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

Tony Duell ard at p850ug1.demon.co.uk
Wed Feb 24 15:03:23 CST 2010

> the 70-LED strings get by with a resistor only.  The 100 LED strings 
> appear to use a diode.   Heaven forfend that a diode bridge be used 
> for them--that could cost additional cents per string.

Ouch!. When I was designing the white LED stroboscope, I looked at 
various LED data sheets on the web. They all emphasised the peak revese 
voltage (which was quite low -- a few volts), and said that the LED would 
be damaged if this was exceeded. Needless to say putting a resistor in 
series and applying 110V AC is going to exceed that revese voltage.

> 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.

The normal trick is to put a diode in inverse parallel. The diode only 
needs to have a PIV equal to the _forward_ voltage of the LED (think 
about it, either the diode of LED is forward biased on each half cycle), 
so just aobut any diode would do.  I'd probably use a 1N4007 since it 
would survive if the LED went open-circut, and anyway, I have them in 
stock :-)


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