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I'm trying to read values from a SCT-013-030 sensor that gives 0-1vac readings . My board analog inputs are 0-10vdc,giving me a value from 0 to 65500 . I need to know what is the best method of amplify this signal to be more accurate possible.

I read something about SCT sensors on Openenergy and Arduino (0-5v inputs) but i don't understand circuit..

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  • \$\begingroup\$ Even if you don't do anything, you get an ADC range of 0 to 6553, better than 12 bit resolution. What resolution do you really need? And have you thought about the difference between accuracy (what you asked for) and precision (aka resolution)? \$\endgroup\$ – The Photon Nov 5 '12 at 18:12
  • \$\begingroup\$ Should say, you do need to do some kind of signal conditioning because you probably can't input -1 V directly to your ADC. But do you really also need to add gain or not? \$\endgroup\$ – The Photon Nov 5 '12 at 18:15
  • \$\begingroup\$ If you're reading 0-1 VAC (volts AC), your voltage is going from about 1.5V to -1.5V. As the Photon points out, your A/D converter may not like that. You'd need some sort of offset amp or rectifier to handle such a situation. But first, is that really what you mean? \$\endgroup\$ – Stephen Collings Nov 5 '12 at 20:41
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You need to amplify your -1 to +1V signal to 0-10V, so a gain of 5 is required (with a input shift of +1V for single rail)
An opamp set up in non-inverting mode is a simple and accurate way of doing this. YOu also need to shift the +1V to -1V AC waveform so it's all positive (if you are using a single rail supply) Here is a circuit similar to the one you linked to, but setup for 10V output. L1 and L2 represent the transformer, V2 supplies 30A at 100V for simulation. R5 is included in your CT, so all you need is R1, R4 and everything to the right of them:

Circuit

Simulation - here are the waveforms, you can see the 30A input, the 1VAC output (shifted to 0V-2V by the 1V output from the R1 and R4 divider) and the opamp output from 0-10V.

Sim

The opamp can be any rail to rail input and output part (unless you are using dual rail) with a supply range of over 10V (Opamp positive supply needs to be >+10V)

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  • \$\begingroup\$ Thanks! :) One question, for people using arduino, this is best that doing this? openenergymonitor.org/emon/buildingblocks/ct-sensors-interface \$\endgroup\$ – Myckie Nov 5 '12 at 17:06
  • \$\begingroup\$ I may need to update this, sorry - just to check is your CT output 0-1 peak to peak AC or +1V/-1V peak to peak AC? \$\endgroup\$ – Oli Glaser Nov 5 '12 at 17:14
  • \$\begingroup\$ I only know that is 1v, and have built-in sampling resistor :(, here specs: ghielectronics.com/downloads/Gadgeteer/Module/… , thanks ! \$\endgroup\$ – Myckie Nov 5 '12 at 17:24
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    \$\begingroup\$ Oli, what issues does OP need to watch out for to be sure he'll get the 16-bit accuracy (or maybe just precision) that he's asking for? \$\endgroup\$ – The Photon Nov 5 '12 at 18:13
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    \$\begingroup\$ @ThePhoton - to be honest I was just answering the gain/shift part here as the accuracy/precision specs have not been well defined, but precision resistors and a proper reference source are an option. However, the CT has a non-linearity of +/- 1%, and no tolerance is given for the burden resistor. So as you note in the comments it's unlikely a 16-bit resolution is really needed here. \$\endgroup\$ – Oli Glaser Nov 5 '12 at 18:53
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This miniature split-core current transformer has specs of;

  • SCT-013-30 (30A max)
  • Turns ratio 1:1500
  • Load = 62 Ω
  • Accuracy = 1%
  • Freq Response= not specified
  • Vout=1V

From above, Vout = 30 A / 1500 *62 Ω = 1.033V.

Of course, the AC output must be biased for your unipolar 0~10Vdc ADC input. If you just want to increase resolution, you can offset with +5 and a gain of 5x and include tolerance of resistors when computing new accuracy.

You can increase accuracy with calibration and noise reduction on grounds and V/2 voltage reference and no need to provide gain with 16 bit resolution and better than 12 bit accuracy.

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