Dangerous knowledge

Sometimes, a little knowledge is a dangerous thing.

I know this one fella who knows a little something about electronics. When I say "a little something," I mean that he knows certain basics, but he fails to grasp a lot of the fundamental principles of electronic design.  Now, I won't claim to be an electronics master; indeed, my practical skills could use some work.  Still, I think there's a difference between needing to improve one's skills and failing to grasp fundamental principles.

For example, I once had to regulate the intensity output of a bank of LEDs.  As LEDs warm up, they tend to draw more current, which in turn changes their intensity output and their color spectrum.  This can be a problem if you want to use LEDs to achieve even, consistent illumination.

So I decided to build a current source -- that is, a device that would hold the current level more-or-less consistent.  When I told this guy of my approach, he said, "Why bother?  Just put a resistor in series with the LEDs."

I said, "No, that won't work. The resistor won't regulate the amount of current, except insofar as it will keep the current draw lower. It's a primitive approach.  Besides, we don't want to reduce the amount of current!  We need to keep the intensity level high, and adding a resistor will have the opposite effect."

"But you're not using any feedback!" he said.  "You need to add a thermal sensor of some sort, so that when the LEDs heat up, you can reduce the amount of current!"

"Of course I'm using feedback" I replied.  "That's exactly what a current source does.  As it senses a change in current draw, it adjusts its voltage output to counteract this change."

"But you need to sense the temperature!" he said.

"No, you don't" I replied.  "The temperature is significant insofar as it affects the amount of current that is drawn.  It makes more sense to regulate the current directly; after all, that is what affects the light output."

As I said, a little knowledge can be a dangerous thing.  It's dangerous when you don't have a decent grasp of the fundamentals.


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