Electronics LED array help pls!

Hi, im going to be doing a keyboard mod to make it glow underneath the keys using LEDs, I have drawn up a circuit diagram for it, but im not sure about the Resistor value i need.

Here is the circuit:

12v
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|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|
|-->|-->|-->|-->|--|--- Gnd


edit : redrew it a bit better



the LEDs are high brightness 120mcd 3mm LEDs, and are rated at Current 20mA and Voltage 1.7V

so acording to my calculations

4x 1.7v = 6.8v 11x20mA=220mA

R=V/I R= 6.8/.22 R= 30.9 so i need a resistor of about 35ohms.

that all seemed ok, but then i looked at the resistor used in this exact same mod on www.heatseekerz.net and they used a much larger (about 500ohms i think it was) for a almost identical array.

so my question is, what Ohms resistor do i need?

cheers,
jamicon

 

yeah u want the total current to be 20 ma still not 220 ma

 

Based on your supplied parameters the resistor needed is 23.63R, the nearest real value is 22 Ohms.

BTW the voltage across the resistor is not 6.8 but 12-6.8=5.2V

 

right the current is correct unlike i stated abv the resistor value stated by macroman is also correct but if the version at heatseekerz works then id use a 200ohm(it is on their site) and theres only 10 led strings so its 200 mA 12 v
12/0.02 = 60 ohm wich is what i would have used but it may be possible to use a 200 ohm wich would be better as it would then draw less current
i dunno

 

Macroman, will it matter if i use a resistor a little under the needed amount? or will it still be fine and not destroy the LEDs

 

That tiny difference will not matter at all, seeing as resistors are usually +/- 10% anyway.

 

It won't matter being a little out either way. 22R will be fine but you could use 27R without losing too much output. I would suggest using at least a 2watt resistor.

 

If you read that article and then work out the current thro' the two sets of LEDs, it turns out to be about 10mA in the blue circuit and only 2.6mA for the red/green one The auther then moans that the red leds are not as bright as the blue ones. LOL

 

thanks guys for the help

 

Stuff up

Sorry to bother ppl again, but due to a stuff up at the electronics store, i had to get my leds elsewhere and instead got 700mcd ones.

i also changed my circuit layout to 13 sets of 3 leds in series
eg.

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|->|->|->|-|
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i worked it out to need a 30 Ohms resistor, and this works (ive built it, looks incredible) but i find the resistor gets very hot very fast (only a matter of seconds). its a big chunky 2watt resistor.

is it ok that it is getting this hot?

thanks for the help

jamicon

 

not sure why thats happening :-/ just try a higher powered resistor . . . rapid do a 100 watt 1 that should qork