current drawn from circuits

[rm -rp ./Matt*]

Now I'm learning as fast as I can about electronics and trying to find a decent book to buy but the connection between amps and resistors eludes me. Especially in relation to a battery vs dc adapter.

I have 3 9v batteries wired up in series so the output is 27v and x amps (500maH?) I want to remove the batteries in place of a regulated 24v 500mA dc power supply. Now this would seem to output alot more current then the batteries. Would I need to put a resistor at the beginning of the cicuit or does a circuit only take the current it needs and nothing more, so it would always take the right amount, even if the adapter was 10A?

I don't want to overload the circuit and it seems that resistors do more to the voltage and not the current, but isn't one affected by the other?

[petemoore]

a circuit only take the current it needs and nothing more

[GibsonGM]

...that's why it's referred to as "how much current a circuit will draw".  Current is there for the taking, it's not "injected" anywhere.  If there isn't enough "current surplus" available (ampacity), the voltage will begin to sag.  I=V/R  and E=I*R, Ohm's Law.  Really important!

Check out resistors in series, in parallel, and how pots are wired as a voltage or current divider - plenty of basic info by Googling.  That will help you understand their behavior better.