LED series vs parallel question

[sycor]

Mods, feel free to move if you wish.

I'm work on a non-prop project (by movie inspired) that requires some 3 blue LEDs powered by a USB port. I know that one of the two options (series or parallel) will cause problems if one of the LEDs died because it would allow all the power to go the other LEDs and potentially cause them to burn out also. But I cannot remember for the life of me which one and I cannot seem to find the information online.

I have an LED resister calculator that can help me figure out the resistor size for the circuit so that's not a problem. I'm more just worried about causing a chain reaction blow out should one of the LEDs die.

 

[primer]

parallel should allow the other LEDs to function if one goes out. Series=one big loop where one point of failure causes everything to fail.

EDIT: Sorry, I realize your question was more complicated than that. In parallel, current stacks up, as opposed to voltage. I'm not sure which one would cause an LED to blow without more research. I found a discussion of this topic here:
http://www.candlepowerforums.com/vb/showthread.php?t=92012&page=1

 

[sycor]

That's what I though. But I read a couple different sites that said you shouldn't hook LEDs up in parallel. But it sounds like parallel is the better option, at least for a wall powered assembly. I can see why it would be bad for battery powered though.

 

[primer]

Yeah I agree it could be problematic for battery power. You're talking about wall powered? Or USB? Incidentally, how often do LEDs even burn out? I've never know that to happen.

 

[exoray]

But I read a couple different sites that said you shouldn't hook LEDs up in parallel.

Actually this is the best* way to wire them up as long as they all have their own resistor... *best because a failure of one LED will not effect the others...

In series all LEDs should be identical color/brand/rating, no mixing or matching... In this configuration a failure of one LED will take out the entire series cluster...

Most efficient combination uses a series parallel array, following the above rules... That being each series cluster is assembled from identical LEDS and each cluster has it's own resistor...

Plug the numbers in and get a picture...

LED series parallel array wizard

A USB port by spec only outputs 5 volts, and most blue LEDs need 3-3.5 volts so in this application you are really limited to a basic parallel application...

 

[exoray]

Incidentally, how often do LEDs even burn out?

Anytime between 0 hours and 100,000 hours on average...

With a proper circuit and no defect in the LED itself they should easily last 1000s of hours, but if anything isn't perfect (aka the real world) anything can happen...

In parallel, current stacks up, as opposed to voltage. I'm not sure which one would cause an LED to blow without more research.

Excess current is what blows LEDs... You either have to limit the current with a resistor or use up all the voltage so that the current can not exceed the threshold of the LEDs...

Ohm's law...

 

[primer]

Thanks for all the insight, exoray. I'm new here and mostly just lurk, absorbing information. Your individual parallel resistor solution made me go DOH!!

 

[tim]

Thanks for all the insight, exoray. I'm new here and mostly just lurk, absorbing information. Your individual parallel resistor solution made me go DOH!!

Just to be clear, he's suggesting that each LED have its own series resistor. The resistor/LED units would then be connected in parallel. So, if you're going to hook up 3 LEDs, you'd need 3 resistors, and so on. See my crappy ASCII-text diagram below (ignore the periods).

+---LED--RESISTOR---+
| . . . . . . . . . . . . . . .|
+---LED--RESISTOR---+
| . . . . . . . . . . . . . . .|
+---LED--RESISTOR---+
| . . . . . . . . . . . . . . .|
| . . . . . . . . . . . . . . .|
+----- USB PORT -----+

 

[sycor]

Hmmm I thought I could get away with only one resister in a parallel setup. I need as few components as possible because this will be illuminating a box from the inside.

 

[exoray]

Hmmm I thought I could get away with only one resister in a parallel setup. I need as few components as possible because this will be illuminating a box from the inside.

Well 'improperly' you could, and it will work, but I personally wouldn't do it and most would advise against it...

The problem with one resistor in a parallel setup is that it provides no balance, there is no way to regulate or control what percentage of resistance/electricity each LED will share in that arrangement...

Resistors are small sized insurance, even smaller if you use say a 1206 (or smaller) surface mount that can still be hand soldered fairly easily... Or you pay extra and get LEDs that have built in resistors but they are hard to find for 5 volt applications...

And as Tim showed that is the 'correct' way to do the LEDs in this application...