Re: Right resistor for high power LED for sound reader?

In article <11fidkn52p9255a@xxxxxxxxxxxxxxxxxx>, M wrote:
>Tom Biasi wrote:
>> "M" <m@xxxxx> wrote in message news:11fgpf8849kds8d@xxxxxxxxxxxxxxxxxxxxx
>>
>>>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 www.film-tech.com (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.
>>
>>
>>
>>
>> Hi,
>> You have a device (LED) that will drop 3.5 volts at 300 mA.
>> Your supply is 7.5 volts. You need a resistor that will drop 4.0 volts at
>> 300 mA. R=E/I
>> P=EI, Closest standard values 10-15 Ohms 2 Watt.
>> Tom
>
>Thanks Tom! So your saying that:
>
>4V / 300mA = 13.33 Ohms, and
>4V * 300mA = 1.2W
>
>and the only resistor I can use is a standard 10-15
>Ohms 2 Watt. Can I use any kind of variable resistor
>and set it at 13.33 Ohms? If I can, what kind would be
>best? Would this be a waste of time, and would the
>fixed value resistor be fine?
>
>I know this is really basic stuff for you people, but
>this is the first time I've ever done this.

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.

- Don Klipstein (don@xxxxxxxxx)
.

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