Science Buddies: "Ask an Expert"

Project Does not work as the LED burns out every time and i have tried to use resistors but does not stop enough electricity. Please help me my project is due this week.

caponejr - Sorry to hear about your LEDs burning out. LEDs are pretty robust, but you have to limit the DC forward current , usually by using a series resistor in the circuit. You say you have tried some series resistance, but clearly they don't appear to have sufficient resistance to prevent the burn outs. As a safe rule of thumb, assume a current of about 20 mADC to light the LED. Depending on your voltage source, the value of resistor must be calculated. The formula R=E/I will be used. For instance, if you use a 6 volt source, your seriies resistor would be R=6/.02, or 300 ohms. If your supply voltage is 9 volts, R=9/.02 or 450 ohms. Try these as starting values and you should be safe.

As an aside, this is a 'rule-of-thumb' approach because we are not taking the voltage drop of the LED into consideration. A typical red, gallium arsenide LED will have approximately 1.5 volts of Vf, or forward voltage drop. In the 9 volt example, 1.5 volts is dropped across the diode and 7.5 volts across the resistor. If you calculate the current flow with the 450 ohm resistor using I=E/R you get I=7.5/450, or about 16.6 mADC, still enough to light the LED, but you could probably get away with a lower value resistor. A blue or green LED will have a higher forward voltage drop, so this must be taken into consideration. Also, lower supply voltages will add further current reduction when using the simple formula.

About the formula, the values of V,I and R are volts, amperes and ohms respectively. That's why 20 mADC is expressed as 0.02 amperes.

Rick Marz

Hi caponejr,

I assume you are referring to this project on our site:

https://www.sciencebuddies.org/science- ... p050.shtml

rmarz gave a good overview of, in general, how you would choose a resistor based on the voltage across an LED. This project is a bit unique because the circuit diagram actually doesn't have a resistor. However, I just built the circuit and tested it, and only measured 10mA through the LED. As rmarz described, most LEDs are designed for about 20mA, so that should not be anywhere near enough to burn it out.

Did you purchase the transistor linked from Jameco Electronics in the materials tab? Are you 100% sure that you connected the circuit correctly, as shown in the breadboard diagram in the procedure? If you accidentally connect the LED directly across the terminals of the 9V battery, that WILL definitely burn the LED out.