Acorn Technology

908 Devonshire Blvd., Brighton, Michigan 48116

kelvinshih@comcast.net

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Principles of operation.

The forward drop of LED is function of drive current and junction temperature. At constant current of 100mA the forward drop of LED is only a function of junction temperature. Because of the very small current self heating effect of LED is negligible.

Regardless the color of LEDs the T.C. (temperature coefficient) of LED is normally between        -1.5mV/^OC to -2.5mV/^OC.

Two LEDs are selected to do the testing. One LED is connected to the reference terminals to measure the ambient temperature and the other LED is connected to the test terminals. The calibrate/run switch should be at calibrate position. The reference LED measures the ambient temperature and the test LED measures the temperature rise.

When both LEDs are seeing ambient temperature DT=0. We adjust the zero potentiometer to read 0 at the center temperature meter.

Place the test LED in an environmental chamber and increase the test LED temperature to 30.0^OC above the ambient. And adjust the gain potentiometer so the DT center meter read 30.0.

Now adjust the drive current and flip the calibrate/run switch to run. When this switch is at run position 99ms out of 100 ms the drive current pass through the LED. At the remaining 1 ms 100mA is flowing through the LED. And the forward drop represents temperature is only measured during that 1 ms time and displayed as temperature in ^OC.

The forward drop displayed at right meter is the average voltage of LED whet the drive current pass through LED.

The following table is the measurement results of a red LED with copper lead frame.