DIYbanter - View Single Post - Why aren't many / most LED light bulbs dimmable?

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On Sat, 25 Dec 2010 21:59:15 -0600, z wrote:

On Sat, 25 Dec 2010 22:29:03 -0500, wrote:

On Sat, 25 Dec 2010 20:04:00 -0600, "
wrote:

Try it. You'll find that you are *very* wrong. LEDs are *not* in any way
linear. You'll also find that the efficiency goes down as you lower the
brightness (the resistor takes more of the line voltage).

Why do you think the voltage across the resistor changes?

Because it does?

You have a
certain voltage dropped across each junction and the resistor takes
the rest.

sorry, new netbook and keyboard/trackpad is driving me nuts

No, that would assume an ideal diode. They are not ideal, the I-V
curve is not a vertical line. The more diodes you put in series and
the lower the voltage across the resistor, the more pronounced this
becomes. From some recent work, a blue LED at about 5mA drops about
3V. At 20mA the drop is closer to 3.3V. Now, put thirty of these in a
string and the difference is 10V. You only have 20V across the
resistor - it's changed 50%. ...and this is quite nonlinear.

The resistor is a current regulator not a voltage regulator.

A resistor regulates nothing. R==V/I. If you say that I is constant
because V is constant and V is constant because I is constant, you're
getting nowhere. ;-).