emonPi, CT1 value = 0

Hi all,
Thanks to forum help, my emonPi North America setup is very close to functional. Thanks for taking the time to help untangle my issue, below:


  • code upgraded to latest version from May 2016
  • config user account, timezone, static IP info, and “calibration=110v”

Also, Setup>emonhub>edit config as follows:

    nodename = emonpi
        names = power1,power2,power1pluspower2,vrms,t1,t2,t3,t4,t5,t6,pulsecount
        datacodes = h, h, h, h, h, h, h, h, h, h, L
        scales = **192.31,192.31**,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
        units = W,W,W,V,C,C,C,C,C,C,p

****end emonhub config edits *********
NOTE: value 192.31 taken from chart - 7500 / 39 ohms = 192.31
Reference: https://openenergymonitor.org/emon/buildingblocks/EmonTx-in-North-America
Connect per instructions, power up, AC-AC adapter recognized, temp probe recognized


Go to Setup>Inputs

  • emonpi power1 = 0
  • emonpi power2 = (varies from ~24,000 and 45,000, depending on whole-house current draw, I believe)
  • (BTW, power1pluspower2 = 127, and vrms = 121.5 to 225…it tracks the CT2 value)
  • (BTW#2 - its a typical Florida “winter” at 21C, so no heat or A/C running)


Swap current sensors (sensor formerly in CT1 moved to CT2 input, vice versa)
CT1 continues to read zero; CT2 reads a negative value
In other words, symptom follows CT1 input, both sensors seem functional
power down emonPi, disconnect everything
Connect meter to male plug, connect to CT1 - reads the expected 39 ohms
Connect same to CT2, reads the expected 39 ohms


the two possibilities I can think of:

  • I damaged the emonPi unit during the desolder 22-ohm / solder 39-ohm (I’m pretty good with a soldering iron, have a nice Hakko soldering station, examined w/magnifying glass, and all looks OK, but still a possibility)
  • My ignorance of emonPi code has led me to misconfigure or fail to configure something.

Of course, I hope for the latter. If so, any pointers?.
If the former, can I purchase just the emonPi board as a replacement?
Thanks much!

In the config file is definitely wrong. Have you really put asterisks in the file?

“CT1” - Do you mean the CT itself, or the input channel on the emonPi? I think you must mean the latter, because you should read less than 39 Ω with the CT secondary and the burden in parallel.

Is that a good solid zero, or a small, “insignificant” value? The reason for asking is, the input is shorted to ground if there’s no plug inserted, so that an empty socket is detectable, and the CT is excluded from the calculations, you get a good zero. If there was a break in the PCB track, or even a dead short across the burden resistor, I’d still expect a small variable “power” from noise pickup - you could try raising the sensitivity in the software ( 192.30 → a small number ) to test that.

The jack socket sleeve is actually the input - that’s not grounded anywhere?

The lastest version is actually 8th Nov 2016 however the May release should still work fine.

I don’t quite understand why you have made the all the changes to emonhub. The only change that should be required is setting the calibration=110v.

The emonPi should work fine in North America without any hardware changes: http://guide.openenergymonitor.org/setup/north-america/

He’s obviously not using the ‘shop’ CT, but a larger one to suit N.American Ali cables.

Ah sorry, missed this. Then yes, changing the burden resistor could (probably) will be required.

thanks for the input.
The “****end emonhub config edits *********” thing was just in there to show those who read my post (you guys) that this was the end of the emon config…that was not included in my actual config.
As for the asterisks in the config, next to “power1” etc are not actually in the config. I attempted to “bold” them in my post, so you could easily see what I changed.
So…there are no asterisks in the config.

For “normal operation” - I plugged my CT’s into the CT1 and CT2 inputs on emonPi, as normal.
For my “resistance test” I unplugged everything from emon pi, then used a simple male jack to plug into where the CT’s would normally plug in, connected meter leads across the wires from the male jack…just an easy way to read ohms without disassembly…and it does read 39 ohms…so, this would be the same as removing unit from housing, measuring across the tip and ring of the female CT1 and CT2 plugs. I was just trying to detect any difference between the non-operational CT1 input and the operational CT2 input (on the emonPi…again, not CT sensors were connected for this resistance test)

The Zero is a solid zero…not some very small number…it is exactly and always and only zero - 0 - zed - nada. :slight_smile:
(I did lower the value for CT1 from 192.71 to 2, as suggested, and rebooted emonPi, CT1 still reads exactly, solidly, and only zero. (this is the “power1” value from setup>inputs.Or from the readout on the emonPI itself.

