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The one in the shop is this: http://www.idealpower.co.uk/external-power-supply/fixed-plug-top-ac-ac-l...

It is the "best" only because we have checked it and know it's properties. If Ideal can supply this with the correct plug - it may be worth asking - then I'd suggest that. Otherwise, you need to find a local equivalent. The actual output voltage (unloaded) is what matters, it must not exceed 15 V rms at your maximum mains voltage, and it should be close to 11.6 V at 240 V mains (11.1 V at 230 V - your nominal mains voltage). If the output voltage is significantly different, the emonTx might not work at the lowest mains voltage, and you might need a programmer to adjust the voltage calibration.

Thanks Robert - I am emailed IdealPower to check. Assuming they can't help then I just need to buy one here and put a voltage meter across the output of the adapter (with no load) and check the voltage against those values you listed?

I have a decent voltmeter (Fluke - from my electricial engineering student days) so I should be able to get a reading I think. How do I check against different mains voltages however? Is that a matter of taking reading during a high PV generation period? Or do I need to monitor the mains voltage for a period of time and wait for higher/lower voltages?

I am a programmer so am more than happy to alter the Arduino sketches - so that is not a problem if I need to alter the voltage calibration.

If you do consider buying an adapter locally, the most awkward question to ask is "what is the phase and harmonic distortion?" No prizes for the likely answer (Don't know) but if they do know, then constancy of phase error and low distortion are preferable. You also need the no-load output voltage at whatever the nominal input voltage is (probably 230 V for you), though it might be phrased as "Output: 9 V, regulation: 18%" meaning the voltage at the rated load is 9 V, under no load it rises by 18% (to 10.62 V). Your electricity supplier will tell you their voltage limits (e.g. in the UK it's 230 V - 6% + 10%) and if you measure the mains voltage and then the adapter output, you can verify the adapter data.

If you want to measure mains voltage, highest will probably be late evening as people go to bed and demand drops, lowest during late afternoon or during an ad break in, or at the end of, a popular TV program!

Is this what you're looking for? http://www.idealpower.co.uk/external-power-supply/fixed-plug-top-ac-ac-l...

Yes - that looks like the ticket! Thanks for finding that Robert. I will try and order one now and see if they will ship to NZ.

Robert - I have tried a couple of times to contact IdealPower.co.uk but haven't had a response yet.

Is there anyway the OEM shop can get one through your supply channels? I want to buy a few other bits and pieces but will wait until I hear from you regarding this, in case I can save some postage by bundling everything up.

Cheers,

Ben

You'll need to contact the shop for that: sales@openenergymonitor.zendesk.com

I heard back from Ideal - they will only deal in orders of 100+ units!

I think I am just going to have to source something locally - or buy the UK version and chuck a plug adapter on it - any downsides with doing that?

Thanks for your help with this Robert.

You need 99 friends in your part of the world!

With an adapter, I suspect it might fall out with a little less provocation than it might otherwise need, otherwise I don't see a problem.

Hi Sumnerboy

I am in NZ and I have used this 9v AC adaptor from Jaycar. http://www.jaycar.co.nz/productView.asp?ID=MP3027&w=9v&form=KEYWORD

I have not done much in the way of calibration but it seems to be reading very close to my solar inverter output so it was good enought for my purposes.  I have also set up a emoncms.org.nz site to host NZ data if required. Let me know if you want to access it.

Regards

Nathan

Hey Nathan,

Would love to have a look if you don't mind - I was hoping another Kiwi would eventually see my pleas for help!

Cheers - whereabouts are you based? I am down in ChCh.

Cheers,

Ben

Hey Robert - well I received my EmonTX shield and have soldered it all together. I also received the 9VAC wall wart recommended by Nathan.

I have been trying to follow the calibration guide and have taken the following readings;

    * multi-meter at a socket in my wall = 240VAC exactly (239.99-240.01)

    * multimeter at output of 9VAC wall wart (i.e. no-load) = 11.33VAC

I can't quite follow how I calculate a new VCAL for my Arduino sketch (I am testing with Shield_CT1234_Voltage_SerialOnly with CT3 and CT4 disabled in the sketch). The VCAL constant in that sketch is 300.6. 

