Series or parallel for multiple LED's?

Series or parallel for multiple LED\'s?

I believe I've seen multiple LED's being hooked up in either series or parallel.
What are the pros and cons to either configuration?

I'd like to drive 3 X 100 ma 5mm LED's from 3 AA's in a single 3 to D holder. I don't know if I'll be using NiMH or E2 lithium batteries yet, but I'll probably be using some kind of regulation if I can ever decide who's circuit to use.

Re: Series or parallel for multiple LED\'s?

Hi my name is mark about your series or parallel I know
in parallel it will need more current say you have 3 led's in parallel each led about 40ma so in that sercet it would be 120ma / in series it would add voltage say you have 3 led's in the sercet led's run about 2 volts so it would be 6 volt in that sercet so you need more batties or a ampfer.

Re: Series or parallel for multiple LED\'s?

Certainly the parallel configuration is the easiest and cheapest. Especially if you use 3 x Li AA, and are at a nominal 4.5 V, put a small resistor on each one and you are finished. Especially if you are only drawing 300ma - easy - and the resistors are more or less free. Pennys really.

If you would like a more sophisticated setup, then use a boost circuit and run them in series, as noted above. This is likely going to be more efficient. Two great sources for these are georges80 and the shoppe. Both sources will run you around 20 dollars.

Last but not least - if you are using 300ma anyway, why not just buy 1 ea Lux I or Lux III, or a Cree X7090, a single resistor, and be done with it.

Re: Series or parallel for multiple LED\'s?

If LED's are connected in series, and one or more LED's fail, then the circuit will become incomplete and all LED's will no longer light up. Therefore, parallel connection is preferred (because one or more LED's failing will not stop remaining LED's from lighting up).

Re: Series or parallel for multiple LED\'s?

one of the main points of LEDs is the unlikeliness of failure.

the research i've done indicates you want to at the least have strings in series.. maybe multiple strands.. the lights i've seen with 5mm and multiple LEDs.. i can see the brightness difference if they are parallel.

I would definitely use some circuit, but on a small ckt like that i'd maybe buy a 'prefab' driver chip that can output 100mA and needs very little supplimental chips.. there are posts all the time in 'bats not included' that show these circuits.. they are typically only like $2-3 for the switcher.. and maybe you need to add in 1 inductor which you can get a sample for free from coilcraft or the likes.

You'll need to typically add in 2-4 small components like caps and resistors but the journey is very satisfying. buy extras if you go SMD. they are very easy to lose.

-awr

Re: Series or parallel for multiple LED\'s?

True, the likelyhood of LED's failing is small, but it does happen; it's happened to me, and it's happened to others (according to forum posts here).

Provided the same current is passed through the LED's, any variations in individual LED brightness will still show up regardless of whether the LED's are series or parallel connected.

Using parallel connection with one current limiting resistor per LED, you can adjust individual LED brightness by changing resistor values. Using just one resistor for all LED's is simpler, but does not allow the option to adjust for variation in individual LED brightness.

So, if LED/circuit failure and variations in LED brightness is NOT a concern, then the series circuit is the better option.
If LED/circuit failure and relative LED brightness IS a concern, then a parallel circuit with one resistor per LED is the best option.
If LED/circuit failure IS a concern, and relative LED brightness is NOT a concern, a parallel circuit with one resistor for all LED's is the better option.
If LED/circuit failure is NOT a concern, and relative LED brightness IS a concern, then you will have to use a parallel circuit with one resistor per LED.

Hope this is not too confusing!?

Re: Series or parallel for multiple LED\'s?

from what i've read and looking at the majority of typical white LED designs.. like backlighting on a cell phone or pda.. they virtually ALL use a combination of parallel/series circuits.. somethign like this.. 3 strands of 4 leds with a small series resistance on each strand.. when mass produced the limit resistor can be chosen to balance wasted power and LED variation..

I've done some monkeying with parallel vs series luxeons and from my experience so far.. it's far easier to have matching output if they are in series... but eyes aren't hyper critical.. and if you can hand-tune the resistance.. there is likely little difference in output, so the battery matching is a more critical consideration.. in other words.. a lot of configurations can be direct drive with parallel but obviously would need a boost driver if series.

Both designs of course have their pros and cons..

I'm absolutely a 'driver LED' designer so i'm biased toward that end, however i do use a lionheart often and have a lion cub on the way.. as well as i'm working on an ELX-6.. 6-up direct-drive lux-3.. no drivers.. check out elektrolumen's page.. http://elektrolumens.com it is absolutely stunning... 15,000 honest lux and over 400 lumens.. for the price of a lion cub... that light i believe has two strings of J bin lux 3s in series (3 in each string)..

