Right resistor for high power LED for sound reader?

[M]

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.

 

[Tom Biasi]

"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...

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

 

[M]

Tom Biasi wrote:

"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...

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.

Thanks, Mitch

---
spamsuckaWell-Worn-Hat at yeahhoo dott caum
Take off the Well-Worn-Hat to respond.

 

[redbelly]

M wrote:

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.

Thanks, Mitch

Mitch,

Best to keep your life simple. Just use 15 ohms, and don't worry about
messing around with potentiometer settings. You'll be well below the
max current (around 270 mA), so burning the thing out shouldn't be a
concern, and you'll still get a reasonable amount of light output.

Regards & good luck,

Mark

 

[Tom Biasi]

"M" <m@m.com> wrote in message news:11fidkn52p9255a@corp.supernews.com...

Tom Biasi wrote:
"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...

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.

Thanks, Mitch

---
spamsuckaWell-Worn-Hat at yeahhoo dott caum
Take off the Well-Worn-Hat to respond.

I agree with Mark (AKA redbelly).
Keep it simple. There is no need to set the resistance to 13.33 ohms. Any
resistor between 10 and 15 ohms will be fine for this application. Two watt
resistors are easy to come by so I would use one.
Tom

 

[Don Klipstein]

In article <11fgpf8849kds8d@corp.supernews.com>, M wrote:

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.

Assuming your power supply will be or can easily be set to your favored
7.5 volts DC when drawing only 300-350 mA, then:

The LED typical voltage drop is 2.95 volts according to the "DS23"
datasheet available from the Lumileds website.

Subtract this from 7.5 volts and this leaves 4.55 volts across the
dropping resistor. Plus .64 volt, minus .56 volt tolerance worked out
from subtracting the minimum and maximum voltage drops mentioned in this
datasheet.

To get 350 mA with 4.55 volts across the dropping resistor: Divide 4.55
by .35, and you get 13 ohms. The next common value up is 15 ohms,
although 13 ohms is a semi-standard value. Power dissipation will
typically be 4.55 volts times .35 amp, or about 1.59 watts. I would use a
5 watt "sandstone" style 15 ohm resistor.

To be extra conservative, let's use 300 mA (.3 amp) and 2.25 volts LED
voltage (minimum at .35 amp is 2.31 volts), leaving a worst case of 5.25
volts across the droping resistor assuming a 7.5V supply.

5.25 volts divided by .3 amps is 17.5 ohms. The next higher "standard"
resistor value is 18 ohms. Again, I recommend the 5 watt "sandstone"
type.

For "bad worst case" designing, use 22 ohms 5 watt rectangular sandstone
style. Expect typical LED current then of about 230 mA with a 7.5V
supply. And the resistor will typically be dissipating 1.15 watts, which
is enough to get a 5 watt rectangular sandstone style resistor very warm.

Another note: Performance of Lumileds LEDs is with the "junction
temperature" at 25 degrees C, and the red one has quite a high sensitivity
of light output to temperature. The DS23 datasheet gives a thermal
resistance of 23 degrees C per watt for this model, and at 2.95 volts .35
amp (1.03 watt) expect to require a heatsink temperature of 1-2 degrees C
to achieve this! And this red model has output slightly less than 70% of
"full" when the "junction temperature" is 25 degrees C warmer than this!

Not that I don't think it will work, but I advise to drive these things
conservatively, heatsink them adequately to excessively, and make your
expectations of light output realistic - I would say 30 lumens at most,
plan on 25 lumens even with a heatsink, and less if you have a
worse-than-average LED - worst case is about 30.4% below "typical"
according to the datasheet. This may well be plenty good for your
application however!

- Don Klipstein (don@misty.com)

 

[Don Klipstein]

In article <11fidkn52p9255a@corp.supernews.com>, M wrote:

Tom Biasi wrote:
"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...

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@misty.com)

 

[Tom LeMense]

"Don Klipstein" <don@manx.misty.com> wrote in message
news:slrndfir51.1vo.don@manx.misty.com...

In article <11fgpf8849kds8d@corp.supernews.com>, M wrote:
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.
[snip]
Assuming your power supply will be or can easily be set to your favored
7.5 volts DC when drawing only 300-350 mA, then:

....and let's hope that that's a fairly well regulated supply, at that: I
would imagine that any variations in the intensity of the LED (due to line
regulation) would show up as audible noise!

