this made no difference so i can eliminate that.

I will increase my burden resistor but where about's would i make calibrations to the software as i have not done this yet.

Many Thanks

Robert Wall's picture

Re: EMON Current Only Incorrect Power Readings at Idle

calypso_rae's picture

Re: EMON Current Only Incorrect Power Readings at Idle

How about running my MinMaxAndRangeChecker tool to see what ADC values you're getting? 

Another constructor recently posted values which showed at most a 1-level movement in the ADC values for his current sensor when idle.  That's nothing like 40 - 50 Watts.

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