The most important thing is that Arduino's analog chips can measure voltage, and convert it to a value between 0 and 1023. But the analog pins cannot handle more than 5V, so I have to use a simple voltage divider to chop off the above 5V parts of the voltage. Testing the circuit on a breadboard the measuring kind of worked, but the accuracy was not the best.
The problem is that the arduino's atmel chip has an internal voltage reference which depends on the power supply. So you cannot rely on that its a constant 5V hence the measurement won't be accurate.
To avoid this problem the atmel chips has an option - the AREF pin - to supply an external reference voltage to the analog converter.
So I went throught the options, an bought an MCP1541 voltage reference IC.
By the way, It has a fantastic data sheet. Full of examples and detailed instructions about how to design a circuit around this chip. The key is you have to put a at least an 1uF capacitor between the Reference Out pin and Ground.
So I hooked it up with and Arduino, and with this code I checked that it working quite accurately - I correlated datas with my multimeter.
Till I got the needed capacitors, I continued to work on other parts of the project.
The instrument need some way to display the actual voltage and perhaps the actual current consumption.
Ebay has tons of similar products for it, from simple 8 digits to fancy oled types.
Another way would be to use an arduino and an lcd display to measure and show it. Because I happened to have an LCD display around, and also recently buyed an i2c interface for it I went this way. I hope to learn more of it.
Read about how to use an i2c interfaced lcd display with an arduino:
I salvaged a transformator from an old broken halogen lamp - anyone remember halogen lights? These were the next big things before LED lights came and ruled the lightning market.
Anyway, the lamp's tranformator has 12V with 1.6A ratings on the secondary (total 20W - those halogens were still juicy ones :). Even better that it also has a switch and mains cable with a plug so it don't need to buy those.
So I decided I can make a variable power supply aka supply lab bench from it with the following parameters:
Variable voltage: 0 - 10V
Variable current: 0-1A ( still not decided that need this function?)