US ACAC Vrms reading low

I’ve got my emonTx v3 + emonESP all set up and installed apart from the garage wall mounting, I’ll post some pictures once it looks pretty.

I’m noticing the power usage is reading a slightly lower than I think it should (compared to the pulses on the meter) and the first thing that comes to my attention is that the Vrms output is a little low as well. I’m seeing 114.48V reported, and my BK Precision TrueRMS multimeter reports 121.6V or 1.0634x.

Looking at the source code I see a comment in the code that Vcal should be (230V x 13) / (9V x 1.2) = 276.9, so US should be half that at 138.42 and it is currently set to 130. The difference between these Vcal constants is1.0648x. We have a match!

So my question is shouldn’t the Vcal_USA be 138.42? Also, any tips for updating the firmware on the emonTx given that I’d need to unplug all the current clamps and bring the device inside? I read here that you shouldn’t have current clamps installed with no burden load on them.

The values given for Vcal are calculated from the data sheet values for the various a.c. adapters. Component tolerances will inevitably affect the calibration. 130.0 is the correct value if you are using the Ideal Power 77DA-10-09 adapter, and if every component that affects the calibration is exactly on the nominal value. I think it’s coincidence that you’ve arrived at a pair of numbers that happen to be more or less the same.

I presume you can’t take a laptop outside? :wink:

You really should drop the CTs off their cables while there’s no burden resistor connected, alternatively you can short-circuit the CT. (Yes, not only is that safe, it’s working under minimum load conditions.)
The CTs from the shop, and many others (BUT NOT ALL) contain suppression components that should prevent danger if the burden is not present. However, I don’t advise relying on that.

I am using the Ideal Power DA-10-09 from the Open Energy Monitor store. I was wondering if it was just sheer coincidence that I arrived at nearly the same value, but the numbers seemed so close that I couldn’t help think they were related. I won’t need to flash new firmware after all, there could be enough error in my wattage measurement procedure (manually timing the meter pulses) that the emonTx is right on anyway. I’m just so excited to have all this data!

Thanks for the reply and the information about the CTs. I bought these off eBay a couple years back-- they claimed they were YHDC 100A SCT-013-000 but who knows how authentic they are but I’ll just play it safe if I ever need to disconnect them from the emonTx.

Where does the data go after it’s been through the emonESP? If it’s ending up in emonCMS, you have the ability to include a multiplier in there to adjust the calibration.
(There’s a 5% tolerance on the output voltage of your adapter alone, then 1% for the two divider resistors, then some more from the voltage regulator. And don’t forget your meter either.)

As long as you detach the CTs from the curent-carrying conductor before you disconnect them from the emonTx


reconnect them to the emonTx before you re-attach them to the current-carrying conductor, you’ll be OK.

Oh snap I totally forgot about that, am doing all the math there to add all the current data together I didn’t even think I could just change it there. Great idea! I’ll need to figure out how to take some longer samples for calibration though; due to it being winter here, my normal power usage is around 500W so the IR pulse doesn’t blink enough to make an adjustment of a few percent.

And thanks @Bill.Thomson, I knew I could do that, but the CTs are inside of the electrical panel behind a bunch of long screws so it isn’t convenient to remove the clamps before unplugging them from the emonTx.

If it’s that the data is going to, you need to add a multiplier in the input process list. If it’s emoncms on your local emonPi, then you can use “scales = …” in emonhub.conf to adjust the calibration before it hits emoncms.

I added the 6.4% multiplication to my emoncms feed and everything was tracking right on today. However when the solar power started to pass ~5.5kW, my power use started to drop. This had me scratching my head for a while until I realized that the 4.3kW cap on CT4 is @230V (or 240V?). The actual specification probably should read 18.33A instead of wattage at unspecified voltage, but I was definitely saturating that input. My “use” is (ct1+ct2+ct3+ct4) * 1.064 so when the CT4 saturated, my use number stated to drop.

I placed an order from mouser for some 27 ohm 1% 1206 resistors to replace the CT1 and CT2 burdens (on the 2 mains lines) and some 47 ohms for the CT3 and CT4. That should be good to 81A per mains and 47A on the solar. When that comes next week I’ll replace the resistors and try again. Can also reflash the emonTx with an updated Vcal_USA so I can remove the 6% math in emoncms while I have the current clamps off. Will check back in later, thanks for the information guys!

Yes, the actual limit is of course a current, as the ‘brochure’ limit is specific to that input with the standard ‘shop’ C.T. and the nominal UK mains voltage. The true limit is the CT’s secondary current, and even that can be changed by changing the burden value (incidentally, you could have bought wire-ended resistors to replace your burdens). So there’s no simple answer to how the input should be labelled.

Yeah that’s a good point. I wasn’t complaining I just had an a-ha! moment when I was confused about why the power usage was dropping until I realized I was saturating the input. I supposed I could have gotten leaded resistors, the through holes by the burden resistors are a nice design touch. 1206 package are massive though so it is no problem to desolder then and put in some different values.

The only sub-100 ohm through-holes I had were 10 ohm. I had 27 ohm 0603 but I was worried about them being too small to fit the footprint and I needed parts from mouser anyway so might as well get 1206 parts. I tell you though this emonTx + emonEsp is amazing and now I am not sure why I waited so long to get one.