OT: white LEDs (was: Re: Solderless breadboarding (and 68010 vs
hilpert at cs.ubc.ca
Wed Feb 24 02:30:42 CST 2010
Dave McGuire wrote:
> On Feb 23, 2010, at 9:14 PM, Brent Hilpert wrote:
> > I have on occasion run single LEDs from line voltage by paralleling
> > the LED
> > with a diode, then putting the pair in series with a C and R. C is
> > calculated
> > to provide most of the voltage drop via capacitive reactance, a
> > small R is
> > still present to limit inrush current. The diode limits the reverse
> > voltage and
> > permits bidirectional current flow so the cap sees AC.
> > Power consumption and heat dissipation are reduced compared to a
> > solution using
> > only R, and a power resistor is not needed.
> > I haven't tested the circuit over many on/off cycles though, to
> > find out
> > whether perhaps the peak in-rush current reduces LED life.
> Wow...that's pretty impressive work! Elegant in its simplicity.
I just looked up my old notes and tried it again:
- two LEDs in reverse parallel, or LED and diode
- C = 0.22 uF, 200V
- R ~= 680 to 1K, 1/4W
This targets around 10mA at 120VAC.
~0.2W consumption. Far more efficient than the straight R solution which
consumes around 1.2W, and hence 2W resistor.
Being pedantic, the power company might not like it as it adds reactive
(out-of-phase, power factor) current to the grid. On the other hand, they might
like it as it is capacitive reactance which helps counteract all the inductive
reactance from all those little unloaded wall-warts and transformers people
I think I've seen such a circuit used in manufactured equipment too, but I
can't remember what.
More information about the cctalk