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RSS >  Led Voltage, Do I need a resistor?
jee #1 30/11/2010 - 03h32

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http://uk.farnell.com/jsp/displayProduct.jsp?sku=1212738&CMP=e-2072-00001000

I found these IR LED (link above) and in the description it states: "Supply Voltage Range:1.5V to 3.8V".

Being the top range 3.8v, does it mean that I do not need to install a resistor?
Or would it work negatively towards the emitters' life expectancy?
Or, than again, I do need a resistor and cut the power to 4.5v anyway?

Thank you in advance for the reply,
your help is much appreciated.
Sasso #2 30/11/2010 - 13h15

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well thats saying what the supply can be, what you need to look at is the Vf=voltage forward and the Forward current.

on this it say the Max Vf is 3v and max current is 100mA so you dont want to be running these at max.

also might help if we knew what supply you were using ;)

basically need to use this equation to work out the resistance needed

R = (Vs - Vf) / I

R=resistance, Vs=voltage from source, Vf=Forward voltage of LED and I=desired current of LED

so Vs what ever your source is, Vf... well a value lower than 3volts xD, and I should be something around the 50-90mA range as you don'#t want to run it at max.

think im right in what ive said but if someone spots a mistake please shout out so we all can learn :)
jee #3 30/11/2010 - 13h26

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Awesome, thanks for the equation and the clarification on forward voltage sasso, I'll be looking for a resistor straight away.

I was going to use a 6v 300mA power supply such as this one:
http://uk.farnell.com/jsp/displayProduct.jsp?sku=1176285&CMP=e-2072-00001000

EDIT: Found 'em  :D

http://www.t2retail.co.uk/Electronic_Components_Resistors/15_Ohm_1_2W_5_Carbon_Film_Resistor_5_Pack

EDIT 2: Well, I wanted to buy the resistors from the same website as all the rest but there are lots of different kinds, the search options are a lot and I do not understand them really.

I think I'll buy the 6th from the top of the list here below (RESISTOR, CARBON COMP, 1/4W 15R) and cross my fingers hoping it will work.

http://uk.farnell.com/jsp/search/browse.jsp?N=500006+1000291+294595&Ntk=gensearch_001&Ntt=15+ohm+resistor&Ntx=mode+matchallpartial&No=0&getResults=true&appliedparametrics=true&locale=en_UK&catalogId=&prevNValues=500006+1000291&filtersHidden=false&appliedHidden=false&originalQueryURL=/jsp/search/browse.jsp%3FN%3D500006%2B1000291%26Ntk%3Dgensearch_001%26Ntt%3D15%2Bohm%2Bresistor%26Ntx%3Dmode%2Bmatchallpartial%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26catalogId%3D%26prevNValues%3D500006%2B1000291
Edited by jee on 30/11/2010 at 15h05.
Sasso #4 01/12/2010 - 01h24

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have you also looked on the main website there is an LED circuit generator... basically asks what is the supply voltage, vf, I and tells you what you need :)

also 6v 300mA..... is 300mA the capacity of it? because depending on how its arranged you might only last 1 hour before needing to charge xD
jee #5 01/12/2010 - 13h29

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Actually I haven't, I'm gonna check it out.

I already know that I need a 15ohm resistor, is just that there are so many of different ratings, materials, tolerance, etc. that gets me a little confused xD.

The power supply will be connected directly to the main of the house and letting out 6v 300mA so there's no problem of recharging, though the idea of having batteries attached is more appealing.

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