3 watt led project problem.

[SunofDoom]

Hello,

i am running a little project in which i want to run 12 3watt leds in series using this power supply..

http://www.dx.com/p/waterproof-670ma-36w-power-constant-current-source-led-driver-85-265v-42906?item=2#.VDmNQGd_sWc

which is supposed to be fine!

The led are 3watts each and 700mA, bought from ebay..

i measured the power supply's voltage and it produces 39.5 volts..so for the 12 leds 3.4(volt for each led)X12 gives 40.8 volts..so generally it should work fine..

what happens is that when i plug it in the leds start to light for 2-3seconds and then they turn off..after some test, one or two of them are burnt..tried to change the burnt ones and tried again but same thing happened again..

Am i doing something wrong?Shouldn't this be working?

Thanks in advance!

[Johan]

You need to limit the current or they will fry. Just like you have experienced. They easiest would be a simple series resistor. But then you don't have high enough voltage.  If you make two parallel strings of leds, 6+6 you only need a little over 20 volts but current will double.  Personally, i would get a ready made led driver from the store and spend my time with my goat instead. ..
J

[Johan]

Guitar. ..Not goat. ..damn phone. .

[bloxstompboxes]

Guitar. ..Not goat. ..damn phone. .

I was getting worried there for a moment. But to each his own, I suppose.

[duck_arse]

what's wrong with goats?

[SunofDoom]

Hello fellow farmers!?

Ok i totally agree with you..i dont wanna mess around with leds all day long but still i have to finish this project.

Why should i limit te current?power supply gives around 670mA and thats what the leds need to work propertly..connecting the leds in series means that you sum up the voltages and the current stays the same..is something wrong in my head?do i miss something?

Just in case..when testing i used aligator clips to connect the power plug with the ac side of the power supply..could this affect the result in any way?

Thanks for the help!

[Johan]

The Leds need 700mA to give three watts. But without a current limiter they are just a short and the weakest part will fry

[SunofDoom]

ok i think i got it.So next question is, do i need a single resistor for all 12 leds in series or each led needs its own resistor?

If the answer is one resistor for all how do i calculate the amount of resistance?Ohms law with power supplies voltage and current or what?

To give more info not all leds are the same coulour but all of them require 700mA.

[SunofDoom]

What does change is the voltage each led needs..

white 3.6~3.8v (1 led)
purple 3.2~3.4v (2 leds)
red 2.2~2.6v (9 leds)

does these voltage changes mean that resistor is needed somewhere?

[SunofDoom]

Personally, i would get a ready made led driver from the store and spend my time with my goat instead. ..
J

As you can see in the link given the power supply is designed for 3watt leds..
Actually it says 3Wx(9-12 leds).

[Johan]

Oh. So it is a storebought psu...sorry. i missed that. Then all i can think of is cooling. .3 watt leds usually need to be mounted with some sort of cooling to not fry... How about yours?

[SunofDoom]

I've made a pretty nice cooling system for the leds..but definately that's not the problem..i think i've found it actually..

how fool am i??i just counted on what the driver said..some of the leds..actually 9 of them..the red ones..need  2.6 volts..so the total Voltage that leds require is much less... around 9 volt actually..so that must be it right?9 more voltage on them could easily fry something..two solutions i guess..put a resistor in series to limit the voltage..(current is not affected in this case as far as i can understand)..or simply add some more leds till i reach 39.5volts which the driver gives out..

how sounds that?