Not that I don't think it will work, but I advise to drive these things
conservatively, heatsink them adequately to excessively, and make your
expectations of light output realistic - I would say 30 lumens at most,
plan on 25 lumens even with a heatsink, and less if you have a
worse-than-average LED - worst case is about 30.4% below "typical"
according to the datasheet. This may well be plenty good for your
application however!

I've worked with the Luxeon Star LEDs and I heartily recommend a heatsink.
The LED life goes down very quickly as Tj rises, so keeping it cool will
improve the reliability of your projector retrofits.

I have a red Luxeon Star on an old i486 (passive) heatsink and that seems to
work pretty well. I also didn't want to worry about a regulated supply, so
I built a "constant current regulator" onto the same heatsink using an LM317
adjustable regulator IC. The circuit is right out of the LM317 datasheet
(see figure 26 of the On Semi datasheet located at
http://www.onsemi.com/pub/Collateral/LM317-D.PDF ) and I used a 1W 3.6 ohm
resistor to set the current at approx 300mA. The benefit of this circuit is
that the LED will be fed with a constant 300mA current without depending on
the input voltage. Not bad for a three component circuit (counting the
LED)!

TJL

 

[M]

Tom Biasi wrote:

"M" <m@m.com> wrote in message news:11fidkn52p9255a@corp.supernews.com...

Tom Biasi wrote:

"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...


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.

Thanks, Mitch

---
spamsuckaWell-Worn-Hat at yeahhoo dott caum
Take off the Well-Worn-Hat to respond.


I agree with Mark (AKA redbelly).
Keep it simple. There is no need to set the resistance to 13.33 ohms. Any
resistor between 10 and 15 ohms will be fine for this application. Two watt
resistors are easy to come by so I would use one.
Tom

Oh man, thanks guys! That's what I was hoping to hear!
I'm going to have at least 300 people counting on me to
put on a good show, so it's a big deal. I've been
trying to figure this out for myself and am making some
small progress. I'm just really stuck for time and
appreciate your help a lot. This is my very first foray
into the world of electronics.

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Someone told me it has to do with "Kirchoff's Law." I'm
trying to learn about that from some badly written web
pages. It's not making a dent yet.

-Mitch

---

spamsuckaWell-Worn-Hat at yeahhoo dott caum
Take off the Well-Worn-Hat to respond.

 

[Mitch]

Tom LeMense wrote:

"Don Klipstein" <don@manx.misty.com> wrote in message
news:slrndfir51.1vo.don@manx.misty.com...

In article <11fgpf8849kds8d@corp.supernews.com>, M wrote:

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.

[snip]

Assuming your power supply will be or can easily be set to your favored
7.5 volts DC when drawing only 300-350 mA, then:


...and let's hope that that's a fairly well regulated supply, at that: I
would imagine that any variations in the intensity of the LED (due to line
regulation) would show up as audible noise!


Not that I don't think it will work, but I advise to drive these things
conservatively, heatsink them adequately to excessively, and make your
expectations of light output realistic - I would say 30 lumens at most,
plan on 25 lumens even with a heatsink, and less if you have a
worse-than-average LED - worst case is about 30.4% below "typical"
according to the datasheet. This may well be plenty good for your
application however!


I've worked with the Luxeon Star LEDs and I heartily recommend a heatsink.
The LED life goes down very quickly as Tj rises, so keeping it cool will
improve the reliability of your projector retrofits.

I have a red Luxeon Star on an old i486 (passive) heatsink and that seems to
work pretty well. I also didn't want to worry about a regulated supply, so
I built a "constant current regulator" onto the same heatsink using an LM317
adjustable regulator IC. The circuit is right out of the LM317 datasheet
(see figure 26 of the On Semi datasheet located at
http://www.onsemi.com/pub/Collateral/LM317-D.PDF ) and I used a 1W 3.6 ohm
resistor to set the current at approx 300mA. The benefit of this circuit is
that the LED will be fed with a constant 300mA current without depending on
the input voltage. Not bad for a three component circuit (counting the
LED)!

