Suuuuuper simple led resistor question

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Timmythekid

Sr Member
Having read a number of google hits on wiring an led circuit with various Vf's, I've realized...I have NO idea what anyone on those threads are talking about. None. Sigh. Guess I've completely forgotten high-school shop class. So, would anyone be willing to try and give a straightforward English explanation for how I approach designing the circuit where I've got leds with different forwarding voltage? Averaged Vf? Highest Vf? Something completely different? Ultimately I want to run 7 leds (three whites, and one each of red, blue, green,and maybe orange) off a 9v battery. Problem is I'm just not sure what to plug into this(http://led.linear1.org/led.wiz), since the leds have different Vf's, to figure out the resistors needed for the circuit. I....hope that made some kind of sense, and thanks!
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Edit.....oooooooooooh, hold on, I think I may buggered the math after all. There's no way a 9v battery is going to run 7 leds, is there? Hmmmm....calls for more thinking.....
 
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b26354

Sr Member
RPF PREMIUM MEMBER
you need resistor for each batch of Vf otherwise some won't light. So work out the resistor you need for the 3 whites and 9v. For the different colored LEDs - I'd use a single resistor on each one - then wire the circuits in parallel to the battery.

A 9v battery should be able to run as many LEDs as you want (within reason) in multiple parallel strings.
 

metal wood

Well-Known Member
i once lit 116 diodes in a ton of parallel strings with a single 9v, drained the power pretty quickly though.
 

Robiwon

Master Member
Gone but not forgotten.
Yes, just plug in the values for each color separately and then wire them in strings together.
 

Timmythekid

Sr Member
Thanks, guys, I was beating my head against the monitor last night trying to visualize this circuit. So LEDs of like voltage (say, whites and blues, reds and greens for example) could be wired together in series (to a maximum of what the source voltage will support obviously, for each of those strings) with an appropriate resitor, then take ALLLLLLL the negative leads, and ALLLLLLL the positive leads, bundle them together and run those to the switch and battery leads? Okay, I....think I'm seeing this now. And b26354, just to be clear, are you saying you'd wire each color with a resistor as its own series string, then tie them all together? Thanks again, guys!
 

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b26354

Sr Member
RPF PREMIUM MEMBER
I think you've kind of got it.

LED's don't behave like a normal light bulb - where you could wire 6 x 1.5v bulbs in series to a 9v battery - The current is what's important and the resistor limits the amount of current though the LED. If you wire 2 different types of LEDs in series that have different Vf most likely one will end up brighter than the other or one won't even light at all.

The LED wizard tells me that a single LED and a 390 ohm resistor is about right for 9V.
 

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