Archived Forum

Short answer - no.  The 'standard' one sold by the shop is 9 V output at rated current, a little over 11 V on no-load as we use it. If the no-load voltage is greater than that, you might need to consider changing the voltage divider resistors. (Read up in Building Blocks about the Mascot Adapter and calibration http://openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-... http://openenergymonitor.org/emon/buildingblocks/report-mascot-9v-acac-a... http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor...). You will not be able to buy a regulated ac-ac adapter, it cannot be done and in fact you don't want a regulated adapter because its whole purpose is to measure the changes in the mains voltage.

(Actually, you can get constant voltage transformers, but it involves clever magnetics and distorts the waveform, something we don't want. And it would be horribly expensive and the only ones I've seen come in at around the 1 kVA size) 

Hi, Thanks for that. 

So what is the maximum voltage that should be coming into the emontx at no-load?

Read my explanation again and do the sums, as explained in the articles I linked to. At worst, you need to change one resistor ( R13 ) to suit your transformer. Failing that, measure the transformer open-circuit output voltage, the mains voltage and tell me the maximum mains voltage your electricity provider is allowed to supply, and with those 3 values I can do the sums. It's not difficult, you just need to take account of all the variables and work methodically.

I've got an old 12V AC-AC adapter that is measuring around 15V on no load. Connector fits the one provided with the emontx kit. What should I change R13 to? Mains voltage is nominally 120V +/- 10V

(however, I intend to use this in Haiti where the mains voltage can dip into the 70s - so I would really need a range of 70-170V)

"so I would really need a range of 70-170V)"  Ouch!

Again, you need to work through the sums. There's a new page in Building Blocks http://openenergymonitor.org/emon/buildingblocks/acac-component-tolerances that might be easier to follow. You must of course substitute your own numbers for the mains and your transformer, and the emonTx voltages in place of the Arduino voltages.

how about using transformer 230vac - to -9vac with current rating 12VA
 

I don't see why not, provided that it is properly boxed and protected, and it doesn't distort the waveform too much or have excessive phase errors. That's all the ac adapter is: an enclosed transformer whose properties we know.