CT Calibration possible or already accurate as possible?


I have my own board up and running (schematic 100% Iotawatt V5 clone).
Before installation I tryed some measurements in my workshop under constant conditions (Germany 230V).

I put 3pcs SCT013 100A/1V + 1pcs SCT013 20A/1V + 1pcs HWCT-004 on a “test-line” with a 2kW heater. I have no burdens for the SCT013 installed but one 20Ohm for the HWCT-004. Further I measured the current with my Fluke 77 multimeter an a cheap clamp-on current meter.

The Fluke says 8.35A the cheap clamp-on says 8.5A
The 5 CT gave me values between 8.7A and 8.85A.

Due to the fact, that I dont know if the Fluke is accurate I will try to get another one the netxt days to verify the measurement.

2 Questions:

  1. If I can confirm, that the Fluke is correct, is it possible to “calibrate” the entire Iotawatt current measurement, so that I can go down the ~400mA? Or is this just the possible accuracy and I dont have to worry?

  2. Is it possible to calibrate the single CTs so that say come closer to the same value? On the SCT013 I can manipulate the “calibration-value” at the Input-configuration, but would this be a good way to change the 100A/1A from Calibration 100 to lets say 98 and so on ?? Or will this bring faulty measurements on other currents, temperatures, etc?
    And what would be the way to calibrate the HWCT? I dont have a Calib. value at the config page because it is pre configured.

Or am I just to squeamish and my readings are as good as they can be?

thanks in advance and best regards.
stay healthy

You have two different issues here.

The spread between the five CTs is a little under 2%. That’s not too bad for YHDC CTs at this very low current. I’d put my money on the HWCT-004 for best accuracy, the others seem to be about +/- 1% of it. What is the tolerance of the burden resistor?

The other issue is that these all vary from the Fluke meter by nearly 5%. Could be the Fluke, but I doubt it.

The primary calibration of the IoTaWatt is via the reference shunt. I use 0.2% components for that. Using the Fluke, measure the voltage after the shunt. If it’s not 2.5 (or whatever value shunt you used), override the shunt reference value with what the Fluke reads in the config file.

thanks for the fast reply.

The 20Ohm burden for the HWCT is 1% tolerance. I measured one => 19.955 Ohm (measured with Fluke 8840A 4-wire-mode).
The YHDC have build in burden, as you know.
So I will not think about the spread any further.

But the difference between Fluke and iotawatt…
The 2.5V shunt I used is 0.1% Tolerance. Measured value on the running board => 2.5006V (measured with Fluke 8840A) EEPROM is set to 2500mV

I just repeated the measurement with a smaller load (400W halogen floodlight). The Fluke77 says 1.71A, the 5 CTs on the iotawatt say 1.71A to 1.72A. My Fluke 8840A confirmed the current reading (has only range up to 2A). So I would say perfect.

But, then changed again the load to the 2kW heater (full power) => Fluke is again ~400mA below the iotawatt readings as described in first post.

Next reduced heater to half power => Fluke is 5.12A and iotawatt is 5.33A to 5.38

so either the fluke is drifting away on higher current or the iotawatt is drifting. Hopefully I can organize another Fluke with a current reading range up to 10A the next days from my company office (thanks covid I am at the homeoffice actually). I will post the measurements as soon as I get them…

If you have any other idea to find and isolate the reason for the problem let me know

Sounds like you know what you’re doing. You do need a more reliable standard as an arbiter. With the three data points, it looks like both the IoTaWatt and the Fluke 77 are pretty linear, it’s just that the Fluke seems to have about 5% less slope.

Have you checked the bias voltage vs. the ADC reference voltage? Here again I use low tolerance resistors for the voltage divider on the op-amp. IoTaWatt has logic to detect and compensate for some drift, but it is restricted to only a few ADC counts. If it is way off from center, it will affect rms current. The production IoTaWatt manufacturing tests check that the bias is within two counts of center as measured on all of the inputs.