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

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

.

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