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Right resistor for high power LED for sound reader?


Rich Grise

Jan 1, 1970
I would use a fixed resistor with value at the high end of the range for
possible answers - at least 15 ohms.

Two reasons:

1. The chips in these LEDs have a nonlinearity, with efficiency being
maximized at currents in the general ballpark of 50-60% of "full current"

2. These LEDs (red Luxeons) have light output very sensitive to
temperature, with output doubling by having the junction 45 degrees C
cooler than the 25 C "characterizing temperature", and halved by having
the junction 45 C warmer than this. This means a 1 degree C temperature
change causes light output to change about 1.5%, with higher temperature
being unfavorable. So I consider it good to operate these LEDs
conservatively, and expect little to gain in light output from pushing
them with current past about 300 mA.
In addition, I recommend heatsinking them to an extent many
would call excessive.
Well, you _are_ the light guy. :)



Jan 1, 1970
Rich said:
This is the kind of question we really like around here - you gave
us what information you have, and said, "This is the goal".

Agreed. No disguised homework questions, just somebody trying to build
a circuit and put it to use.

I'm curious as to how Mitch made out. The thing was (hopefully) to
have been built and working by yesterday, Aug. 11.

M said:
Hi, I'm a projectionist in a movie theater. I need to
convert our old white light incandescent sound
"exciter" lamps over to red LED.

The entire movie industry is changing over to having
the soundtracks on film based on cyan dye, instead of
the traditional silver based soundtrack. The big
difference is that the new way is cheaper for the
studios to make. Another big difference is that almost
every movie projector in the world will need the new
red LED readers. Most have already changed over.

The soundtrack runs along one side of the film, and the
sound information is a tranparent wavy stripe which
runs down the middle of the track. The old style
soundtracks are opaque black, and the new tracks are
semi-transparent cyan blue. There is an "exciter" lamp
on one side of the film which shines through a barrel
with lenses in it, which only lets a slit of light hit
the film. On the other side of the film is a solar cell
which picks up the light information.

We have a movie starting Thursday which is our first
cyan film. I need to rig up a red LED light source
ASAP. I saw instructions on how to do this on (search using word "homebrew"), but I
need more info. I need your help in choosing a proper
resistor for the LED.

I ordered and received 2 Luxeon Star red LEDs. Here are
their specs:

Part # LXHL-MD1D

Max Current - 350ma (300ma or under may be preferable
so I won't have to add a heat sink)

Max Voltage - 3.5vdc

Color - Red

Wavelength - 625 NM

Light Dispersal - LAMBERTIAN

Typical Flux (Lumens) - 44

The incandescent exciter lamp it is replacing is rated
at 9v 4a 36w.

I've checked the power going to the light, and it is
only 7.5dc. That's OK because most film technicians set
the power lower on exciter lamps to make them last
longer. That's what I've heard, anyway. That power
supply is adjustable, but I'd prefer to leave it at
7.5v so I can still put in the old lamps whenever I
need them. The power supply is rated at 5-10vdc and 5a.

So, what kind of resistor would I use to get this LED
to work? I wouldn't want to burn it out or run it too
dim either. I've been wracking my brain trying to
figure this out, and I'm not having much luck so far.
Not sure which formulas in Ohm's Law to use.

Thank you, Mitch

spamsuckawell-wornhat at yeahwhoo dott caum
Take off the well-wornhat to respond.

I was wondering, since you're trying to use the cyan audio, does the
light source HAVE to be a red LED or could you put a red filter in
front of your incandescent lamp?

I've worked on the photo lamp house used on a Cintel (Rank) telecine
and at least for that machine, adequate light alignment is difficult.
Dolby labs makes LED replacements for the Cintel machine.