resisitor question

[atomicareprise]

i'm about to use 12 blue leds on my mods..

and i will make it sound reactive...

so i go to http://ledcalc.com/#calc to know what

resistor i'll be using..

supply voltage                        = 5v
voltage drop across led           = 4v
desired led current                  = 20 mA
how many leds connected        = 12

4.7 ohm is the nearest higher rated resistor  so that's what i'm about to use..

what i want to know is this correct??


my concern is that how i put the resisitors..will this work??

[psp339]

I don't think you need that much resistors and what kind of LEDs use 4v?

[bustinthejustin]

Wait whaaaat? 4v LEDs? Since when? Also, your resistor dageam confuses me. Why is there like 10 resistors in the middle of nothing? Or am I reading your diagra
 wrong? Either way your diagram makes no sense.

[rceckspurt13]

Yeah you only need one resistor. It goes on the shared + line of all the LEDs. In other words, all of the LEDs will connect to one resistor. The way you have in the picture will short circuit and you will end up with a big problem.

[GhoSt]

bro that caculator works it out for one resistor :P, you just need one 4.7ohm resistor not 12. Btw you sure 4v is typical voltage?

[atomicareprise]

the led cal says when its blue its 4v..

[atomicareprise]

Wait whaaaat? 4v LEDs? Since when? Also, your resistor dageam confuses me. Why is there like 10 resistors in the middle of nothing? Or am I reading your diagra
 wrong? Either way your diagram makes no sense.

 i thought for every leds that i'm going to use like 12 leds that i've mentioned i thought it will

need 12 resistors as well..sorry guys if i confuse you..noob here...

[GhoSt]

show me the data sheet, and no when you use 12 leds you dont need 12 resistors as long as you take the all the wires from all the selected side of the leds to that resistor. Im pretty sure wiring up 12x 4.7Ohm resistors in paralell gives a resistance of 0.391666'ohm . Is this what you want??? that is a minute resistance.

[atomicareprise]

the website says the blue led says 3.5- 4v..

that's why assume it's 4v..sorry if i confuse you guys..

[rceckspurt13]

That diagram on the website is of a parallel circuit with one led per resistor. I think you just got mixed up on how the LEDs attach to the resistor. You could actually use one resistor and get the same result.

[atomicareprise]

thanks rceck..but i don't know how to place my resisitor..

i don't know which is side to place towards the leds...

i' don't know which is the positive side of the resistor..

[GhoSt]

thanks rceck..but i don't know how to place my resisitor..

i don't know which is side to place towards the leds...

i' don't know which is the positive side of the resistor..

resistors dont have polarity they can go both ways around, and it doesn't matter if they are on the positive or negitive wire as long as they complete the circut at one point.

[bustinthejustin]

Excuse my crappy diagram, but that's how you should wire it. Green blocks are leds. I'm assuming your forward voltage of your leds is actually 3v because that's what typical blue leds are, and so you'd need a 10 ohm resistor. Only one.

[Jumbo]

That led calc things is a average. There are different leds. There are low power high power all different. Dont go by that go by the datasheet.

[GhoSt]

10ohm is wrong, by my caculations assuming the leds are 3v 25mA and the lm386 outputs 3.6v? you should use aprox 27 ohm resistor.

Plz man show me the data sheet so we can figure it out exactly.

[bustinthejustin]

*shrug* yeah we need the data sheet. But idk he said he's using 5v point, and most  leds are 20ma. But to OP, where'd you get your leds?

Edit: Ooops I think my original calculations gave me like 120ohm. But yeah, 5v voltage with 3v forward voltage, and 20 ma gives you 120ohm. 3.6v with 3v forward and 25ma gives you 27ohm, and 3.6v with 3v forward and 20ma gives you 33ohm.

[GhoSt]

yea you should take more care when working things out becuase he is using a lm386 which runs on 5v, the output from it isn't 5v .

[atomicareprise]

thanks guys for all your replies...i get my leds at phenoptix..

[rceckspurt13]

Yeah there really isn't a set output from the lm386. It goes up and down depending on the loudness and frequency of the audio it's amplifying. You are gonna have to make a ballpark estimate. It should be close enough to go on though