what size resistors for LED's?

[emaxxjeremy]

ok i know its simple, but for some reason i'm just not thibking i'm correct. granted its been 15 plus years since electronics class.
i have LED's rated @ 3.6v and i am powering them with a 4.5v supply. what size resistor do i need? i wanna say 25 ohm-ish

[JohnRobHolmes]

This might help http://www.electronics2000.co.uk/calc/calcled.htm

What is the forward current of them?

[TLTCrawlin]

125-150---175 ohms, depending on the brightness.
^.....^......^
Low mid-Hi Ultra brite

Id say go 150ohm resistor (I did science last year in eight grade and we studied on resistors and ohms for a long time!)

[skellyo]

For the sake of learning, here's how it's done:

Supply Voltage - LED Forward Voltage = Voltage Drop Across Resistor

Voltage Drop Across Resistor / LED Forward Current = Resistor Value (In Ohms)

So, if the LED you are using is say 20mA of forward current, it goes:

4.5 - 3.6 = 0.9

0.9 / 0.020 = 45 ohms

A 25ohm resistor would come out to 36mA. Typically, the more current, the brighter the LED, so long as you're within the limits of the LED.

[TwistedCreations]

here ya go....... easy and simple.. click wiring diagram and it will tell ya how to wire them up

http://led.linear1.org/led.wiz

[TLTCrawlin]

Originally Posted by skellyo

For the sake of learning, here's how it's done:

Supply Voltage - LED Forward Voltage = Voltage Drop Across Resistor

Voltage Drop Across Resistor / LED Forward Current = Resistor Value (In Ohms)

So, if the LED you are using is say 20mA of forward current, it goes:

4.5 - 3.6 = 0.9

0.9 / 0.020 = 45 ohms

A 25ohm resistor would come out to 36mA. Typically, the more current, the brighter the LED, so long as you're within the limits of the LED.

I edited my post, my first post said 50ohms but I saw Johnrobs post and hes got more knowledge so I went off that site!

[emaxxjeremy]

ya thats what i was looking for...an idiot calculator.
its funny i came across my old electronics notebook from YEARS ago and its amazing all that i have forgotten. it sucks getting old!

[maxboostbusa]

It also depends on supply voltage. Alot of the leds are just over 1 volt but others are 3 volts and some are 6 volts. You can buy them that the impedience is already set for a 12 volt system. I have used alot and majored in electronics design. The 3800's I used in my truck are for 1.3 volt and running off a 9 volt battery. The resister I am using is a 470 ohm. You could probibly get by with a 400 but I dont like having to redo stuff because I over amped it and the led blew up or shorted. Although when you overvoltage one and it blows up its cool. Just my 2 pennys for what its worth. Just check your specs when you buy them. Jameco is an online company that has killer prices on things like that. Radio shack 5 resistors 2 bucks and change. Jameco 100 for a buck and some pennys.

[BlueMonster]

post something like that without giving us a link!!!

It's funny, I logged on today specifically to ask this exact question!

[TwistedCreations]

I bought all mine from LSdiodes.com

[BlueMonster]

Twisted, you rock!

I paid $28 for 5 bright whites 5mm at Radio Shack today, I am ordering 20 for $15 from LSD! Good thing I haven't opened the RS ones yet!

[maxboostbusa]

radio cracks prices blow. They dont have a selection like they used to either. It should be called Home electronics, cell phone shack lol

[emaxxjeremy]

Originally Posted by maxboostbusa

radio cracks prices blow. They dont have a selection like they used to either. It should be called Home electronics, cell phone shack lol

ya they are down to like 1 small drawer cabinet for ALL components