You really should read the basics of electronics... But cos' I'm in a good mood i'll try to explain.

All basic electronics is based on the great god of almighty the "Ohm's law". Ohm's law helps to determine the third value from the existing two.

U=voltage, I=current and R=Resistance

Ohm's law states:

I=U/R

R=U/I

I=U*R

So if we know for example the current and the voltage we can calculate the resistance.

To calculate the needed resistance of a circuit that "lowers" the voltage we need to know two of the components. The current and the voltage. This is hard to explain so i'll demonstrate.

If a basic Finnish man (luultavasti amis

) wants to light led to his Toyota Corolla he'll calculate the circuit

Basic led needs about 1,2V to light up and needs a 20mA current. If the power suply supplies 12V we need a resistor in front of the led.

12V-1,2V=10,8V so 10,8V is the voltage that we need to be lowered. Cause the circuit is in series we know that the current flowing thru the resistor is the same that is flowing thru the led we can calculate the resistors resistance.

Ohm's law... R=U/I so -> resistance=10,8V/0,020A(20mA)=540ohms.

So we need a resistor that is 540ohms...

Hope this helps.