If you want mega reliability.. you could use 3 NIMH or ALK and calculate the DD or 'resistored' parallel circuit for your setup.. I am guessing that you can't get max output from a fatman in series on a 3D solution into 3 lights.. it's close.. from a single LiON i could only get 700mA because of the current limit on the switch in fatman..

I think george is working on a 'big dude' version of fatman driver that can handle higher voltage differences and current which would maybe be a perfect solution for this type of light. go check out taskled's site. http://taskled.com

if the fatman would work for your design i would use it for sure i love the driver.

-awr

Re: Series or parallel for multiple LED\'s?

Hi there,

I always go with series whenever possible, as direct parallelling can have adverse effects as outlined in my
EE course here on CPF.
There are times when series isnt practical because the
higher voltage isnt available. In this case there's no
other way -- you have to parallel. Using small matching
resistors helps to some extent, but eats up some eff, so
it's a tradeoff.

Take care,
Al

Re: Series or parallel for multiple LED\'s?

i see what andrew is saying, in series, they will all have the same current going through them.
so if one is more or less sensitive in its voltage vrses current , in series it would not matter.

wheras if you parellel them you can set the same voltage through the lot, and different ones can react to that voltage differentally.
if they are all of the same BIN, or you just sort them yourself, and they have relative sensitivity , then parellel will work just fine.

if you got about 3.6V comming off the batteries, parelleling them is going to more efficient and cheaper to slop together, as opposed to boosting it, then seriesing it, and then risking a single unit failure.

most of the stuff you see in stuff is resisters on each row, or resisters on each led. like he said.
i think if you are controlling the input completly, wasting power through a resister is a waste of power, IF you can design it clean without it.

Re: Series or parallel for multiple LED\'s?

in series, a led failure is not always the whole series row dying. sometimes the resistance of the failed one goes way down. and instead of a series of dead ones, you can end up with one dead or dying one, and the rest of the row going balisticly bright.
in other words, in series there is the possibility of burning the whole rack out, when the resistance on one goes wackey, and the rest thermally run away.

i think the resister on the row, in a lot of these devices is enough to prevent that.
one led goes low resistance, and the rest go high brightness, but they were underdriven, and resisted too, so even with ONE dead shorted the row still works.

but with flashlights we dont usually allow things to run extra low, so they cant handle one in the series shorting.

Re: Series or parallel for multiple LED\'s?

So there's alot of to think about series vs parallel.

How much does your 100ma LED cost? Are they the multiple die type where 5 to 6 die wired together to make 100ma? If you put 3 of them together you will be paralleling(no typo) already paralleled LEDs!

If your 100ma LEDs are cheap why not get a bunch and match the forward voltage, then you won't have risk of overdriving any of them. Having said that, wiring 3 resistors in series with the LEDs will be much more economical but that also depends on your design.

I have seen brake light LEDs fail in vehicles. Whether it is a cold solder joint failure or the LED itself but as long as you don't overdrive them you will be fine. Of course good design means one or two LEDs fail instead of the whole row.

Now again, back to the LED itself. If it is already a parallel LED with dies in it, then it's better not to put it into overdrive.

Re: Series or parallel for multiple LED\'s?

oooo thats right, if its them 8mm 4xparellel things, before you put them in series, make sure you check the current at a voltage.
because 1 or more of the gates of the 4 parellel gates in it can be dead. and when that happens, any current going through it hits whatever is left.

EX:
your feeding 120ma into a 4 die 8mm
2 of the dies die
now your feeding 120ma into the last 2
in series, the dead one can be as bright, as much current is flowing through it, but 2x as much current is hitting the last set of gates.

that is how i fried a few of the 8mm, till i got smart and sorted them based on current at a specific voltage.
which can somewhat determine if a gate on them is dead.
also you can magnify the front, and project the 4 gates onto the wall.

Re: Series or parallel for multiple LED\'s?

Oh I'm so confuuuuuused... /ubbthreads/images/graemlins/Christo_pull_hair.gif
I don't have all the design details worked out, but I envision the mod as this:
Three 100 ma MJLED's poking up through the top surface of a cylinder made of exotic wood (somwhere around 2 1/2" diameter and 4" high). It's to be more of a a pretty mod as opposed to the butt ugly other one I'm making which is for pure functionality. Should wind up looking like a three wick candle made of wood and LED's. I'm using the MJ's in the hope that their side emitter-like properties will give off a more candle-like effect, and they're bright.
The reasons for the 3 to D holder is that it will nicely allow for the form factor I want while being able to at least pump 4.5V from a small area, and I'm desperately looking for a home for my one EL/Waion 3 to D that I have left. If 6V using a 4 to D holder really would be a much better thing for this mod, then I'll get one.

