OpenEnergyMonitor Community

Trouble calibrating SCT-013-000

Hey all,

I’m having a little trouble calibrating the SCT-013-000 current transformer with the emon library. I understand that the calibration value is supposed to be however many amps equal 1V at the ADC, but I seem to have a strange problem.

I calibrated the CT to accurately read the current from a 1500W hair dryer on low (about 9.5A). However, when I tried to read the current either of the hair dryer on high, or of a different device like as toaster or space heater or something, the amperage was slightly off.

Using a Kill-A-Watt, I re-calibrated it for the hair dryer on high, but then it would read incorret amperage for the hair dryer on low. It’s like the calibration only works for one set current reading.

Any thoughts on why this is? I can’t seem to figure it out…


How does your hair dryer switch from high to low? And did you go through setting the PHASECAL variable?

It’s possible that the hair dryer uses the trick of inserting a rectifier diode in series with the heating element on the ‘low’ setting. While the emonLib software can handle this, the current transformer can’t pass the d.c. component through to whatever emonLib is running on, with the result that the calculated value is wrong.

It’s also quite possible that the motor is presenting a reactive load, and if the phase error of the transformers isn’t negated, that too will introduce an error in the power reading - and the proportion that this represents will be different on the two settings. I advise using something without a motor (like a kettle or convector heater) when you are trying to calibrate.

yes, PHASECAL is set to the default 1.7

Also it’s not just the hair dryer, I’ve tested a convection oven, a toaster, and laptop charger as well. I calibrate the CT, and then it is only accurate for that exact current reading. It deviates almost exponentially the farther away you get from that reading.

I appreciate your suggestions!

I don’t expect an accurate reading from a laptop charger. The current for that alone is almost certainly below the specified 10% - 120% flc where the c.t. accuracy is guaranteed, and the power factor and the shape of the current waveform is anybody’s bet.

But I would expect reasonably accurate readings from something like 2 A up to 100 A. Are you following the calibration procedure in ‘Learn’, and are you using the word “exponentially” in the figurative sense, or a strictly accurate engineering sense? Because I would expect any error, when you’ve calibrated at a sensible current for the c.t. rating (and I’d aim for 30 - 50 A) to be quite small between about 5 A and 100 A, and I’d expect it to increase outside those limits. At the top end, I’d expect the output to be low due to saturation, and at the bottom, it could go either way but I wouldn’t be surprised if it read high due to noise pickup. You haven’t yet said what hardware you’re using and running emonLib on, and that could be having a significant effect.

Whenever I’ve checked a c.t. and an emonTx against my true rms multimeter (after calibration of course), it’s been within the multimeter’s specified accuracy, so I’ve been unable to say with certainty that there is a linearity error.

You’re assuming your Kill-A-Watt is accurate. Have you any guarantee of that? The spec. that I’ve found says 2%, but whether that’s 2% of reading, or 2% of full scale, or 2% of reading + n digits (as one would expect for a digital meter), we don’t know.

Note that you can multiply your load by using a multi-turn primary winding for your c.t., e.g. your 9.5 A load with 4 turns through the c.t. will look like something close to 38 A.

P3 International - the manufacturer, or at least, the company whose name is on the device - says 0.2%
Ref: Kill A Watt Meter - Electricity Usage Monitor | P3

Still no reference regarding percentage of reading/fullscale/FS + digits, nor how
long that spec is good for. Given their low cost (15 to 30 USD) one wouldn’t expect
any of that, anyway. At that price point, it’s a throw-away item.

Thanks again for the response! I did follow the calibration procedure. Also I was using the word “exponentially” in the figurative sense haha. I likely have some slight error that is throwing off my measurements (my burden resistor might not be exactly 33 ohms, I’ll look for a more accurate one) or something else.

Thanks for finding that! The measurements from my CT were definetly worse than 2% accuracy, but I will keep trying to improve it

I don’t see the funny side of that. I’m trying to help you, but if you tell me things that are inaccurate, I cannot.

The calibration procedure is intended to compensate for component tolerances such as that.

I did ask what hardware you are using, but you haven’t answered that.
“33 Ω” suggests that you are using a 5 V Arduino - and your own design of front-end hardware? Experience says that many calibration problems are due to noise picked up by the ADC, either through the external wiring or from the processor itself, often the source can be a poor (i.e. electrically noisy) power supply. What do you read at what is nominally zero current?