OT: white LEDs (was: Re: Solderless breadboarding (and 68010 vs

Tony Duell ard at p850ug1.demon.co.uk
Wed Feb 24 15:06:43 CST 2010


> 
> Chuck Guzis wrote:
> 
> > There's some fellow on the web who's been making various line/mains-
> > operated lights using only a resistor in series with a single LED.
> > Weird--I would have at least paralleled the LED with another
> > resistor.  I'll try to find his web page, if anyone's interested.
> 
> 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.

If you use a 'capacitive dropper' (as they tend to be known in the UK), 
it's worth putting a high value resistor (1M or so) in parallel with the 
capacitor to discharge it when you unplug the device. Otherwise, if you 
unplug it at the right (wrong ?) point on the AC cycle, you'll get zapped 
if you touch the plug pins.

-tony




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