Sunday, August 22, 2010

Measuring the Characteristic Curve for an LED

Last month I was forced to break hard to avoid hitting a doe on the road at 3 am one morning near Oakhurst, CA and the developed a "shimmy" or jerking motion in the steering afterwards. I took the car in to have the front end realigned and put new tires on and now its back to normal.

While waiting for the repairs to be done I picked up some LEDs at RadioShack. There wasn't much information on the LEDs except forward voltage 2.1 V, current 30 mA, luminous intensity 630 mcd, and wavelength 565 nm. So I tried to measure the characteristic curve using two 1.5 V AA batteries, a resistance box and a digital multimeter to measure the current but the curves proved to be useless even though I corrected for the voltage drop across the resistance box. I decided to get a second digital multimeter and this time measured both the voltage across and current through the LED. The results were better.

The function for the characteristic curve is empirical and gives the current in milliamps. The -1 is needed so that the current will be 0 mA when the voltage is 0 V. A cubic exponent gives a good fit over the range of the voltages measured.

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