🚀 go-pugleaf

In article <p8ivpu4vt7apo04r6thdrufutn3l7mqc4a@4ax.com>, 
JoeBloe@thebarattheendoftheuniverse.org talked about...
> On Fri, 04 Oct 2002 10:51:27 -0700, Lizard Blizzard <NOSPAM@rsccd.org>
> Gave us:
> 
> >Keith Wootten wrote:
> >> In message <3d9d452f.4128065@news.xs4all.nl>, "Wouter van Ooijen 
> >> (www.voti.nl)" <wouter@voti.nl> writes
> >
> >>>> Have assembled an array of LEDs for use as a 3rd brake light in my car,
> >>>> configured as 4 rows of 9 jumbo LEDs, as of yet unwired. Of course 
> >>>> the power
> >>>> supply will be 12 volts nominal (13.8 volts, maximum).
> >
> >>>> What is the best method to power these from an intermittent (whenever 
> >>>> I press
> >>>> the brake pedal) power source?
> >
> >>>
> >>> The crucial point is that when you put a resistor and N LEDs in
> >>> series, the LEDs will take roughly a constant voltage so all voltage
> >>> variation from your battery will be over the resistor, thus causing
> >>> current variation.
> >
> >
> >> <Snip>
> >
> >> You can mitigate this to a useful extent by using a small filament bulb 
> >> instead of a resistor.  The positive temperature coefficient of the bulb 
> >> will help to keep the current variation reasonable as the supply voltage 
> >> changes.  The bulb should be selected to run at more or less normal 
> >> brightness at maximum supply voltage while passing maximum LED current.
> >
> >
> >> My bicycle rear light is an array of 50 LEDs from RS components which 
> >> are arranged in a series/parallel configuration with a nominal Vf of 9V5 
> >> IIRC.  Using two small paralleled 5V lamps (can't remember the type - 
> >> selected from a junk box) the current swings 2:1 for a 14V-10V supply.
> >
> >> In my case, it's also a useful low battery warning - the filament lamp 
> >> goes out long before the LEDs.
> >
> >> Cheers
> >
> >Using a lamp to serve as a more or less constant current 'ballast' is a 
> >good idea, but if it is run at anywhere near normal brightness, the 
> >lamp's lifetime will be short in relation to the LEDs.  And of course 
> >when the lamp goes open, the LEDs no longer get current.  So it would be 
> >much wiser to use it at much lower than normal brightness so that the 
> >lifetime is extended considerably, comparable to the LEDs.
> 
> 
>   Not only that, but they emit IR like wild when they are at "normal
> brightness"!  Doh!  Does nobody care that the enclosure is going to
> pull a Three Mile Island here?
 
See my previous post re your lame statement.


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