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LED resistor calculation

Tezza1

Dec 9, 2011
18
Joined
Dec 9, 2011
Messages
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Hi there Guy's:
I want to run a 3v Bright blue LED from a 3xAA battery pack = (4.5v).
The vf is 3.2v - 3.8v
forward current 30MA.
I know how to do the calculation but am a little unsure about which of the two vf voltages to use in the calculation!.
Would I use the (3.2v) the (3.8v) or a voltage in between.
By using the vf (3.2v) the resistor needed would be (47ohm) using the vf (3.8v) the resistor would be (27ohm)? any help would be very much appreciated.

I don't need another sleepless night HaHa.


Thanks in Advance
Tezza1
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
12,615
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Nov 17, 2011
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12,615
I'd use an intermediate Vf. This will balance lifetime vs. brightness. If you want to control the current very tightly, use a current source, not a resistor.
 

Tezza1

Dec 9, 2011
18
Joined
Dec 9, 2011
Messages
18
Thank you all:

A big thank you to everyone how took the time to help me out with my query' I'm quite confident now of how to do the correct calculation.
Guy's like you make this a great worth while site for us novices.
Thanks
Tezza1

(No more sleepless nights)
 
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