New User / Ordering Questions

Before I order or wait to order, I have a few questions.

Is there an availability update for the 9VAC power supplies for the USA? I’d prefer not to have to use EU/USA adapters if it can be avoided. Or can the EmonTX3 use a 12VAC / 1A adapter with calibration, since I already have plenty of these on hand?

Same questions for the HeatPump Dev Kit.


Welcome, Jeff, to the OEM forum.

Possibly not, without a hardware change also. Our USA 9 V adapter actually gives 12 V when it is very lightly loaded - as is the case with the emonTx. However, your 12 V adapter is also likely to give more than the nominal 12 V, so I’d need to know the actual no-load voltage before I can give you a definitive answer. Apart from the question of the rating of the components, clearly once the input is overloaded and the measured wave is distorted, calibration isn’t a solution.

And the same answer!


Thank you for the reply.

The no load voltage for these are pretty consistent between 14.75 and 14.8 VAC. I had used these for a ATM90 project in the past and have a few on hand.

If thats too high a value then I can wait for the U.S. plug version to be back in stock.


That’s too high without changing components in the voltage divider circuit. The divider ratio is 13, but you need to add 5% for your supply voltage tolerance, and another 1% approx. for other component tolerances within the emonTx. That takes you to about 3.5 V peak-peak - and that’s exceeding the ADC’s input range. If you like, you could decrease the value of R14 (10 kΩ) so that the output of the voltage divider gets back to what it would be with the “shop” adapter. You could do that relatively easily by soldering a 39 kΩ in parallel.

Thanks Robert. That may be the best approach since I have plenty of these power supplies on hand and they can readily be sourced in the states. One last question on this topic. Is R14 an 0805 or 0603?

According to the BOM I have (not necessarily up to date):
Vishay CRCW060310K0FKEA