Odd CT readings with emonTX V2.2

I’ve just brought into use an old emonTx V2.2 board that I bought ages ago. Currently it’s running the emonTX_CT123 sketch as I can’t feed it with AC volts right now. I’ve been trying to calibrate it with the two CTs I currently have connected and I’m getting odd readings.

I have a kettle plugged into a smart switch that reports voltage, power, PF etc.

  1. With no CTs connected, the serial is stating 22 and 19 for CT1 and CT2.
  2. If I connect the CTs but without the CTs installed on wiring, I get 212 and 250.
  3. If I install them on the live of the kettle lead (kettle off) I get 1015 and 1195
  4. If I switch the kettle on I get 2693 and 2751 - which is fairly close to the 2800W reported from the smart plug. The smart plug reports zero in all other circumstances.

I can adjust the calibration factors to get the power on values more accurate, but I’m bothered by the abnormally high values reported with no current flowing.

That isn’t what my emonTx V2 does.

Are there strong alternating magnetic fields where you have your c.t’s? If I want a quick test to see if a c.t. is the one I think it is, I hold it close to the a.c. adapter.

Is there a strong electric field where your emonTx is?

Have you tried with a different 5 V USB power supply?

The other thought - is the processor bedded correctly in the socket, and are the c.t. plugs and their sockets clean? A twiddle and a few insertions is usually enough.

Using the same sketch as you, I see numbers like this (c.t. plugged into CT1, no current flowing)

19 40 20 3342
18 40 20 3342
17 39 17 3342
19 41 16 3342
18 40 19 3342
18 40 17 3342
18 40 17 3342
20 43 18 3342

And if I put a bit of current through the c.t, (and swap it around):

8239 40 18 3342
8238 38 19 3342
8238 40 16 3342
8238 40 19 3342
99 37 18 3342
31 8217 19 3342
19 8231 16 3342
17 8211 17 3342
18 8205 18 3342
18 78 8206 3342
21 43 8221 3342
17 39 8207 3342
18 41 8184 3342
19 40 8184 3342

The very nature of the maths means that any electrical noise picked up is rectified by the rms calculation, then multiplied by the voltage to give a spurious power reading. However, 1 kW is a bit too much.

Robert, in another thread, you wrote:

Unfortunately, this is the expected behaviour because what you are seeing under no-load conditions is noise from the digital circuits in the processor getting into the analogue input. Then, when you calculate the rms current, the noise is rectified by the maths and appears as a (relatively) large apparent power, whereas when you multiply each sample of voltage and current to get real power, the noise, not being rectified, tends to get averaged out.


As he’s calculating only apparent power,

is that essentially what he’s seeing?

The problem is, I’ve never seen power supply noise or anything remotely close to 1 kW, which is why I suspect something is seriously amiss. My first thoughts turned to external influences. Where it’s going wrong is the hard part. A near-correct reading on his kettle indicates it’s not a stupid calibration value that’s crept in, and the burden has not fallen off.

Those are sensible numbers.

Those aren’t sensible. They indicate the c.t. cable, or the c.t. itself, is picking up something.

Now that’s ridiculous. It seems to be seeing an electric field on the kettle lead.

There should be no difference between the c.t. being unplugged, plugged in and not on a cable, and on a not energised cable. In practice, it’s hard to get away from electromagnetic fields, so you do expect a small increase when a c.t. is connected.

@Guff666 might want to update his sketch: In The emonPiCM I published a sketch for the emonTx V2 using emonLibCM and the native RFM69 radio message format. It’s relatively easy to change it to use the classic JeeLib message format - simply change #include <rfm69nTxLib.h> to #include <rfmTxLib.h> and most likely also change #define RFM69CW to #define RFM12B and I think it should work. I don’t think this will solve the present problem.

Thanks for the prompt responses, guys; I appreciate it.

I can’t think of any immediate electric fields. The emonTX was installed in the outside electrical supply cabinet, so there’s only the mains wiring in there. Under test, the equipment was on the bench in my radio shack, but none of the radios were on.

I’ll check the CT connections etc and try introducing a deliberate field and report back. It may be a week or so though. I’m currently preparing for departure to Le Mans for the 24H race next weekend :smile: