Determine how to get the maximum brightness of LED

[wolfman hal]

I am building a project using Ardunio. I need MAXIMUM brightness from the LED'S

1. The LED's are RED and show a voltage of 2.5 v

2. Maximum current 20ma

LED's are 3mm

3. Output from Ardunio 5v

What size Resistor do I need to get the Maximum brightness without blowing the LED's?

When using OHMS LAW do I subtract the 2.5v from 5v to put in the voltage?

[RR_Mel]

From my experience 2.5 volts is a bit high for a red LED.  Most of the red LEDs I use peak at 20ma at 2 volts.

I use 180Ω resistors from my Arduinos for the max LED brightness.  I rarely run any LEDs over 10ma and they are still bright enough for me.


Mel



 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.

[speedybee]

In addition to the previous excellent advice re: 180ohms minimum, I notice you said LEDs plural. When powering things directly from Arduino pins you have to keep in mind two things:

1. The Atmega328 chip that powers most Arduinos is rated for no more than 200mA total current. The chip itself uses up to 40-50 mA, leaving you with 150, or no more than seven LEDs at maximum current.

2. If you're using a small Arduino, like the Pro Mini or its numerous eBay clones (which I love), the onboard regulator (if you're using it) may not even be able to produce the 200mA. So if you're using an onboard regulator, check what the regulator is, and look up its maximum rating.

 

And an extra, keep in mind, that at high currents like anywhere near 20mA you won't see the full 5V, there'll be some drop in the internal circuitry. Maybe more like 4.5, plus or minus a bit. Shouldn't actually matter for most purposes.

[RR_Mel]

speedybee

In addition to the previous excellent advice re: 180ohms minimum, I notice you said LEDs plural. When powering things directly from Arduino pins you have to keep in mind two things:

1. The Atmega328 chip that powers most Arduinos is rated for no more than 200mA total current. The chip itself uses up to 40-50 mA, leaving you with 150, or no more than seven LEDs at maximum current.

2. If you're using a small Arduino, like the Pro Mini or its numerous eBay clones (which I love), the onboard regulator (if you're using it) may not even be able to produce the 200mA. So if you're using an onboard regulator, check what the regulator is, and look up its maximum rating.

 

And an extra, keep in mind, that at high currents like anywhere near 20mA you won't see the full 5V, there'll be some drop in the internal circuitry. Maybe more like 4.5, plus or minus a bit. Shouldn't actually matter for most purposes.

 

Good advice Speedybee!

I rarely use the Arduino outputs for over 10ma to insure not dinging them.  I make driver boards that will handle upwards to 500ma at up to 45 volts using seven channel driver chips.  Three chips per expansion boards will handle all 20 usable outputs.  At a bit over 75¢ per chip and a $1 for the board makes for a pretty cheap high current driver for the Arduino UNO.

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.

[wolfman hal]

Sorry about that. I did not have the box with me. I am using a Ardunio UNO.

Every thing is working perfectly. Running off a old computer

Voltage Red 2.0-2.2  Yellow 2.0-2.2  Green 3.0-3.4  White 3.0-3.4  Blue 3.0-3.4

I currently have 8 LED's in Projects. 

5 Yellows, 1 Red, 1 White, 1 Blue

I have 330 ohm resistor on all.

I am stating a project with 2 Red LED's. I need to calculate how much I have already used.

I understand how to use OHM's law .

My question is.

What is amount of voltage in the formula.

Do I subtract the volts (RED 2.1) from the 5v output of the Arduino to use in the OHM's Law formula?

 

[BATMAN]

http://led.linear1.org/led.wiz 

 

[RR_Mel]

wolfman hal

 

Do I subtract the volts (RED 2.1) from the 5v output of the Arduino to use in the OHM's Law formula?

 

 

Yes.  The voltage out of the Arduino outputs will be slightly under 5 volts, around 4½ volts under load.


Mel



 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.

[Overmod]

And remember to really, really, really overdesign to avoid spikes if you're working near the limits of these devices!

Remember why the old joke isn't so funny: 'How do I get maximum brightness out of an LED?' "Crank up the voltage until you hear a pop, then turn it down a RCH and you've found the setting"

(we need to start a contest for 21st Century backronyms for certain precise measurement terms... )

[rrinker]

 That's the electronic equivvalent to how it was said Colin Chapman designed Lotus race cars - keep removing stuff to make it lighter, when something breaks, go back to the last version.

There will likely be little perceived difference between an LED running on the bleeding edge at 20ma and one running close at say 18ma, which gives a slight amount of headroom.

Something else to consider - there are LEDs, and there are LEDs - instead of ordering the cheapest Chinese ones on eBay, look for high efficiency red LEDs, if you really want bright. They can be blinding when run at half of their rated current, so running at 3/4 would amke them super bright yet not in danger of a slight voltage bump turning them into flash bulbs. Be sure to read the data sheet on the LEDs you use, the numbers tossed about are general rules of thumb and MAY be correct for the LEDs you use, but the right way is to be absolutely sure and get the information from the manufacturer or the seller.

To calculate the resistor, it's (Source voltage - LED voltage) / current in amps = resistor value. Since resistors have tolerences, they don't come in every possible number, so always pick the nearest HIGHER standard value. 

                                    --Randy

 

[Overmod]

rrinker

instead of ordering the cheapest Chinese ones on eBay, look for high efficiency red LEDs, if you really want bright.

And on top of that, if you want to go brighter still, consider LEDs with effective heatsinking, like those in full-scale headlight or home bulb replacement applications.  I'm more familiar with these putting the die directly on something with high thermal conductivity instead of 'potted' in clear plastic, so you may wind up with something like SMDs mounted on the end of a stick of material which is then heat-sinked itself rather than 'carving into' a plastic one along the leads and adding thermal-transfer material there.

[wvg_ca]

the spec sheet that you got with the leds should show thier rated voltage and maximum current already ...

just follow that ??