Make CT4 the same as 1-3?

drewzarn · 2020-11-19T20:15:32.247Z

I’m in the USA, using a recently purchased emonTX (w/ default sketch) to monitor mains input (1 & 2) and solar production (3 & 4). At high solar production levels, I’m seeing a mismatch between CT3 & 4 that doesn’t match what the inverter is saying (around 200 watts less on CT4, when it’s only a few watts difference according to the inverter). I’m also seeing CT4 drop to 0 intermittently during these episodes. This is a 3.68kW array, so shouldn’t be exceeding the max input for CT4, and I’ve swapped the CTs to make sure it wasn’t the sensor.

I’d like to adjust the CT4 input so it matches the other 3 in terms of sensitivity. I saw another post about adding a resistor inline to do this, but that post was a couple years old and now I cannot find it anywhere. Can someone tell me what I’d need to do to make input 4 match inputs 1-3?

Thanks!

Robert.Wall · 2020-11-19T21:04:27.313Z

It’s not an inline (series) resistor, it’s one in parallel. The internal burden for channels 1 - 3 is 22 Ω, for input 4 it is 120 Ω. So you need to add a parallel resistor to bring channel 4 down to 22 Ω - that’s 27 Ω. You will find the 120 Ω resistor and two holes suitable for your new wire-ended resistor just behind the channel 4 socket.

David_Steketee · 2021-11-22T01:36:26.752Z

I added the 27 ohm resistor by soldering it to the back of the board. But, the kW I’m showing for p4 is around 760W where I would expect around 4.2kW. Is there a calibration setting somewhere which needs to be adjusted? I read through your notes in the Learn section, but I’m still a bit unclear on what needs to be changed.

Robert.Wall · 2021-11-22T12:18:09.828Z

David_Steketee:

Is there a calibration setting somewhere which needs to be adjusted?

Yes - make the calibration constants the same as for the other three c.t’s. It should be somewhere near the top of the main sketch file, or you can change it ‘live’ and store the calibration values in EEPROM.
You’re looking for i4Cal & i4Lead, (in the default sketch on Github).