# help with leds

Discussion in 'The Projects Forum' started by DR1, Mar 4, 2013.

1. ### DR1 Thread Starter New Member

Sep 13, 2012
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0
Im powering two 3 watt rgb leds with my arduino using a 13.8V/3A power supply. The are the led specs:

Forward Voltage: 2.5V/3.4V
Continuous Forward Current:350/400
Peak Forward Current: 800mA
Max Forward Voltage: 2.8V/3.8V

Would I need a constant current driver even though the leds can source the needed current from the power supply?

2. ### #12 Expert

Nov 30, 2010
16,665
7,310
Electronic devices don't use light bulbs very much now because LEDs are way more efficient (lumens per watt) and durable. Drop an LED on the floor and it just bounces, no harm, no foul. The important part of “Light Emitting Diode” is, “diode”. LEDs aren't resistors. They don't work like Ohm's Law. An LED that is barely on at three volts will be completely smoked if you apply four volts to it. It has this “breakover” characteristic like a rectifier diode, because it IS a diode. Once you get to the breakover voltage it starts letting current through in an almost unlimited manner. That is why you must use some way to limit the current through an LED. The easiest way is to use a resistor. Say you have a 9 volt battery and an LED that needs about 3 volts to start up. You use Ohm's Law to calculate a resistance that will keep the current under the usual .02 amp limit. Calculating from the fact that you have 6 leftover volts that the LED doesn't need, the math goes like this:

R = V/I
R = 6 volts/.02 amps
R = 300 ohms.

Any resistor more than 300 ohms will protect that LED from smoking.

Another way is to use a constant current circuit, but that's getting a bit complicated for beginners. The way it works is that it allows .02 amps to flow and the LED just uses up the amount of voltage it needs. That's the opposite of a radio where you give it some voltage and it only takes the amount of current it needs. The cool part is that an LED gives you some result besides numbers on a meter. They actually DO something. Bill Marsden has posted a lot of LED circuits on this site: LEDs, 555s, Flashers, and Light Chasers. It is also a more comprehensive tutorial on LEDs.

3. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
How are they matching together with the power supply? Totally not. You need LED drivers.

Especially RBG LEDs need 3 slightly different voltages. Or the current will be off, if you run them from one voltage. One solution are resistors, but not so good for 3W.

4. ### DR1 Thread Starter New Member

Sep 13, 2012
23
0
Im familiar with the basics of leds, however, this my first time using High Powered LEDs. I know ppl use constant current drivers for high powered leds. Right now Im powering my arduino project off a benchtop ATX power supply. But now I'm trying to move the project to a perfboard so I can free up the Arduino. I have a spare power supply(13.8V/3A) that I will be using for now. My first question would be, since the power supply can output 3A, that's plenty of current for the leds to draw from, correct? Or am I thinking about this the wrong way? If I add the correct resistor to bring down the 13.8V to the forward voltage(3.8V) I wont have that 3A anymore, that number will come way down, correct? This is the part Im confused about, when I bring down the voltage using a resistor will I still have access to the full 3A my power supply can push out?

5. ### DR1 Thread Starter New Member

Sep 13, 2012
23
0
I am using N Channel Mosfets, I have the Gate of each to the neg of the RGB hooked up to the PWM pins on my Arduino, Drain to 3.3V, Source to Ground. They're common Anode leds

6. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Get some of these LM2576 boards. Then at first turn down to zero volt. Carefully crank up. Does the LED explode? No. you can then increase as much as you think it is good.

Or get a real LED driver.

Think: 12V 1A in -> 3V, 4A out.

Only the watts are important.

If nothing is connected, your powersupply will supply zero Watts.

I use such a driver here, from 32 volts, for a large LED panel from 5mm LEDs. Everything heats up, the LEDs, the LM2577 chip. But nothing has burned out so far.

Get these LED driver modules, this is the regular way.
If they are too expensive, use dc/dc converters.
Resistors are really the last resort.
I use sometimes a very small resistor for LEDs with large Wattage, and direct connection. Only to offset the current a little, not to drop off meaningful power. For instance 2.1 Amps which is too high, to 1.7 Amps.

