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Hi:

This email has some wrong information.
LEDs can be operated in a pulsed mode, but for human viewing the
brightness will always be lower than with Direct Current drive. That's
due to a drop in efficiency as the LED heats up.  You can greatly
increase the peak output intensity by pulsing and it does make a
difference when using some type of photoelectric detector.

LEDs do NOT want a constant voltage, but rather they run best from a
current source.  The voltage across the LED depends on it's color and
the temperature.  If you use a voltage drive and the temperature changes
the light output will change a large amount.

The common way to drive LEDs is to use a series dropping resistor to
limit the current.  This is a good solution when power consumption is
not an issue.  But devices like PDAs or lap top computers use active
regulators to power the LED back lights without any dropping resistors
to be more efficient.

Have Fun,

Brooke Clarke, N6GCE
Products I make including a battery adapter for the SDU-5/E Emergency
Strobe Light http://www.PRC68.com



Royer, Doug wrote:

>Jared,this is good stuff.I'm going to FW it to the group.I do hope that is
>ok with you.Thanks for the info.I'll write Mon. about what I find out
>reguarding these.
>
>-----Original Message-----
>From: Jared Sherman [mailto:jared.sherman@verizon.net]
>Sent: Friday, October 10, 2003 14:32
>To: Royer, Doug
>Subject: Re: Lights etc.
>
>
>Doug-
> appear as always on and current consumption would go down.>
>
>That's the theory, somewhere between 32 and 60 cycles and the eye thinks
>they are always on. There's more of a debate about subjective/objective
>brightness, but also apparently a real time factor that an LED takes a mucj
>longer fraction of a second to come to full brightness. That is, it is "on"
>right away, but it doesn't reach full brightness for something like 1/10th
>second--way longer than the blink.
>
>I don't know, I've just heard "he said she said" on this.
>
>The main spike that gets thrwon in a boat is from the starter motor. When
>you stop cranking the starter, it continues to spin for a short time and in
>that time it is acting as a generator, so it can add +14V to the 14V on the
>power lines, spiking 28VDC for less than a second--but that's eternity for
>some solid state devices. Those caps &tc you have are good protection. I'd
>batted this around with some ham operators and electronics techs and there
>was some debate, especially since a boat may be running 10VDC (low battery
>limping home and hoping the lights are still on) to 12.8V (full battery) and
>13.8-14.4 when the alt/generator is running. In order to get optimum light
>from the LED's they want a specific voltage, not a range. So...
>
>The jury was out on whether to use a single-chip DC regulator at the LED's
>(more expensive, best controller), or just spike protection (zener diode or
>other simple device) with resistors to limit current. I was cleaning up the
>instrumentation power lead on a boat last year and wound up building a small
>box with a circuit board in it. One external fuse for the instrument power,
>two sets of overvolt protection (fast spike protection + zener diodes since
>no one would swear which one was best, and the spike protectors DO wear out
>over the years) and then a tiny buzzer that should only sound if there is an
>overvoltage, allowing us to shut the instruments manually if the electronics
>all go out or the alternator goes berserk.
>
>Unfortunately I could only find an aluminum mounting box, and LORD! does
>that shop-grade aluminum get pitted fast in a salty atmosphere.
>
>
>
>

   

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