Suppose we have a bulb or a forward-biased LED (or any other component) which needs no more than say 3 V across it to operate. And we want to connect it in series with a voltage source of say 12 V, and a resistor. So without this resistor, the LED would quickly burn out and be destroyed.(adsbygoogle = window.adsbygoogle || []).push({});

I've edited a diagram to illustrate the situation:

[PLAIN]http://img225.imageshack.us/img225/6515/circt2.jpg [Broken]

How can we calculate the value of this resistor in series with the LED and the battery, that limits the current flow through the LED to a safe value, and reduce the 12 V to 3 V? I know that there many websites and softwares that can calculate the value of the resistor, but I just want to know how engineers would do the calculation without using them. Is there a quick method or a formula?

And what if there are two (or more) LEDs instead of just one?

I couldn't find any methods for this in my physics textbook so any guidance is greatly appreciated.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Resistor values for a series circuit

**Physics Forums | Science Articles, Homework Help, Discussion**