TJL

Excellent info! I can dig some heatsinks out of my old
computers. The power supply is a Kelmar 8604-B 5-10vdc
5a. It is made for sound "exciter" lamps, so I'm fairly
certain it is regulated.

I also got 4 LXHL-LD3C Luxeon Star III LEDs to
experiment with. They are 140 max lumens, 3.51v 1400ma.
If I can figure out the proper resistors tomorrow
morning, I may make the lamp with those instead. I was
just planning on using the older LXHL-MD1Ds because
others had done it and it seemed safer to follow in
their footsteps.

 

[John Fields]

On Wed, 10 Aug 2005 22:51:56 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

---
No, you don't. You get a 15 ohm 2 watt resistor.
---

Put four 10 ohm 1 watt ones in series-parallel 2-by-2 arrangement and
you get a 10 ohm 4 watt resistor.

With a 2-by-2 series-parallel arrangement of four identical resistors,
it does not matter whether or not you put two parallel pairs in series or
put two series pairs in parallel.

--
John Fields
Professional Circuit Designer

 

[Don Klipstein]

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

Put four 10 ohm 1 watt ones in series-parallel 2-by-2 arrangement and
you get a 10 ohm 4 watt resistor.

With a 2-by-2 series-parallel arrangement of four identical resistors,
it does not matter whether or not you put two parallel pairs in series or
put two series pairs in parallel.

- Don Klipstein (don@misty.com)

 

[John Fields]

On Tue, 09 Aug 2005 20:46:42 -0700, M <m@m.com> wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

---

Two 10 ohm half watters in series, in parallel with two 10 ohm half
watters in series, like this:


+---[10R]-----[10R]---+
| |
+---[10R]-----[10R]---+

|<--10ohms, 2 watts-->|


Or, two 10 ohm half watters in parallel, in series with two 10 ohm
half watters in parallel, like this:



+---[10R]--+--[10R]---+
| | |
+---[10R]--+--[10R]---+

|<--10ohms, 2 watts-->|


--
John Fields
Professional Circuit Designer

 

[John Fields]

On Wed, 10 Aug 2005 18:37:23 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Tue, 09 Aug 2005 20:46:42 -0700, M <m@m.com> wrote:


I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

---

Two 10 ohm half watters in series, in parallel with two 10 ohm half
watters in series, like this:


+---[10R]-----[10R]---+
| |
+---[10R]-----[10R]---+

|<--10ohms, 2 watts-->|


Or, two 10 ohm half watters in parallel, in series with two 10 ohm
half watters in parallel, like this:



+---[10R]--+--[10R]---+
| | |
+---[10R]--+--[10R]---+

|<--10ohms, 2 watts-->|

If the're out of 10 ohm half watters when you get there you can do
this:

+---[15R]--+--[15R]---+
| |
+---[15R]--+--[15R]---+
| |
+---[15R]--+--[15R]---+

|<--10ohms, 3 watts-->|

Or this:

+---[15R]--+--[15R]---+
| | |
+---[15R]--+--[15R]---+
| | |
+---[15R]--+--[15R]---+

|<--10ohms, 3 watts-->|


--
John Fields
Professional Circuit Designer

 

[John Fields]

On Wed, 10 Aug 2005 18:25:37 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 10 Aug 2005 22:51:56 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

---
No, you don't. You get a 15 ohm 2 watt resistor.
---

---
Oops...

You get a 15 ohm 1.5W resistor.


+--[10R]--+--[10R]---
| |
+--[10R]--+

equals

---[5R]---[10R]---

so, since:


P = I˛R

rearranging and solving for the current in the 10 ohm resistor when
it's dissipating 1 watt:

P 1W
I = sqrt ---(PR) = sqrt ----- = 0.316 amperes
R 10R


Now, with 316mA through 5 ohms,


P = I˛R = 0.316A˛ * 5R = 0.5W

So, since the 10 ohm resistor is dissipating one watt and the set of
paralleled resistors is dissipating half a watt, the array is
dissipating 1.5 watts.

--
John Fields
Professional Circuit Designer

 

[John Fields]

On Wed, 10 Aug 2005 19:02:51 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 10 Aug 2005 18:25:37 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 10 Aug 2005 22:51:56 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

---
No, you don't. You get a 15 ohm 2 watt resistor.
---

---
Oops...

