View Full Version : More Resistor info plz

09-03-2010, 03:19 PM
Hello my name is Derek. I am currently enrolled in college and am interested in electrical engineering. I just got my hands on the stamps in class "Whats a Microcontroller?". I was wondering if anyone could give me some information on how to calculate what resistors I need with certain voltages or direct me to a source where I can find the information. An example would be...the book says to use a 470 ohm resistor with an LED and I'm assuming that this size resistor is based off of the voltage that my 9v battery produces. How would I calculate what resistor I need if the book did not tell me what resistor to use? Any help would be appreciated.

09-03-2010, 03:29 PM
Assume 5V (the output voltage of a Stamp pin that is set high) is across the resistor and LED, which are in series and connected to ground. You want, say, 20 mA through the LED, which will have a voltage of 1.2V across it. Just use simple arithmetic and Ohm's Law to calculate the resistor value.

Jessica Uelmen
09-03-2010, 03:43 PM
Hi Derek!

Depending on the book version that you have, there is more information on calculating current and voltage in the Appendices. It's in Appendix B in version 3.0, and Appendix F in version 2.2.

Happy Developing! :]

-- Jessica

09-03-2010, 09:21 PM
Thanks for the help. I think I have it figured out. Pretty simple math really.