Author Topic: Lots of LEDs to be powered directly without resistors, should I ?  (Read 10693 times)

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pGrnde

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Lots of LEDs to be powered directly without resistors, should I ?
« on: September 28, 2012, 02:46:25 PM »
Hello

I have been searching in the web for ways to power LEDs without the always-needed resistor, and I found this great article here :

http://www.fieldlines.com/index.php?topic=131287.0

and this other one where a good explanation of LEDs without limiting resistors :

http://tinkerlog.com/2009/04/05/driving-an-led-with-or-without-a-resistor/



I would like to, if it is not so much to ask, for confirmation that my design could work, and will not burn the LEDs in matter of hours

Please let me take a couple of minutes of your time

the design a  C I I C letters, first one horizontally flipped, the Cs are (intended to be) made of 50 LEDs and the Is of 40 LEDs, the LEDs are to be mounted in tiny plastic stripe, and it should keep as clear design as possible (therefore the intention of not adding more components than just the necessary ones)

the circuit should be powered from a car battery, sometime, form the car itself

so, to power it I thought to use a DC to DC conversor to apply exactly the voltage that LEDs need (I will play in the safe side, and keep 80% of it)

The LEDs I intent to use are  :  http://www.ebay.com/itm/190522746092
    Forward Voltage (V) : 3.0~3.2
    Forward Current (mA):20

And the DCtoDC converter : http://www.ebay.com/itm/260757400857
     Wide range of input 3.0-33.5v
     Stable & High efficient up to 92%
     Powerful max.3A / Adjustable output 4.5-35v
     Max. input up to 35v and output up to 35v
   
And the design, (sorry for the kindergarden-paint-style) something like :

     



Mathematically,
     50 LED in parallel, 20mAh * 50 =  1 A
     40 LED in parallel, 20mAh * 40 =  0.8 A

     so load for the DCtoDCD   1.8 A
     voltage output for the DCtoDC 3V + 3V = 6V

     and as the DCtoDC range is 3A



Should this design works ?

Should I split both branches, the Cs to one DCtoDC converter, and the Is to another DCtoDC conveter ?

Should I add some resistors (please do not ask me to place 180 resistors in the circuit)  ?
u other components ?

Should I aim for another configuration that would work better ?



thanks a lot for your help,
I would not like to spend long time soldering to discover the whole thing burns when powering up.

OperaHouse

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Re: Lots of LEDs to be powered directly without resistors, should I ?
« Reply #1 on: September 28, 2012, 03:05:10 PM »
I would be interested to know if you ever get 1.8A out of that module or one like it.  I say like it because you chose a module that will not produce 6V from a 12V source.  You want a buck not a boost.  Keep us informed.

Why don't you solve a couple problems at one time and set up strings powered by something like 24V.
« Last Edit: September 28, 2012, 03:47:45 PM by OperaHouse »

DamonHD

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Re: Lots of LEDs to be powered directly without resistors, should I ?
« Reply #2 on: September 28, 2012, 03:34:41 PM »
0) Welcome! (Well, I'm nearly an engineer and definitely a computer person!)

1) LEDs directly in parallel won't be exactly matched so some will draw too much current and overheat while others too little and be dim.

2) You much always current-limit LEDs one way or another, though you can limit an entire series string with a single device if your supply voltage is high enough.  A single resistor is often good, but a constant-current source can be used instead, indeed a constant-current switching driver from a higher voltage is an elegant solution but will dissipate some power just like a resistor would.

Rgds

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OperaHouse

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Re: Lots of LEDs to be powered directly without resistors, should I ?
« Reply #3 on: September 29, 2012, 12:58:44 PM »
One can also ask why.  I've grown fond of the 5 meter 300 LED strip lights that you can get shipped to you for less than $10.  I have wrapped these around aluminum chime tubes as replacements for screw in lamps.  Attached to a flat plate they also get rid of heat buildup.  Already 12V they are a lot less work, cheaper and no need for a power supply.

richhagen

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Re: Lots of LEDs to be powered directly without resistors, should I ?
« Reply #4 on: March 11, 2013, 01:07:58 AM »
I would not do it the way you have it listed because the LED's will not be current limited.  If I were doing it, and I'm not the expert, I would run out a 12v and a ground lead and then put three led's in series connected through Commanda's circuit 4 from her master class.  ( http://www.fieldlines.com/index.php/topic,131213.0.html ) That way the current and brightness is held relatively constant.  Since I generally power single strings of Cree LED's with them I do not add equalizing resistors, so there are only four components in the ones I build.  It protects the LED's from over current conditions if there are fluctuations in voltage.  I have LED's that have been running daily through that circuit for many years now without problems, and I have built in the hundreds like that in total to this point since some time in 2007.  In those small systems, the batteries have failed, one guy baked the whole thing ruining the LED's anyway, and other things such as disconnected solder leads have happened, but that circuit is a tank and keeps on ticking as long as the FET is heat sunk.  The lights I built without current limit circuit generally have issues and failures.  Just my 2 cents worth.  Rich
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