Re: Led forward Vdrop anomaly

On Mar 20, 9:54 am, default <defa...@xxxxxxxxxxxxx> wrote:
On Thu, 19 Mar 2009 23:56:14 +0000 (UTC), d...@xxxxxxxxxxxxxx (Don





Klipstein) wrote:
In article <2qf5s49dv9phut223ovjuos61vr9bjh...@xxxxxxx>, default wrote:
I got some tricolor leds for a project from "superbrite" in Hong Kong.
The general specifications are what you'd expect typical values: RED
2V, GREEN and BLUE 3.4

The leds I got are at full current (20 ma superflux style package)
with Red 2 Volts, Green 1.7 volts, and Blue 1.1 volts (with some
variability in the green ones - one tested at 3.4 volts but it was the
exception the rest were much lower.

I don't think I've ever seen a blue led work down that low before.  Is
there some new technology afoot, or does the normal variability extend
down that low?

 If a blue LED without an internal boost converter glows with 1.1 volts
measured across it, I would suspect much higher voltage not being
indicated by your voltage measuring means.  I would question your way of
determining this voltage.

 Blue LEDs and green ones of related chemistry can drop much lower
voltages after static damage - but without glowing.

- Don Klipstein (d...@xxxxxxxxx)

I've got a breadboard with 2 AA alkaline batteries in one holder, and
a third in a separate holder - power to the board is via a SPDT center
off switch.  Can run on 3V or 4.5 volts. Absolute max is 5V for the
part I'm using but it (the pic) works from 2-5V.  

The meter might be off, but the batteries have been in a year.  The
pair measure 1.56 volts each and the extra battery 1.71 V.

The leds work well at the 3V setting, with a 1.1 volt knee at 3 or 4.5
volt, and the pic probably eats a few tenths.  I took them out of the
circuit and applied the full 20 ma and the voltage drop was still ~1.1
little higher initially, then dropped as it ran.  (in circuit I'm
running with ~3 milliamps at 3 volts and the leds are plenty bright
and except for the rogue green one, well balanced output wise -
blue/red yields magenta etc.

I'm just quoting the manufacturer they say green at 3.4 typical to 3.8
maximum, so perhaps they are using a phosphor.  

The final application will use a 4.5 volt 100 ma linear regulator and
the common anode has a 100 ohm in series, each cathode 470 ohms to a
totem pole output.  
--- Hide quoted text -

- Show quoted text -

I agree with Don, you must be measuring something wrong. With 1.1
Volts and no voltage boost, energy conservation says you can not have
a photon with more than 1.1 eV of energy, that is a frequency of
2.7E14 Hz or a wavelength of 1.1 um., somewhere in the infrared. A
phosphor is not going to help because they give off light at
wavelength that is longer than the incident photon.

George Herold
.

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