Glyn - as to the version, I just went to setup>administration: Update emonPI. hit the “Update Now” button, and that’s what I got. (the may 2016 version.) I agree, this is not likely relevant, but thanks for letting me know I can upgrade again.
To ensure clarity… I read thoroughly the north american thing, bought the magnalab SCT-0750-000 because they would fit around the larger diameter main feed wires in my panel. I read the chart on the reference link provided, which indicated that these CTs require a 39-ohm burden resistor.

Does this all make sense?

It was originally written to have the new scale values in bold, but since then Jon has edited the post, making it a code block and

scales = 192.31,192.31,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1


   scales = **192.31,192.31**,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1

Likewise for the input names.

As for the value of 192.31, whilst it probably won’t be the main issue here, that is unlikely to be correct. Since the sketch already applies an Ical of 90.9 (for the 22R burdens) your new “scale” will need to factor that in, ie for a target Ical of 192.31 you would “scale” by 2.12 (192.31 / 90.9), as a rough check, since the burden is approximately doubled, you would need to double the scaling unless you edit Ical in the sketch.

Did you power cycle (or reset the emonpi board) when the CT’s were swapped, note that rebooting the Pi doesn’t reset the AVR, you would need to disconnect the power for that.

EDIT - I see you have posted whilst I’ve been typing! The value of “2” is closer to the correct value to use in scales, try power cycling the emonPi with both the CT’s connected and if you still get nothing on ct1 you should try checking for continuity between the burden and the midrail component etc to see if the burden swap has caused an open-circuit on the inboard side of the burden.

Can you measure resistance from the sleeve of your jack plug to GND. An easy place to find that is the shell of the aerial SMA connector. If you can see a very high resistance, that’s OK. If however you see the burden resistor (39 Ω), then it looks as if there’s a short to ground somewhere.

Yes, the values on the page “EmonTx - Use in North America” are for the calibration constant in the emonTx, or in the sketch in the Atmel 328P inside the Pi.

guys. you rock.
I changed the scale value to 2.12 for both power1 and power2.
Then I powered down the unit (5…4…3…2…1…wait 30 seconds…remove DC power)
THEN I swapped the CT/sensors between CT1 and CT2 inputs on emonPi.
powered back up…BAM!
Now I read between 76 and 120 on CT1, and around -468.5 on CT2.
I powered down, re-oriented the other sensor, and now BOTH CT1 and CT2 (power1 and power2) read positive!

Weird thing: shouldn’t power1 plus power2 actually equal…you know…power1 plus power2? Please see below:

1 Like

That’s good to hear, the power1pluspower2 also needs scaling, 71 x 2.12 = 150.52 = 78.44 + 72.08.

cool. should’ve figured that.
I did a search on the forums about calibration, and couldn’t find a straightforward answer.
I’d like to ensure the values reported by emonpi are accurate.
I guess I could buy an “amp clamp” type device and check its readings against those of the emonpi?
Then adjust calibration values accordingly?

The information about calibration hasn’t yet caught up with the emonPi. It’s all in the melting pot and a complete revision is planned. Calibration isn’t in the forum, it is in Resources > Building Blocks.

The Power1 & Power2 are added before the values reach emonHub, so that explains why the 3 are adjusted there separately.

It might not be necessary to buy a clamp ammeter, but you could do that. One method that’s been used is to compare the long-term kWh values against your supplier’s meter, and tweak the calibration based on that. Better than 1% has been claimed, and a clamp ammeter good to at least 1% won’t be cheap.

Sorry! My bad! :cold_sweat:

Not a problem! it was an easy spot for me as I have previously done the same thing. i just keep an eye open for bold and/or italics (or html tags) when editing code blocks and remove them if they cause confusion.