OpenEnergyMonitor Community

Hacked AC-AC adaptor for Voltage measurement

Hi, have been reading through some documentation here for Power measurement. I see that i might need a AC-AC 9V adapter to measure voltages across different Phases. Looked around here online in India and can’t really find a 230v - 9V AC-AC adapter and International ones might end up costing a bomb.

Just trying to understand what my chances are if i just open up a 230>>9V AC to DC adapter and just solder the output wire to the Transformer in it and skip the Bridge rectifier circuit. Any experience, suggestions?

Or even better if i just buy a 230-9v transformer separately and just use that. Will be way more cost effective.

Any suggestions on pros and cons of each approac above (I’ll take care of safety aspects etc, and am not worried about it) vs getting a factory build AC-AC adapter only.

Thanks in advance.

Welcome to the OEM forum.

Safety is a primary concern for us here at OEM. I read that it doesn’t concern you, but if the transformer is not properly enclosed, and the frame earthed, and an appropriate fuse is used, then you are putting yourself and anyone who might be tempted to touch it in significant danger.

Bearing that in mind, the simplest solution is going to be to use a d.c. adaptor and remove the rectifier (assuming it is an unregulated one and NOT a switched-mode) and connect to the transformer secondary winding. The advantages are you have a plug, a case, and most probably thermal protection buried in the windings.

Whichever route you take, the open-circuit voltage needs to be around 11.5 - 12 V rms if you are going to power an emonTx, and the VA rating wants to be about 5 VA or more. (Yes, 9 V is at full load, and the emonTx is designed for the no-load voltage of the adapters in the OEM Shop.)

If you’re building your own circuit based on an Arduino, then you only need 1.6 V rms at the Arduino input (for a 5 V analogue reference) and for a higher voltage transformer, adjust the voltage divider ratio accordingly.

What I can’t tell you are the phase errors you might encounter, which might be significant – assuming you’ll be calculating real power.

1 Like

Thank you, that was helpful. I will circle back if i hit any issues.