Multiple LED questions

S

Steel Chicken

Supreme [H]ardness
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Messages
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OK, question about using multiple LED's in a 12V environment.

OK, i bought a whole bunch of LED's w/included resistors to drop the 12v down to the roughly 3.5v the LED needs. If I wanted to run a bunch of LED's off of one 12V rail, in paralell, could I use just one resistor? Do I have to figure out the wattage capacity of the resistor in order to figure out how many LED's I can out on one resistor?

Example:

*---------*---------*---------------<resistor>--------12v+
led led led
*---------*---------*--------------------------------------12v-

or, do I have to use one resistor per LED?

*-------------------*------------------*---------------------12V+
led led led
resistor resistor resistor
*-------------------*------------------*---------------------12V-

sorry about the diagram, doesn;t like extra spaces
 
If you run the circuit in a parallel circuit, yes you will need one resistor per LED. If you run it in serial, than you will only need one resistor (but I'm sure that would be a bitch to do all the math).
 
Forward voltage is the voltage that is given to you for your leds. For my blue ones it's 3.0V. You'd probably be better off just doing 3 in series with a resistor, but it's worth a try just putting 4 in series without one, no guarantees it'll work though.
 
Steel Chicken said:
and forward voltage is the amount of voltage the LED requires to light up?
So if I got 4 3.0V LED's, I can use 4 in series with no resistor, is that right?

PS thats an awesome calculator, pooooifect thank you!
Click to expand...

that is correct... but not recommended.
 
plot said:
that is correct... but not recommended.
Click to expand...
why is it not reccomended? if they are realy 3v avrage foward voltage LEDs, they can be hooked up in series with no problems
 
theshadow27 said:
why is it not reccomended? if they are realy 3v avrage foward voltage LEDs, they can be hooked up in series with no problems
Click to expand...

Because they are not necessarily 3V exactly. You run the risk of having excess current and burning all three of them up.
 
theshadow27 said:
why is it not reccomended? if they are realy 3v avrage foward voltage LEDs, they can be hooked up in series with no problems
Click to expand...

as already mentioned, you risk having to much current going through the first LED... need a current limiting resistor. If you want to reduce the number of resistors used, i'd recommend using 1 resistor in series with a couple of LED's in parallel.
 
thanks for all the advice. i got plenty of resistors, its no big deal to do a 1:1 ratio. I was just trying to understand the logic behind the different solutions.

WOOT for enlightenment!
 
plot said:
to much current going through the first LED...
Click to expand...

circuits in series divide voltage equally amongst themselves. The first load gets exactly the same voltage as the last one, no matter how many or how few loads there are.

EDIT: but everyones gotta start somewhere, so dont give up :cool:
EDIT2: :eek: yeah yeah yeah so shoot me - - i fixed it
 
Speak for yourself, theshadow27. Series circuits don't divide current among the components. They divide voltage, with the current through all of them being the same. Components in parallel divide the currently, and then not necessarily equally. However, the voltage across all of them is the same.

So take it from an EE, and go read an electronics book yourself. And don't flame around here.

Back to the topic on hand, it would be better to put one resistor in series with 3 LEDs, all in series. The reason you need a resistor is because of a phenomenon called thermal runaway. As most electronic components heat up, they draw more current for the same voltage. LEDs are the opposite--the hotter they get, the more current they draw. So if you don't have a resistor in series (called a current-limiting resistor), they can pull more and more current until they burn out.

If you put a whole bunch of LEDs in parallel, their currents add up, so you'll be pulling more current through one resistor dropping more voltage from +12V, and pushing more current through it, so you'll need a higher-power resistor. Instead, put one resistor in series with 3 LEDs--you'll get the same current as for one LED, but far less voltage across the resistor.
 
Teancum said:
Speak for yourself, theshadow27. Series circuits don't divide current among the components. They divide voltage, with the current through all of them being the same. Components in parallel divide the currently, and then not necessarily equally. However, the voltage across all of them is the same.
Click to expand...
This is correct, just done an ALevel in Physics.
 
Teancum said:
As most electronic components heat up, they draw more current for the same voltage. LEDs are the opposite--the hotter they get, the more current they draw. .
Click to expand...

did you mean most draw LESS, and LED's draw more?
 
theshadow27 said:
circuits in series divide voltage equally amongst themselves. The first load gets exactly the same voltage as the last one, no matter how many or how few loads there are.

EDIT: but everyones gotta start somewhere, so dont give up :cool:
EDIT2: :eek: yeah yeah yeah so shoot me - - i fixed it
Click to expand...

*shoots*


in 10 months i'll have my bachelors in EET, hopefully i know what i'm talking about by now ;)
 
plot said:
*shoots*


in 10 months i'll have my bachelors in EET, hopefully i know what i'm talking about by now ;)
Click to expand...
:cool: thats what happens when you get tiard and angry... = angry bullsh!t
my bad lol i realy ment voltage but watever :rolleyes:
no more rants
 
To summarize:

Take
one 12 volt DC supply
four LEDs with 3 volt forward voltage drops

You can:

1. Connect them all in parallel, with a resistor in series with the parallel combination (use the LED calculator, but quadruple the power dissipation at the resistor).
2. Connect them in parallel, each with a accompanying resistor (the LED calculator works as is).
3. Connect them in series without a resistor (not recommended due to lack of overcurrent protection).
4. Connect them in series with a current-limiting resistor (use the LED calculator as is).
 
xonik said:
To summarize:

Take
one 12 volt DC supply
four LEDs with 3 volt forward voltage drops

You can:

1. Connect them all in parallel, with a resistor in series with the parallel combination (use the LED calculator, but quadruple the power dissipation at the resistor).
2. Connect them in parallel, each with a accompanying resistor (the LED calculator works as is).
3. Connect them in series without a resistor (not recommended due to lack of overcurrent protection).
4. Connect them in series with a current-limiting resistor (use the LED calculator as is).
Click to expand...


OK that makes perfect sense. It also explains why the calculator said to use a single 1 ohm resistor, for current limiting
 
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