My project is to build a digital watt meter, to measure the Voltage, Current, Power, etc. from an appliance connected.

My question is, is there a way I could measure voltage from a 240V/50Hz mains connecting to an appliance?

 

Generally speaking, a voltmeter works extremely well. In your case, using a transformer to reduce the voltage by some known factor and to provide galvanic isolation might be a start. Rectifying and filtering the voltage makes applying it to an ADC much simpler.

Use a current transformer for that function, and you have your instrument under way.

 

Voltage, Current, Power, etc

Don't forget you also need the phase angle for the power.

You can buy these for next to nothing.

http://cpc.farnell.com/jsp/search/productdetail.jsp?sku=PL1005312&_requestid=264004

 

The easiest route for building it yourself is to use an energy measurement IC. Have a look at these http://www.analog.com/en/analog-to-digital-converters/energy-measurement/products/index.html from Analog Device and these http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11029&mid=11&lang=en&pageId=79 from Microchip.

Download the datasheets and related app notes for these devices and have a read.

 

You can use a sense resistor that creates a specific voltage across it at a specific current. Use the formula V=IR, thus R = V/I.
Then amplify this voltage using a non-inverting amplifier and then send this to a PIC to sense the voltage, thus you know how much current passed through. Then sense the voltage before the resistor. Amplify and send to PIC. Now you know the voltage. Use software to calculate the Wattage and you know everything.
Hope this helps.

 

Now you know the voltage. Use software to calculate the Wattage and you know everything.

Phase angle?