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

On Wed, 10 Aug 2005 03:06:02 +0000, Don Klipstein wrote:

> 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.
>
Well, you _are_ the light guy. :-)

Cheers!
Rich


.

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