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Topic: Driving POWER LEDs ... (Read 3258 times) previous topic - next topic

simkard

Hi,

I'm actually in the begining of a project where i will use a led panel counting not less than ~150 power leds (read 3w leds).

The LEDs have a forward voltage of 2.4V and they run fine @ 700mA.
I would like to use an ATX Power supply because i will need :

  • +3.3V for bluetooth communications (i could even use the +3.3V provided by the arduino mega board)

  • +5V for quite all of my electronics that will include some relays, servomotors, etc ...

  • +12V x2 rails (like on quite all of the PC power supplies) for driving liquid pumps, solenoids, and ... the LED PANEL !!! which will consume a lot of Watts !



Take a look at this LED scheme i will (probably) use if it can do the job fine :
Done with th LED CALCULATOR : http://led.linear1.org/led.wiz
Code: [Select]
Solution 0: 5 x 28 array uses 140 LEDs exactly
    +----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 1 ohms
    +----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 1 ohms
    +----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 1 ohms
    +----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 1 ohms
    +----|>|----|>|----|>|----|>|----|>|---/\/\/----+  R = 1 ohms
... x28 (total)

The wizard says: In solution 0:•each 1 ohm resistor dissipates 490 mW
•the wizard thinks 1W resistors are needed for your application
•together, all resistors dissipate [b]13720 mW[/b]
•together, the diodes dissipate [b]235200 mW[/b]
•total power dissipated by the array is [b]248920 mW[/b]
•the array draws current of [b]19600 mA [/b] from the source.


The resistors i want to use are like those : http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&item=320667800523
2512 SMD resistor ; 1W ; 1ohm +/-5% ; runs from -55°C to +125°C

I will use Aluminum PCB optimising heat dissipation, big heat sinks and fans to dissipate the heat.

I would like to know if this will drive the LEDs safely.
I will equilibrate the power consummed from the LEDs on the 2 rails of +12V of the power supply.
It seems that it will consume about 20A constantly so having 10A on each rail seems safe.


Hope that some electronic-people will give me the bests advices i can get !

Please let me know !


LONG LIFE TO ARDUINOs and their community !!!!!!!

Grumpy_Mike

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Hope that some electronic-people will give me the bests advices i can get !


The best advice I can give is don't power your LEDs like this. For power LEDs you need a constant current drive. If you do make what you put with 1R resistors you will end up with a molten mess of metal.
When any LED calculation turns up with a value less than about 33R it is time to throw it away and consider some other path.

Look up constant current drivers and also be prepared to use large heat sinks.

retrolefty

Here is a E-bay search for 3watt (700ma) constant current LED drivers:

http://www.ebay.com/sch/i.html?rt=nc&LH_BIN=1&_nkw=3%20watt%20LED%20driver&_fln=1&_trksid=p3286.c0.m283

simkard

I know of these drivers ... but i have to power ~150 LEDs !!! i can't use them ! it is a waste of time/space.

Is there any other way of building a power driver (constant current source) with some cheap electronics instead of buying small ones ?

If it is cheaper, why not building it ?!

Grumpy_Mike

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it is a waste of time/space.

Well not if it actually works as opposed to your solution that would not.

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Is there any other way of building a power driver

Yes but it brings it's own heat sink problems as it is not electrically efficient.
http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/step2/Specs-Function/
This is because that is a linear regulator, the ones posted were switching regulators, they take much less power and produce much less heat. And before you ask why you can't make a switching regulator, it is because they take quite a bit of skill to design ad lay out correctly on a PCB it is not the sort of thing you can knock up on a prototype board.


retrolefty

http://www.ebay.com/itm/10x-Power-Driver-MR16-1x-3-Watt-LED-Light-Lamp-12V-/300601422018?pt=LH_DefaultDomain_0&hash=item45fd3db0c2

These are about $1 per driver and you can probably drive two leds in series with each driver. You will not be able to build anything cheaper.

Lefty

simkard

OUCH !