Back to the subject, and thanks for dealing with the newbie questions... just want to make sure I get this straight.
So say I've got my 4.5V. If the LED's are in series, and they each have a Vf of (average) 3.3V @ 100 ma, it'll require 9.9V to drive them but only 100 ma of current? This would require a boost circuit I imagine?
Conversely, in parallel it would take only 3.3V but 300 ma to light 'em up? This would need a step down circuit?

Oh, and also... with a little practice I can do small soldering within reason, but I've never done SMD. Checking out the Fatman, it not only looks like it would work nicely, but it also doesn't appear to use a length of wire for a sense resistor. That works, but I'd like the neatness of a little resistor on the board instead. I have to find out about how to set the current on it. It's also a neat, little circular thing which also fits the form factor.
Andrew mentioned the Lionheart driver which I don't know a lot about except that it has multiple brightness levels which could be a cool thing for this, and I don't know how to set the current for it either.
Either way, are they ready to go as soon as I hook up the batt and LED connections, and set the current? Looks like I need to get a DMM.

The pros and cons of not only whether to go series or parallel are confusing, but also what circuit to use as apparently there are different choices no matter what configuration. Still a little foggy on all this, but all your answers are helping to put the picture together (I still need a lot more though... hehe). This is much appreciated.

EDIT: In response to the last two posts, these are the MJLED single large die 5mm's running 100 ma each.

Re: Series or parallel for multiple LED\'s?

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So say I've got my 4.5V. If the LED's are in series, and they each have a Vf of (average) 3.3V @ 100 ma, it'll require 9.9V to drive them but only 100 ma of current? This would require a boost circuit I imagine?
Conversely, in parallel it would take only 3.3V but 300 ma to light 'em up? This would need a step down circuit?
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me thinks you said all this 100% correctly.
to be simple, just toss in a resister on each led, the resister wouldnt have much drop across it, and therfore you wouldnt lose much.

+ ----------[resister]------>led|------- -
+ ----------[resister]------>led|------- -
+ ----------[resister]------>led|------- -

bing bang boom your all done. the losses would be whatever you lose in heat through the resisters.
so if you drop 1V of the 4.5V total, your efficency will be a loss of 1/4.5 , only a good converter would be better than that.
BUT, a good converter could also run your batteries all the way down, without reducing the output much.
BUT
look at the specs on the converters :-( they DO reduce the output, and some MUCH .

i dont know how much the MJLED is, but a cheapo 100ma 10mm, or 100ma 8mm can be sanded on the top, and its like a micro soft white bulb. all the light goes all over the place. little OF it in one place, but it going everywhere.

(some of the above is opinion)

Stupid side notes:
a to blue led, can have a cheap yellow gift wrapping transparency put over it, or be dipped in yellow transparent paint, and the output is relative to the lower output warm whites. for that "candle" look.
a amber, could be added to balance the blue when behind frost, and warm occurs

Re: Series or parallel for multiple LED\'s?

There's some interesting ideas for getting a yellower, warmer look. Thanks, Vid!

I could save a lot of time using direct drive... then I'd only have to be asking how to figure the right resistor to use. But the runtime to 50% brightness thing is a bit annoying, and I also wanna' show off the "complexity" of my electronic candle to techie friends... hehe.

My next set of guesses/questions starts out with efficiency.
IIUC, whether using direct drive OR regulation, more efficiency is gained when there's less voltage differential between what the batts put out (Vbatt?) and the LED's Vf? This I would guess, produces less heat and gives a longer runtime due to less energy being wasted.
If so, I'd be tempted to use 3 NiMH AA's at 3.6V (which is right near the top end of the LED's Vf range). With say, 2100 mah batteries, 300ma going to three paralleled LED's, and rounding off to perhaps 90% efficiency, the light should get approx. 6 hrs. 10 min. runtime until the batteries are totally dead?
Last but not least, say my circuit is putting 300 ma to it's output connection. For parallel, I simply hook up the + side of each LED to the circuit's + connection, each - side to the - connection, and each one will suck up 100 ma?

Building a circuit myself, although I'm sure I could do it, realistically I probably can't match the design, small size, and quality of the Fatman or Lionheart drivers. It most likely will be worth the money to buy something and I just hook up the power. In fact, I'll buy two... one for the other mod using a 6V battery driving a Lux III. Won't have to worry about series/parallel in that one. /ubbthreads/images/graemlins/wink.gif

Re: Series or parallel for multiple LED\'s?