You get a 15 ohm 1.5W resistor.


+--[10R]--+--[10R]---
| |
+--[10R]--+

equals

---[5R]---[10R]---

so, since:


P = I˛R

rearranging and solving for the current in the 10 ohm resistor when
it's dissipating 1 watt:

P 1W
I = sqrt ---(PR) = sqrt ----- = 0.316 amperes
R 10R
^

--- |
Oops again... (PR)----+ shouldn't be there.
---

Now, with 316mA through 5 ohms,


P = I˛R = 0.316A˛ * 5R = 0.5W

So, since the 10 ohm resistor is dissipating one watt and the set of
paralleled resistors is dissipating half a watt, the array is
dissipating 1.5 watts.

--
John Fields
Professional Circuit Designer

 

[Don Klipstein]

In article <4q2lf1hvoii78n42jqoqhto6fj23ec42bg@4ax.com>, John Fields wrote:

On Wed, 10 Aug 2005 22:51:56 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

---
No, you don't. You get a 15 ohm 2 watt resistor.

Oops, better keep my eye on the screen better. It's obviously 15 ohms,
and that's what I meant to type.

- Don (don@misty.com)

 

[Don Klipstein]

In article <qi4lf1pa68potjgkt3ep6oj58jlbshqp59@4ax.com>, John Fields wrote:

On Wed, 10 Aug 2005 18:25:37 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 10 Aug 2005 22:51:56 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <11fiu5hmnlvb544@corp.supernews.com>, M wrote:

I live in a small town and went to the local Radio
Shack. They don't have any 2 watt resistors. They have
15 ohm 1/2 watt, 10 ohm 1/2 watt, and 10 ohm 1 watt.
What combinations of common resistors can I put
together to get a 10 ohm 2 watt?

Four 15 ohm ones in a 2-by-2 series-parallel arrangement is a 15 ohm 2
watt resistor.

Put two 10 ohm 1 watt ones in parallel with each other and put that
combo in series with a third 10 ohm one, and you get a 10 ohm 2 watt
resistor.

---
No, you don't. You get a 15 ohm 2 watt resistor.
---

---
Oops...

You get a 15 ohm 1.5W resistor.


+--[10R]--+--[10R]---
| |
+--[10R]--+

equals

---[5R]---[10R]---

so, since:


P = I˛R

rearranging and solving for the current in the 10 ohm resistor when
it's dissipating 1 watt:

P 1W
I = sqrt ---(PR) = sqrt ----- = 0.316 amperes
R 10R


Now, with 316mA through 5 ohms,


P = I˛R = 0.316A˛ * 5R = 0.5W

So, since the 10 ohm resistor is dissipating one watt and the set of
paralleled resistors is dissipating half a watt, the array is
dissipating 1.5 watts.

Thanks... I did screw that one up.

- Don (don@misty.com)

 

[Rich Grise]

On Tue, 09 Aug 2005 16:04:42 -0700, M wrote:

Tom Biasi wrote:
"M" <m@m.com> wrote in message news:11fgpf8849kds8d@corp.supernews.com...

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 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

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.

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

Yes, a power rheostat (variable resistor) would work here, but
a new one would be astronomical:
http://dkc3.digikey.com/PDF/T052/1102-1103.pdf for an example.

But you should be able to find a 12 ohm, 2 watt resistor! I'm
sure that will be fine.
http://dkc3.digikey.com/PDF/T052/1078.pdf

Break a Leg!
Rich

 

[Rich Grise]

On Thu, 11 Aug 2005 01:10:04 +0000, Don Klipstein wrote:

In article <qi4lf1pa68potjgkt3ep6oj58jlbshqp59@4ax.com>, John Fields wrote:
....
P = I˛R = 0.316A˛ * 5R = 0.5W

So, since the 10 ohm resistor is dissipating one watt and the set of
paralleled resistors is dissipating half a watt, the array is
dissipating 1.5 watts.

Thanks... I did screw that one up.

But Ten Ohms will fry his LED! Take SIX pcs. 10 ohm, two watt, and
put them 3x2 for 15 ohms at SIX watts!

Cheers!
Rich