by Butch Shadwell
We bought a new 61” HDTV last Christmas. It is the DLP type, though it has some interesting innovations. Instead of the usual projector lamp and color wheel for the light source, this set uses three high output LEDs (red, green, and blue). No color wheel and motor and no projector lamp to replace. Incase you are not familiar with DLP TVs, DLP stands for digital light processing. It is based on the use of a MEMS (micro electro mechanical system) device with 2,073,600 tiny mirrors that can be electro-statically moved. The mirrors are adjusted 360 times per second successively displaying the red, green, or blue components of the image, so that we get up to 120 complete display frames per second. The amount of red, green or blue light that is added to an individual pixel (picture element) by its respective mirror, is controlled by how long the mirror allows that color light to project to the screen. Each mirror pulse width modulates the light to control how much red is blended with how much green and blue. Your eye integrates these pulsing light sources into 10,000 levels of brightness for each of the three color components.Let’s say that my high output LEDs have a forward voltage drop of 4 volts at 25 degrees C at the junction and the forward voltage drops 3mV per degree C. Then I supply forward current to one of these LEDS through a 100 ohm resistor and a 10VDC source. So tell me the current through the LED when the junction gets up to 100 degrees C? I know this is a simple one, but I have been struggling with writers block. Good luck.
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The names of correct respondents may be mentioned in the solution column.
* We seemed to have gotten behind on the challenges and answers, so here is the last few. As of September, the July edition was the last one that Butch had sent out. Figure that one out:) - Webmaster.
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Brain Teaser Solution - July
Last month’s BTC was a bit easier than I like, but I was having a worse time than usual coming up with an appropriate problem. I decided to ask about LED biasing as follows: “…LEDs have a forward voltage drop of 4 volts at 25 degrees C at the junction and the forward voltage drops 3mV per degree C. Then I supply forward current to one of these LEDS through a 100 ohm resistor and a 10VDC source. So tell me the current through the LED when the junction gets up to 100 degrees C?”
Though the problem was very easy, it does point out a common design error. LEDs are usually biased through a current limiting resistor. When a higher voltage supply is available, there are often multiple LEDS in series biased by a resistor. Here is the error. When you have many LEDs in series, and they all have negative temperature coefficients, as the temperature rises the total forward voltage changes by the sum of all of the individual LED drops. This will cause more current to flow through the LEDs (and so on) and you can get a thermal runaway condition. In this example a 75 degree rise in junction temperature causes a 0.225 volt drop in the LED forward voltage. So you get 6.225 volts across a 100 ohm resistor, resulting in 62.3mA of current. But I bet you already knew that.
-- Butch Shadwell
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