Arduino CT Sensor low voltage

Hello,

I got an arduino non-invasive current sensor that outputs voltage instead of current, Measuring the voltage gives 1mVAC and up.... I want to be able to read this signal from the arduino however it is too small. I tried building an inverting amplifier, but I couldnt know how to flip the output voltage into a positive voltage. Moreover, my output voltage is 1m*100 (gain of op-amp) = 0.1V. Am I able to read it from the arduino.

Do you recommend a simpler method for this sensor input ?

Note : I have UA741 op-amp and can't use any other op-amp.

Sensor Datasheet : http://garden.seeedstudio.com/images/b/bc/SCT013-030V.pdf

Thank you!

That is a classic simple current transformer. Before you amplify and wire to an arduino analog input pin you first must convert the AC output voltage to DC voltage (with a rectifier diode and filter capacitor). There is no real need to add amplification as you can simply add a analogReference() command to enable the 1.1vdc ADC reference, and then you can read the voltage directly. You will have to adjust in software from peak current to rms current as the (take reading and multiply by .707) as the diode/ filter charges up to peak value.

The 1N4001 Diode for rectification is good for this small signal ? Also, analogRefrence(type) takes a parameter... What should this parameter be in my case ?

Actually my suggestion to use a simple diode rectifier for such a small voltage range was incorrect as standard diodes have a forward voltage drop of around .6vdc so you will lose most of your measurement range. You are going to have to research op-amp circuits for 'precision diode' application so you can measure all the way down to 0 volts DC.

As far as the parameter to use for the analogReference command, it depends on the specific arduino board type you have. But the answer is in here:

According to the data sheet, the CT outputs 1 VAC when 30 amperes AC is flowing in the primary circuit. The peak voltage will be 1.4 times that, so 2.8 Vpeak-to-peak at 30 amps. A 5V Arduino can easily measure this output voltage range if it is offset by 2.5 V, as described in this link: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface

If you want to use that sensor with much smaller primary currents then you will need a precision rectifier. I’ve attached an example that I used with a CT60 from CoilCraft. It has a gain of about 22, so you would probably want to reduce that. The gain is set by the ratio of R2/R1, so reduce R2 to reduce the gain.

The 741 op amp is an ancient design and not very well suited for your purpose. It would need about an 18V ( +/- 9 V) power source to have the full swing of 0-5V on its output. You can get free samples of modern op amps from many distributors. Choose a rail-to-rail single supply op amp.

current_sense_amp.png