I have been very confused with resistors because I have not found a great, although simple explanation about if resistors reduce or limit voltage. Also, I want a explinaton on why you subtract Vf from V when finding the right resistor for a LED. I get that V=IR, but I don't get how to get the resistor value. I take (V-Vf)=I*R, that means that if I want to get 5v to 3.3v@25mA, I take 1.7=0.025*r, meaning that the resistor would REDUCE VOLTAGE by 1.7 volts, correct? Thanks! *Feel free to make any comments and questions, this is quite vague.
Resistors resist the amount of current flowing through them. When they do this, a voltage difference is developed across the resistor.
LED are usually characterized by the amount of voltage & current they need in order to light up. So a 3.3 volt 25mA LED needs to develop 3.3 volts across the LED.
In closed loop circuit all the voltages need to add up. This is Thévenin's theorem. So when calculating which resistor is needed for a given constant voltage power supply, say 5 volts, and given LED which requires, say, 3.3 volts, you subtract 3.3 from 5 and get 1.7. See, the power supply adds 5 and the LED takes away 3.3 and then the resister needs to take away the remaining 1.7. All the while you want 25mA running through the loop. So, using your equation you have:
5 - 3.3 = I x R 1.7 = 0.025 x R R = 1.7 / 0.025 R = 68 ohms
Do check to make sure these calculations are correct and that you really want 25mA running through the LED. Most LEDs these days take only a fraction of that amount of current!