I understand that driving LEDs on +12V with resistors will be a disaster ... OK !

Now, i have to make a choice on which current source i will use on my project ...
I found some useful things on eBay like this one (but it is used...) : http://www.ebay.com/itm/90W-LED-Driver-Power-Supply-Constant-Current-HPLED-/160654435132

In fact, i would prefer to buy it, but i need a one case at all, i just don't want to have x modules around my led panel.
Only one would be a nice thing : :)

Grumpy_Mike

The problem with one big controller is that all the LEDs have to be wired in series which will require a voltage greater than the capability of the unit. The other problem is that all the LEDs would then be on or off together, unless that is what you want.

At 34V output max and you LEDs being you can only drive 14 in series.
It will also output 2.6A so that is too much for your LEDs and they will blow.

simkard

#8
Sep 29, 2011, 05:52 pm Last Edit: Sep 29, 2011, 05:54 pm by simkard Reason: 1
Ok ... well well well ....

In fact, what i need is something like a LED projector.
I need to power all the LEDs at the same time and @ 700mA (knowing that @700mA , they have a forward voltage of 2.4v).
I need about 200watts of LEDs ; knowing that a 3W LED will never run at this power consumption, they will mostly run @70% of their capacity, this is why i wanted to have ~150x 3W LEDs ...

By the way, i could buy some led drivers on eBay, but i don't know if these models are reliable enough (in time).
I even don't know how many heat they produce.
There is a lot of "i don't know" in this history.

Rolling over on eBay, i found this item : http://www.ebay.com/itm/320584715555

3A ... of constant current on about 100W ... i could drive a lot of LEDs with this !
If i drive 5 similars rails of LEDs, i could expect approx 600mA of current, which is good for me. (i could optimize their lastlife).

What do you think of this ?



PS : after reading your post, now, i doubt it is even possible ... OR, is it possible to drive the LEDs on a parallel basis with this kind of constant current power supply ?

retrolefty

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3A ... of constant current on about 100W ... i could drive a lot of LEDs with this !
If i drive 5 similars rails of LEDs, i could expect approx 600mA of current, which is good for me. (i could optimize their lastlife).

What do you think of this ?


You seem to miss the concept and need that each LED needs constantly controlled 700ma of current. How is the 3 amp constant current power source you linked to able to partial out constant 700ma current paths to each specific led?

Lefty


simkard

#10
Sep 29, 2011, 06:04 pm Last Edit: Sep 29, 2011, 06:10 pm by simkard Reason: 1
I know that i have too much less knowledge about LEDs ... and i'm here to learn some useful informations.

these 3A can't be divided if i split my LEDs on some parallel rails ?


OR, are you telling me that i will only be able to drive my LEDs with a constant current source that is on the same forward current (700mA for the ones i choosed) ?

Grumpy_Mike

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is it possible to drive the LEDs on a parallel basis with this kind of constant current power supply ?

No you can't drive LEDs in parallel with a constant current supply. If you try one LED takes more current than the others because the forward voltage drops are all slightly different. This then gets hot forcing the forward volts drop to reduce even more forcing even more current through the LED which eventually burns out and so the process starts on the next one.

So for an LED with a 700mA current requirement you need a 700mA constant current drive. You can put these in series up to the maximum voltage the constant current driver will supply.

simkard

Ok, now i understand perfectly the situation.

I want to thank you for your precious informations (and patience  :smiley-mr-green: ) for explaining this to me !

The problem i'm now facing is that i need a constant current source of 700mA (max) but that can handle a lot of watts ! (more than the ones we can find on eBay from shenzen ...)

By the way, i think i will not be able to find what i want because more than 50v DC is dangerous for human body !


Now ... i'm lost  :smiley-mr-green:

Grumpy_Mike

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i think i will not be able to find what i want

I would have to agree with you. In electronics things rated over 100V tend to be quite expensive as well and so designers like to keep the volts down.

retrolefty

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Now ... i'm lost 


No problem, just move on to another project.  ;)

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