Have four circuits. One takes 9Vdc, two 5Vdc, and last one takes 12Vdc. Have variable transformer for source. How do I calculate or choose the voltage needed to run all four circuits with out blowing them all up, and at the same time run each circuit by themselves or two at a time or three, etc. How do I know how much current the source needs and where do I place leads to measure current and voltage for each individual circuit?