There is an alternative way of doing it. You already have a 12V DC supply so you could feed that into a buck converter such as this one:
http://www.ebay.co.uk/itm/5-30V-to-0-8- ... 3a8900d308You would then be able to adjust the output voltage to anywhere between 0.8V and about 10V. The maximum current of that one is 5A, though several models are available on Ebay.
A buck converter does for DC what a transformer does for AC, ie. changes the input voltage without changing the power used (I use a buck boost converter to give an adjustable 0.8V to 30V power supply). Let's know if you decide to go that route, and I will give you a diagram showing how to wire up the meters.