LEDs in series?

[vanillasnake21]

Hey I've built an LED light for my aquarium as a supplement to my existing light but I'm having some issues with it. The light runs fine, but I noticed that the first LED that I'm powering is significantly more brightly lit then the last one, the middle 2 are in between. I'm using Cree XT-E Cool White/Soft White combo (3 of each). I built a driver by using LM317. I have a 1 ohm resistor running from control pin to the out pin, then I just ran a single wire from the outpin to all the + on the led, and ran a single wire through the - and into ground. Is this the right way to do it? Just to save you trouble here are the specs for the LED:

wattage:
2.1w (700mA), 3.1w (1,000mA), 5w (1,500mA)
max drive current:
1500mA
forward voltage:
3.0v (700mA), 3.15v (1,000mA), 3.3v (1,500mA)

Thanks!

 

[BobK]

If the LEDs are in series, then they all get the same current. If they are the same model of LED their brightness should be close enough to not be distinguishable by eye.

Show us a circuit diagram. Your explanation sounds more like they are in parallel.

Bob

 

[vanillasnake21]

In 12v - 1A ---- LM317 --out----LED1+ ---- LED2+ ----- LED5+
Ground -- LED1- ----- LED5-
I have only 2 wires one wire is soldered to all the +s and the other to all the -, the LM317 voltage regulator OUT pin is soldered to the beginning of the + line. The first LED that LM plugs into is the brightest, all the other leds get weaker as they move along the wire. The thing I just realized is that my power supply is only providing 12volts, each of these leds needs at least 3 volts I believe, but since my current is 1a they take up 3.15 volts. One led misfunctioned so I only have 5, totaling 15-16 volts needed, can that be why? But again I'm afraid if I hook up the highter volt supply the first LED will burn out since it's so much brighter then theh rest.. here are pictures, starting at top, running right to left, bottom left led is the last in the series. (I took these in manual mode so the shutter speed and aperture are constant, the brightness is how I see them)

 

[vanillasnake21]

about the power supply, like I said my current one is 12v - 1A, I didn't realize the LEDs drew 3v each. I have a laptop power brick, it's out is 18.5v - 6.5A, I'll be ok the voltage, if I get the 6th led to work (I think it just lost connection), but how do I lower the current to 1A, as that's the max that I can plug into LM317, can I use a bunch of resistors?

 

[Colin Mitchell]

Try each LED on 6v with a 10R resistor (all at the same time - they are in parallel - all are classified as separate circuits) to see if they all have the same brightness. This is where you start.

 

[vanillasnake21]

why wouldn't they be the same brightness? these are cree bulbs, not some generic. As you can see they are up and running at the moment soldered and everything, you know how much time this will take me? Only as a last resort. Besides I can see a linear drop in brightness from first to last so I doubt its a conicidence.

 

[Colin Mitchell]

You can simply try my suggestion by measuring each one in turn without desoldering anything.

 

[vanillasnake21]

You can simply try my suggestion by measuring each one in turn without desoldering anything.

what do you mean? With a multimiter? Ok, gimme a minute...

 

[vanillasnake21]

ok, i hope I did this right, I placed the multimiter on the pos and neg line AFTER each led, first one read 3.03v, after the 2nd it was 2.87 after the 3rd about 2.87 also after the fourth it was 2.71 and round same for last.

 

[BobK]

You have the LEDs wired in parallel with no resistors. This is not the way to do it. As you have discovered, they do not share the current equally. There are minor variations between all the LEDs which causing the unequal sharing. You lucked out in that your power supply was a current source at 1A and this current was shared among the various LEDs, so it was not high enough to burn them out.

Please read this:

https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

Edit: Also, if you are running these at 1A, I hope you have a heat sink.

Bob

 

[vanillasnake21]

@BobK but... how can be running them in parallel? they are wired one after the other on the same exact wire? I'll be back if I can't find the issue after reading that article. ps. I have do have large heatsinks as you can see in the pics.

 

[BobK]

Perhaps this will help:

http://www.allaboutcircuits.com/vol_1/chpt_5/1.html

Bob

 

[vanillasnake21]

ok read that whole thing, while iit's very interesting I don't really see how it helps me out here. I have these wired exactly how the diagram in that article show. edit: again, as far as I can tell there is a single path that the elictricity can take. This will be easier if you tell me why you think I have it wired in parallel?

edit: maybe the sloppy diagram i made was confusing, by ground I meant - on the power supply. So its like this

power ---- +----- LM317 ----out-------------------------------------------------- + led --- + led ----- + led ---- +led ---+led
-----ctrl---- 1 ohm ---- | (soldered to out)
power ----- - ------- -led ----- -led -------- -led ---------- -led ---------- -led

 

[BobK]

That article has two diagrams, one series and one parallel.

What you have described is parallel.

All of the + leads from the LED are going to the power + and all of the minus leads are going to the power -, right?

Bob

 

[vanillasnake21]

That article has two diagrams, one series and one parallel.

What you have described is parallel.

All of the + leads from the LED are going to the power + and all of the minus leads are going to the power -, right?

Bob

I understand now, the diagrams on the site your provided just showed one led after another with no + or -. Another diagram I found explained it, I have to solder the - of one led to the + of another in one single string, right now I have -s solder to other -s. Thank you, I will get back to update.

 

[BobK]

Yep, that is how you put them in series.

Bob

 

[vanillasnake21]

Allright, I set it up right the lighting is even now. I can only use 3 leds now though I'm assuming because I'm no longer sharing the voltage. Thanks @BobK and everyone, I would never have caught that.

 

[BobK]

Good to hear you got it working.

You can make 2 strings of 3 each and have a separate LM317 regulating the current to each of the two strings.

Bob