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
Lets start simple..
LED's are like fat people.. Put a giant mound of food (3 amps of current) in front of them and they will try to eat it all and explode.
Spoon feed them (with a resistor or CC supply) and they do just fine.

Check out the Meanwell LDD series for some low cost CC drivers. Like the LDD-700HW

thatoneguy and #12 like this.
8. ### #12 Expert

Nov 30, 2010
16,665
7,310
First, you don't want to give YOUR leds the maximum possible forward voltage for the worst led in the batch that the factory made. Second, there is no law that says you have to drive YOUR leds at the highest current they will survive. Third, you are not lowering the voltage to 2.5 volts or 3.8 volts, you are limiting the current to 350 or 400 milliamps...or less.

After you create a circuit that will provide 350 ma to one of your LEDs, that current will no longer be available to the other leds. The rest of the 3 amps you have will be available for other leds. All you have to do is attach another wire to the 13.8 volt supply line and make a second circuit to give current to a second led.

9. ### DR1 Thread Starter New Member

Sep 13, 2012
23
0
I know pretty much nothing about switching regulators so Im gonna have to read up on them. Money is an issue atm, so I'm trying to use what components I have. I do have one driver though:
http://www.superbrightleds.com/more.../700ma-constant-current-led-driver/1323/3045/

Great analogy, lol. Trying to continue with that, I thought leds only draw what they need. So if they're only hungry for 400mA, wouldn't they leave the rest for everyone else? I thought it was only voltage, that if you give them too much, out comes the magic smoke.

I'll stick to the recommended specs. But wait, if Im using a resistor, wouldnt I be dropping the voltage? I know its not the ideal way and it will produce a lot of heat. The part in RED, see this is where its just going over my head. Im fairly new to electronics and I've messed around with regular 3mm, 5mm leds, Im just now stepping up to high powered leds. In BLUE, this will probably sound dumb, but your saying I need to create a circuit to provide that 350mA but why do I have to provide it 350mA, there's 3A there for it to draw? why wouldnt the led just draw the 350mA/ as long as Im not feeding it the full 13V its not gonna burn out on me. I'll keep reading, and sorry if it seems like Im being difficult, its just Im tryna get this down and Ive had this setup on my breadboard with the benchtop powering it and I havent had any problems yet. But then again its a benchtop and has way more power than the circuit requires. But turning this into a embedded circuit.. man I need a break.

10. ### #12 Expert

Nov 30, 2010
16,665
7,310
The important part of Light Emitting Diode is, diode. LEDs aren't resistors. They don't work like Ohm's Law. An LED that is barely on at three volts will be completely smoked if you apply four volts to it. It has this breakover characteristic like a rectifier diode, because it IS a diode. Once you get to the breakover voltage it starts letting current through in an almost unlimited manner. That is why you must use some way to limit the current through an LED.

11. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
No.. no.. no...
You have to keep remembering they are like fat people. They are ALWAYS hungry. Have you ever seen a fat person leave food for others.. They are inconsiderate, they try to shove it ALL in their face at one time. Then they just puke (aka..let out the magic smoke). An LED/fatty will even take some voltage but unlike with current its only takes enough to fill its "forward voltage" pocket.

They have NO manners.

Its simply ohms law
Lets assume the following
A 15V power supply

An LED with a forward voltage of 5 and a max current of 1A

Ohms law says
R= V/I
So we know our V is 15 but that chubby LED already filled its "forward voltage" pocket with 5 V so we have 10V left.
And we know how much food we can safely feed this fatty LED till it starts to bloat.. 1 Amp as it said in its "fatty care manual" aka datasheet.

So V = (15-5) = 10V and
I = 1A
So back to R = V/I the resistance we need around this LED's neck to keep him from piggin out is 10/1 or 10 ohms.

Now we need to see how big that resistors muscles need to be (power or watts) to choke that LED's throat to prevent him from eating everything.

