Measure voltage Arduino - External power


In first place sorry for my english, I am not native speaker.

I am measuring the voltage with Arduino using the circuit that you can see here:

The voltage measured was perfect (like a multimeter). I was powered my arduino from USB. The problem was when I powered my arduino with an external power. The same measure obtained powered arduino with external power was 508V and powered with USB was 211V (Correct).

The external power is an power adapter DC 9v~2 amp. Really is AC (the same power adapter that I’m using to measure) converted to DC with Rectifier Circuits)

The circuit is exactly the same with both of powers.

Can anybody help me?

Thanks a lot!!

I suspect your problem might be one of isolation between the low voltage AC signal you’re trying to measure and the DC being used to power the Arduino. Check out this thread from the old forum:

dBC, Thank You!!!

I read all that topic, but unfortunaly i couldnt get any idea. If i dont understand bad, there is’nt way to use only one power…:frowning:

I apreciate your help.


There is a way to use only one transformer, but you cannot use a bridge rectifier. The amount of current/power that you can take is strictly limited, and you must be very careful when calculating the component values to be sure that you choose the optimal values. Look at the circuit diagram of the emonTx V3.4 to see how it is done. If you are using an Arduino, it is possible that the Arduino demands too much current for this to work. If that is the case, then you must use two transformers.[quote=“Leonel_Rodriguez, post:3, topic:531”]
I read all that topic, but unfortunaly i couldnt get any idea

What happens, when you use one transformer and a bridge rectifier for both power and voltage measurement, is you short-circuit part of the bridge rectifier and at the same time you distort the voltage waveform.
So as you found, you get the wrong voltage.

Thank you Robert!!!, unfortunately is the second case. My arduino has an ethernet shield plus a LCD, so it need a lot of current…

Thank you so much for you time!!!