Design voltage divider circuit

[ProPatto16]

Homework Statement

The memory components in a personal computer require voltages of -12 V, 4 V and 12 V all with respect to a common reference terminal (you can assume this to be the ground). Design a circuit to achieve this if the total power supplied to the divider circuit is 80 W and the supply voltage is 24 V.

The Attempt at a Solution

its simple to make the voltage divider for the positive terminal voltages, but I am not sure, mathematically, how to include a negative terminal.

Ive been placing a resistor between the negative terminal on the source and the ground but whatever resistor i put in there changes the rest.

i suppose my question is
How do you achieve negative voltages from a one sourced voltage divider?

thanks

 

[rude man]

Think switching regulators!

 

[CWatters]

Is that really the entire question?

You would normally need to know the power required for each rail. It's trivial to make a 4V rail if you only need a few uA from it.

The problem doesn't appear to stop you making the +12 and -12 by "center tapping" the 24V ??