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Connect Different Leds

camilozk

Apr 20, 2014
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Hi!

I am trying to figure out the best way to connect 3 Leds with different characteristics. All of them are 3 Watts; 1 blue, 1green, 1red.

red = 2.2 - 2.8V // 700mA
green = 3.4 - 3.8V // 700mA
blue = 3.6 - 3.8V // 700mA

I would like to use a 5 volts power supply for this. Also, the red Led is more bright than the other 2, so it is probably the best to provide less Voltage to it?

I would appreciate to read how you would approach this issue.

thanks in advance!
 

duke37

Jan 9, 2011
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To use 5V, each of the leds must be supplied separately. Calculate a resistance to give the required current from 5V. For example R = V/I so the red would need a resistor of 2.2/0.7 or about 3.3Ω. If you wish to reduce the brightness, try 5 or even 10Ω.

Power in the resistor will be I*I*R or V*V/R

Leds have a fairly constant voltage and to set the correct operating condition, current needs to be controlled.
 

camilozk

Apr 20, 2014
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thanks for your answer duke. and also harald for the post about leds, very usefull.

I am trying out the Leds with a "LM2596 DC-DC adjustable step-down". http://www.ebay.com/itm/221920170517?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

and this is the ebay link for the leds: http://www.ebay.com/itm/10-50-100-p...hash=item338bac36d0:m:myFT9DgSHWaMQVdx-30JfJQ

I am using 2.2Volts for the red, 3.7Volts for the blue and the green. When I connect them, they work for few seconds correctly, and the either they stop working at all, or they start flickering


am I doing something wrong or they are very bad quality?
 
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duke37

Jan 9, 2011
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You are doing something wrong!

A led needs a certain voltage to work. If you exceed this voltage by a very small amount, then the current will be very high and it will go pop.
You can control the current with a constant current supply rather than a constant voltage supply or use a resistor as I suggested.

Did you read the link that Harald suggested?
 

camilozk

Apr 20, 2014
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thanks for your prompt answer duke.

I did not exceed in any case the voltage range in which each led should work. Being their specs

red = 2.2 - 2.8V // 700mA
green = 3.4 - 3.8V // 700mA
blue = 3.6 - 3.8V // 700mA

I gave 2.2V to the red, and 3.7V to the green and blue.

This guy suggests in this video


that if they are not mounted in a "star board" they may overheat and burn themselves out.

I still didnt try the resistor because I dont have the resistors I need (1W and 2W). I thought to use this voltage regulator in order to see how much voltage I want to provide to the Led, and afterwards I can buy the resistors I need, in accordance to my experimentation.
 
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camilozk

Apr 20, 2014
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Is it anyhow possible that the current was too much for the Leds if I didnt exceed their working voltage range?

I am reading the link, but of course it is not possible to apprehend all this knowledge at once without having previous electronical training.

thanks for your support
 

duke37

Jan 9, 2011
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The video shows the led being driven by a current source, you have a voltage source. It is a different convertor, controlling voltage and current.
If the led needs 2.3V and you drive it with 2.3V it may work OK but it will get hot and then need 2.2V and the current will rise to destruction as shown on the video you linked to.

The voltage range will doubless be due to the tolerance in the production process or the voltage required at different temperatures.
 

camilozk

Apr 20, 2014
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ok.

so a resistor is limiting both voltage and current?
 

duke37

Jan 9, 2011
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Yes, it limits the current by dropping the voltage.
The led voltage and the voltage across the resistor must add up to your supply voltage. With a 5V supply and 2.2V led, the resistor must drop 5-2.2 = 2.8V. The resistor will then need to have a value of 2.8/0.7 = 4Ω. The power in the resistor will be 2.8*0.7 = 2W
 

Harald Kapp

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Nov 17, 2011
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I did not exceed in any case the voltage range in which each led should work.
Don't be so sure. The rated voltage ofd an LED is a typical value which may vary by a few 10s or 10s of millivolts depending on temperature, batch etc. You always have to control the current through an LED, not the voltage!. This is explained in our resource.
 

camilozk

Apr 20, 2014
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ok. I will have this much more present from now on, and I will read back the recommended website until I incorporate all the knowledge!
 

camilozk

Apr 20, 2014
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are you guys aware of a good source to control brightness of 3W / 10W single color or RGB Leds with Arduino?
 

BobK

Jan 5, 2010
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To control brightness use a 5V supply, a resistor and PWM. You will need a MOSFET to turn the LED on an off with the PWM signal.

Bob
 

camilozk

Apr 20, 2014
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thanks bob for the feedback. do you know any resource where I could get a deeper insight on the issue?

I found this guy who did a guide with a tip122, but he says he would rather use a mosfet.
http://henrysbench.capnfatz.com/hen...vices/10-watt-high-power-led-with-an-arduino/

I also found this video on mosfets which mentions the FQP30N06L as an appropriate mosfet to be used with arduino. (
)

can I rely on the guide mentioned above, but using the FQP30N06L?
 
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BobK

Jan 5, 2010
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That MOSFET is way wrong. It requires 10V on the gate, which the Arduino cannot provide, and can handle 32A, when all you need is < 1A.

What you need is a logic level N channel MOSFET that handles at least 1A of continuous current.

The IRLD014, used in this thread;

https://www.electronicspoint.com/th...chematic-yet-another-work-in-progress.256843/

Is appropriate. I built that project and it worked well.

Notice also how the MOSFET is connected in that project. You would do the same, but use only 1 LED and an appropriately calculated resistor.

Do you understand PWM on the Arduino?

Bob
 
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