How to Safely Scale 220V AC to 5V for Arduino Mega Without Using a Transformer

Hello everybody.

I'm building a system able to log the voltage and current from a power outlet (up to 250V 60Hz). I'm already done with
the current part, but I'm not being able to find a solution for the voltage sensing part.
I would like to condition my 220V signal into a 5V signal suitable for an arduino Mega uC.

I've already searched some circuits, but all of them involve using a transformer of some sort, which I would like to avoid.

 

There is a good reason to use a transformer, safety. The cheapest and simplest way is a transformer, there is no power transfer, so the core can be quite small, provided it still meets the safety requirements (a flash over test has been done at a high voltage). Some circuits use the reactance of a capacitor to provide voltage division, but any failure of the capacitor might put the full mains voltage on your low voltage system.
You have to consider that the voltage you are working with could prove fatal, and a properly rated transformer is still the simplest way to minimize the possibility of an accident. The transformer can go on a PCB, but you must meet creepage and clearance standards between the primary and secondary sides of the transformer, this often means a slot must be cut in the PCB between the two if the layout is tight.,
Cheers,
Richard Gabric

Hello Richard,

I came up with this simple circuit, what do you think?

Thanks for your answer, by the way.

There is an art in building equipment that works with mains voltages, thinking through a system, either using double insulation, or isolation, takes some care.
When you are working with mains voltages, you always ask the question what if a component fails( and in checking it out, a two component failure is normally carried out during testing), you look at the failure modes, and figure out what the worst case answer is. On that score your circuit fails, since you could get mains voltages at the output of the circuit.
If I were doing this, I would stick with a transformer, they are quite small, and provide good isolation.
If you decided to go with an insulated case, you can get away without isolation, but no metal parts are allowed to come out through the casing, and quite a bit more engineering is required.
I appreciate that you are probably only making one unit, and it is an interesting project, but safety should come first when working with mains voltages. For that reason, current measurement using a shunt resistor can be dangerous as well, and a current transformer is the safest way to go. Power meters are doubly insulated, and the electronics can operate at mains voltages, any signals needed externally would usually be in a digital format, and optically isolated, for example.
Cheers,
Richard