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

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On Sun, 26 Dec 2010 00:45:29 -0500, wrote:

On Sat, 25 Dec 2010 21:56:12 -0600, z wrote:

On Sat, 25 Dec 2010 22:26:43 -0500, wrote:

On Sat, 25 Dec 2010 19:56:44 -0600, "
wrote:

On Sat, 25 Dec 2010 11:48:23 -0500, wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, "
wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

How do you "just vary the current" efficiently, without chopping it? It
doesn't have to be a lot faster but you're right, 60Hz doesn't cut it IMO.

Since the currents are so low, a rheostat may not be out of the
question.

Huh? Sure they are. You might just as well use incandescent lights.

OK you have a string of LEDs dropping about 95% of the line voltage
and a resistor dropping the rest and limiting current now. How can
making that resistance more by adding a rheostat in series be more
inefficient?

Resistors heat == inefficient

But the resistor will always be there. You are just making a bigger
resistor, the current will drop and the light will dim in a vary
linear way.
The voltage you drop across your resistor will be the same no matter
how big it is. That is not like a rheostat on an incandescent where
you are changing the voltage applied todrop the filament.

The voltage across the resistor *does* change. Also, P=I^2R.
The voltage drop across a diode is relatively constant, so the
voltage across the resistor has to also be relatively constant, with
only the current being changed by the change of resistance. This is
not 100% accurate, but for this discussion I believe it is close
enough.

It is *NOT* "relatively constant" when you multiply the change times
the number of LEDs in a 120V string and compare that to the voltage
across the balast resistor. If you string a few together and have a
large balast resistor it matters less but you're simply wasting that
much more power, losing gains you made by using LEDs in the forst
place.

Not agueing with krw

Of course not. You could *NEVER* admit that you're wrong.

- just agreeing (to a point) with gfretwell. I
say "to a point" because it is not totally linear. Much more linear
than some would have you believe in a DC circuit.

Of course you would be WRONG, as usual.