You are on the right track. - or tracks. Using your design idea of 3 x NiMH and 3.6 V, you pretty much do not have enough voltage excess over the Vf to do more than a resistored arrangement. Most downconverter boards need at least 1 V of headroom to work well.

You may or may not actually obtain the 100ma per LED you desire at V battery of 3.6 V - depends on the Vf of the LEDs - easy enough to test on a bench.

Given the soft glow you are after - you might actually really like the Cree 1 watt part - if has that "soft white" glow vs a Lux III which is more of a "clear white" bulb analog.

Re: Series or parallel for multiple LED\'s?

The fatman and VIP drivers are both excellent and i use them all the time on the bench and in lights.. you can pre-set the exact current before hooking it up with an ohmmeter.. fatman has a built-in trimpot and the VIP has solder tabs for putting on SMD resistors but you can just wire to them a normal sized resistor (or better: potentiometer!) or better better yet.. a digital potentiometer.

I have never used the taskled drivers for non-lux but they sure would do the trick.. just make sure you get the resistor set properly first.. you don't use 'sense resistors' like the typical sammie board.. it's preset at .1ohm.. you change other much higher level resistors to affect the feed-back loop to the op-amp.. it makes it waaaaay easier to dial in a precise value.. and i mean to 1 or 3 mA easily.

They are kinda expensive.. especially for a little 'candle' light.. since they power capability is so high..

check out this thread: http://ldo.rouse.com you could build your own LDO circuit it will put out a pretty constant output with just 4 chips.. you could make one circuit and hook up all in parallel (leds).. maybe put a tiny inline resistance to make them more even ( like 1ohm or somethign).... maybe less. did you say 100mA..

if you run from 3 alkalines through the LDO it'll hold the voltage back to 3.3V or whatever it takes to make the current whatever you set it.. if some heat kicks back from the LEDs onto the chips.. it counteracts the head loss of the LED (brightness loss from heat) and the light output is more stable.

http://rouse.com/nano has some charts of drivers using the circuit.. of particular interest would be the 350mA test.

you could run it from LiON batteries :-D.. one R123 would probably run 2-3 hrs depending on the current level you decide on... 3x AA Lion would get you.. well.. probably jsut about the same as 3 NiMH.. probably not worth it.. LiON is better for power in small space for a shorter time i guess.

-awr

Oh here I go, digging up an old thread again...

I have read somewhere (and as indicated by posts in this thread) that connecting many LEDs in parallel is not a great idea, as the slight variations in binning of the LEDs will cause some to take more of the power than others, resulting in some of them brighter than others.
Does anybody know how pronounced this is?

I am interested in applying more than 7 (this is the voltage blackout range of the boost drivers I am aware of (22V+)) SSC/Cree LEDs in a single light, to be run on batteries (read "Limited space"). This could be done in strings of series, one long series, or X*parallel...

Parallel would be easiest, and could be achieved at voltages low enough for a driver. The other options would be slightly more difficult, but are they worth it?

I'm also suspicious to a very ignorant extent that having high currents through the entire circuit may be less efficient than a high voltage circuit, if anybody knows enough to comment on that aspect of it.

Thanks,

Xiorcal

I would never consider running LEDs in parallel but I haven't ever tried it so it might work - sort of. You may need to hand pick the LEDs to make sure that they are balanced. But will they stay balanced over time?

I've a better solution. Create some sort of constant current regulator that provides enough current to run all the LEDs in parallel. Connect that to the high-side of all the LEDs. On the low-side, create a ring counter with some beefy current sinks - one for each LED. That way you'd multiplex the LEDs. If there were N of them then each would experience N * max-current but for only 1 / N of the time. The average power would be right and all the LEDs would be balanced. You could even create an efficient dimmer by introducing extra steps in the ring counter.

Ha! Cool to see this old thread! I've been running a number of lights with all parallel 5mm LED's latelyas shown in this thread, and they've been working great so far. The wooden thing wound up with 9 LED's in parallel, and a couple weeks ago, I FINALLY made use of those 3 MJLED's a couple weeks ago. Both lights are direct drive, no resistors. The PVC mod in there doesn't count because it uses a voltage regulator, and a resistor on each LED.

Sure, they're overdriven, but for what they are, they're really bright, and there's no angry blue at all in either light. In fact, tint for both types of LED's became supremely better, especially the MJ's! It might be hard to tell the long term effects of the overdrive for a good while, as they see very light duty. I'll keep an eye out though, to see if any or all emitters start to dim over time.

Two more all parallel mods are coming soon... another pure direct drive, and one with an overvolted boost circuit. :devil:

Thanks for digging up this oldie! Good to read through all the useful info again.