Led's and ohms law (newbie question)

[dolomite]

Hi,

I am learning electronics so please go easy on me. I want to start by making an UV Led exposure box ( i am getting fed up of using strip board) and this is where my question comes from. It will use 84 UV LED's in groups of 3 and i am getting a little confused as to what resistors to use, let me explain. i was going to use a 12V supply and the LED's have a forward voltage of 3.4 which times by 3 give 10.2V leaving 1.8V so would i calculate R as this
R = 1.8/0.02 (assuming i want 20mA for the LED's) or as there are 3 would i use this
R = 1.8/0.06 (20mA per led) or would it be
R = 12/0.02
R = 12/0.06

I hope someone can understand my confusion and i would like to ask that you please don't reply with ideas on how to make this work better or tell me this is the wrong way to do this as i am interested in the theory. Thanks for any help i receive on this.

 

[LTX71CM]

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

 

[ChosunOne]

The power source you use could make a difference if it's not strictly voltage regulated. I work a lot with "12" VDC power sources, i.e., DC power supplies with a 12V lead-acid battery backup attached. The 12VDC rating is "nominal" and approximate.
The output off either the power supply or the battery direct (fully charged) is usually over 13 VDC, and close to 13.5 VDC if the load is light.
So if you're using that kind of power supply, after dropping 10.2 V over the LED's, you'd wind up with nearly twice the voltage to calculate for your resistor.

Now if your voltage source is strictly voltage-regulated at 12.0 VDC, that's not a problem; but I thought I'd better mention it because all the Power Supplies I work with name "12 VDC" in the specs. If your PS does the same, you could wind up running twice the current you want in your circuit---briefly, anyway.

 

[dolomite]

Thanks for the link, most helpful,

ChosunOne - Thanks for advise i do intend to use regulated supplys but it's something to remember in the future in case i use that setup, Thanks