Well that crazy ohm guy also figured out that
P = I^2 x R
Well we know I is 1 and R from the last fun fun math lesson was 10
So we need to head over to Muscle Beach and pick a big meat head because we now know we need a 10W resistor.. But being the smart guys we are we know that if you only get a muscle head with "just" enough power to choke that LED's neck he is gonna get tired (or heat up) pretty fast. So for "safety" we always get a muscle head thats twice as strong as we need to keep him from getting tired. So you need a 20W resistor with a resistance of 10 ohms.

That was kinda fun.. stop thinking an LED has manners.

A constant current power supply is one that is typically more efficient at choking that throat versus a resistor so when you get into higher current LEDs you want to be more efficient and use (or waste) less energy. It uses other methods..spoon feading.. to keep that LED from going gonzo.

Last edited: Mar 4, 2013
12. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
For LEDs with higher power you can not get proper resistors. Or to have an array of exotic and expensive resistors.

So I have a few main voltages, and if I get new LEDs (or LEDs burn out ) I try to match them together so they can use one of the voltages.

Sometimes they heat up just a little too much, and need a 0.3 Ohms resistors or something. I use toroid inductors for that...

In some cases I have to install another dc/dc converter.

If it would be permanent, very likely I'd use LED driver modules.

Having 10 LEDs in series is comfortable to adjust, they do not burn out easily or heat up so fast.

They do have some internal resistance, which adds up the more LEDs you add in series.

Thermal runaway does exist, but the current is far too high at the starting point in such cases. Normally the cooling system should keep the LEDs from overheating. One way is to crank up fan voltage- reducing their lifetime expectation.

13. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Although the LED and fat people analogy made me LOL..

LEDs can be effectively modeled as a short circuit with a voltage above their Vf.

Once the Vf is met, LEDs are current controlled devices. The more current, the brighter, and hotter (up to, and including, fire).

Read the datasheet. There should be a rating for Voltage@current. That is usually the suggested operating current. LEDs CAN be run far below the maximum current. Think "flashlight" vs. "indicator light". The same LED can fulfill both roles, depending on current fed through them.

Ohm's Law with LEDs to find resistance when using a voltage source:

$
R=\frac{V_{suupply}-V_{F_{LED}}}{LED_{current}}
$

14. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
If you write "LEDs", this is not saying anything.
Also an "effective short circuit" is not saying anything.

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15. ### patricktoday Member

Feb 12, 2013
157
42
But posting a picture of a piece of wood with a bunch of toothpaste on it will resolve all OP's questions?

thatoneguy, mcgyvr and tshuck like this.
16. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Certainly it will be beneficiary for OP. I hope.

Last edited: Mar 5, 2013
17. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,787
973
Come on man thats "high-tech" No More Nails used by carpenters... or hack LED assemblies in Ireland.

That looks HORRIBLE. Not to mention a 20% shrink ratio .. There is NO way you are getting a proper/consistent thermal bond with a 20% shrink ratio. And a max temp of 70 deg C..

18. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Hmm I was thinking about that too.

It does not shrink.
The LEDs are attached 100%.

I have run some LEDs very hot, more than 100C.

No signs of weakening.

I have LEDs under full load since January. All OK, normal temperature.

It is vibration resistant, and remains in place. It is adhesive.

The 2 Euro variant works same good, is a little softer, but smells the same, so it is a No more Nails knockoff.

Also used for LM2576, and a large bridge rectifier.

All works excellent.

I use thermal grease of course. Essential for power LEDs. And I can ensure you they run at full Watts.

Last edited: Mar 5, 2013
19. ### DR1 Thread Starter New Member

Sep 13, 2012
23
0
Ok I think I have it down now. I'm looking to make cc drivers for my two 3W rgb's. The simplest Ive seen was a LM317 with a resistor. Any reason why this wouldnt work?

20. ### DR1 Thread Starter New Member

Sep 13, 2012
23
0
I should be able to order components at the end of the week. I plan on trying all three, a resistor, LM317 and an LM2576. Can someone take a look at my cart for digikey make sure I have everything I need.

Resistor 16ohm/10W:
http://www.digikey.com/product-detail/en/SQP10AJB-16R/16W-10-ND/18740

LM2576: