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Using High Sensitivity (CT4) port on EmonTx

Having recently gotten my emonTx up and running, I’m wondering about the implications and use of the high sensitivity CT4 port. I’m in the US and using CT1 and CT2 for mains, with CT3 and CT4 tracking branch circuits of interest. The readings on CT4 appear to be similar in nature (baring differences in load, of course) to what comes from the other ports.

I can’t find any docs in Learn or Resources that cover what “high sensitivity” means from a practical effect. Is this just a limit on the max power monitored on that port? Is this port capable of higher-period samples and, if so, how is that enabled in either the emonTx sketch or in the corresponding emonBase unit?

Thank in advance for the help and clarification!

Yes. Or more to the point, current. About 18 A with the 100 A c.t from the shop.

I don’t understand what you mean by that. If you’re up to the programming, you can read (average) over as long a period as you want. But you’ll need to be careful the numbers don’t overflow internally.

The emonBase / emonPi doesn’t communicate back to the emonTx at all, it’s a one-way data flow.

Ah, so the port is strictly more limited and not more precise than CTs 1-3. I should have expected it to do “just what it says on the tin”. :stuck_out_tongue:

At US voltages, that gives me around 36 A to play with (if my high school electronics math isn’t failing me). That’s more than enough for branch circuit monitoring (as would be 18 A, to be honest).

Thanks for the fast response!

No, it’s current limited, so still 18 A, but only 2.2 kW or so on a 120 V circuit. You can add a wire-ended resistor in parallel with the existing burden to reduce the sensitivity if you need to. And you can remove the existing burden and put a higher-valued wire-ended resistor in its place to increase the sensitivity of the other channels.

But remember, we’re talking about a 100 A : 50 mA c.t. With anything else, all the numbers are wrong. The really useful thing to know here is the input can see 1.1 V rms maximum across the burden resistor, and you’ll need to recalculate everything from that.

Your maths wasn’t failing you - it was the wrong maths that you did. :slight_smile:

Doh. P = I x V. Of course. Thanks again. :smiley:

As this thread comes up when searching for more information about the “high sensitivity” CT4, I’d like to add this to it:

CT4 is described as “1 x high sensitivity single-phase CT current sensor input channel”, and it appears that it is actually more sensitive (not just “more limited and not more precise than CTs 1-3” as described above).

It has a higher burden resistor (120 ohms instead of 22 ohms), so any given current in the CT is converted to a higher voltage input to the ADC input port.

The following lines of code enable the software to treat it differently than the other CTs, and interpret the readings correctly:

float i1Cal = 90.9;         // (2000 turns / 22 Ohm burden) = 90.9
float i1Lead = 4.2;
float i2Cal = 90.9;         // (2000 turns / 22 Ohm burden) = 90.9
float i2Lead = 4.2;
float i3Cal = 90.9;         // (2000 turns / 22 Ohm burden) = 90.9
float i3Lead = 4.2;
float i4Cal = 16.67;        // (2000 turns / 120 Ohm burden) = 16.67
float i4Lead = 6.0;

No changes are needed in the emonCMS configuration, as the compensation is done entirely on the emonTX.

Correct. And you’ve quoted the code from the “standard” sketch, so what is the point you’re trying to make?

But it is actually more limited - in performance, because the phase errors and distortion increase as the value of the burden is increased. The general idea is that c.t’s should ideally work into a dead short. Anything that means the c.t. has to transfer more power adds distortion and errors. Or to put it another way, using the higher-valued burden is a fudge to avoid using a lower-rated c.t. that would almost certainly provide a better performance.

The OP wanted to know what “high sensitivity” actually means regarding the CT4 port, but I don’t think this thread answered that question until now.

When I read it, I assumed that this was correct, as nobody said otherwise:

But I think that it is in fact more precise, as it has lower quantisation error in the ADC, due to the higher burden resistance. If not, then why would the designers bother making the CT4 port different to CT 1-3? It would be a design mistake?

I also thought the OP was looking at how to enable this extra precision, so I wanted to clarify that it’s already enabled by default, nothing extra needs to be done. The official documentation still says almost nothing about the CT4 channel!

I didn’t know that a higher burden resistance would make the CT less accurate, thanks for educating me. It would be nice if we had a wider variety of CTs to choose from in the shop. I can see why distortion could increase, but why do phase errors increase? And how large are those errors likely to be?

The phase errors part is inherent in the way the magnetics work.
CT Theory.pdf (87.8 KB)

If you look at the test reports in ‘Learn’, you’ll see the effect of the 120 Ω burden on phase error.