Based on the formula (copied verbatim from http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor...);

    voltage constant = 230 × 11 ÷ (9 × 1.20) =

I am just not sure where these values come from. 230 is obviously mains VAC. But where is the 11, 9 and 1.2 from?

Any help would be greatly appreciated, I am happy to change the sketch and measure voltages, but I just am a bit lost where to go from here.

Ok - so further testing;

Hooked up CT1 around my 3kW hot water cylinder feed and turned it on. I am getting the follwing out of the ShieldCT1234_Voltage_SerialOnly sketch;

emonTX Shield CT123 Voltage Serial Only example
OpenEnergyMonitor.org
28250.33 439.43
3366.62 261.75
3093.65 259.51
3096.61 259.52
3103.76 259.79
3110.60 260.16
3114.85 260.28
3112.58 260.23
3110.37 260.12
3104.99 259.91
3101.55 259.73
3109.43 259.90
3104.65 259.81
3110.61 260.08
3105.78 259.88
3109.65 260.05
 

So it is a little higher than 3kW but is that to be expected? This is still using the default VCAL = 300.6.

I am just not sure if I need to calibrate anything - sorry for the noob questions!

You haven't read the rest of the calibration theory page, have you ;-)

The adapter has a regulation of 20%, meaning that unloaded, the output voltage is 9 V x 1.2, always assuming that the adapter gives 9 V fully loaded at 230 V in. Yours may be different. That voltage is divided down by a potential divider  R13 & R14 by a factor of  ........   wait for it  .........  11, again assuming the resistors are exactly on the nominal value.  The assumptions mean it's perfectly acceptable to tweak the number to get the right answer for the voltage (and the current for that matter). I assume you've seen http://openenergymonitor.org/emon/buildingblocks/calibration for blow-by-blow instructions. That was written for the emonTx V2 (running on 3.3 V) and the lines in your sketch won't be identical, but it should help you through the complete calibration process.

I did read it - believe me - multiple times! This stuff is all a bit new to me so I am finding it a little difficult to follow. I really appreciate you taking the time to respond tho!

So based on what you have said, my equation should look something like;

    VCAL = 240 * 11 / 11.33

Since I measure my (unloaded) mains supply as 240V exactly, and my 9V adapter as 11.33 (also unloaded).

This gives me a value for VCAL of 233.01. This doesn't account for variations in the potential divider as you mentioned.

I did read the blow-by-blow instructions but I am not too confident about putting a multimeter in series with a 240VAC load. Happy to read voltages across things, but putting in series is another matter!

So does that now look about right (in terms of the equation)?

I will update my sketch with 233 as the VCAL and try this out again tomorrow - this time on my PV feed since I have a digital readout on the inverter showing what it is generating. Hopefully there will be enough sun to make it worth while! 

Thanks for your patience :).

That VCAL looks reasonable.

"I am not too confident about putting a multimeter in series with a 240VAC load"  Obviously you need to do your homework first. Find out the maximum current that your meter will handle - there are likely to be two values, continuous and short-term with a time limit. Also check the current rating of your test leads. Choose an appliance whose rating you know that will give you a suitable current, and you should be OK. If you are using the short-term rating, make sure you switch off the load within that time limit and allow the meter to cool completely before you measure again. You will probably have to wait some tens of minutes.

I guess what threw my off in the first place was why the shield example sketches all have a VCAL of 300.6. Wasn't sure why that was deemed the default when the calcs in the calibration page seem to suggest it should be around 234.26 (from equation "voltage constant = 230 × 11 ÷ (9 × 1.20) = 234.26" on http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor-installation-and-calibration-theory).

After hooking up the CTs around my solar inverter output and adjusting the VCAL based on what the inverter output is saying I think I have now got it calibrated pretty right!

Thanks for your help. Now to integrate with the ethernet shield and start publishing MQTT messages...