LED Forward Voltage

[jrz126]

I just want to make sure that I'm correct. When I bought my blue leds, I wasnt given a forward voltage or anything. So I did a few tests to try to figure it out.
I used 3 different leds, with a 1k series resistor. I adjusted the supply voltage until I got the desired currents. Then I measured the voltage drop across the LED. So I'm assuming my Vd is 3.4V~3.5V? also, is it possible that these leds are rated for more than 20mA?

LED 1 5mA- 3.164 10mA- 3.331 15mA- 3.412 20mA- 3.462

LED 2 5mA- 3.221 10mA- 3.364 15mA- 3.444 20mA- 3.488

LED 3 5mA- 3.178 10mA- 3.327 15mA- 3.412 20mA- 3.478

 

[JohnBrown]

A quick look at the Maplin catalogue shows blue LED's having a forward voltage anywhere between 3.4 and 4.5 volts, so I'd say that your results make sense. As far as current is concerned, some LED's have absolute maximum ratings of around 50mA, but it's going to be harder for you to test, unless you have a lot of LED's and are prepared to burn a few out!
Most light output figures are quoted at 20mA.

 

[jrz126]

I have about 115 of them, so I can afford to lose a few. how would the brightness increase from 20mA to 30mA? would there be a large increase? Is there any sort of formula for the brightness output? Also, would running the LED's at greater than 20mA cause them to fail quicker?
I'll probably test the max current and brightness this thursday in one of my EE labs.

Thanks for the replies.

 

[JohnBrown]

I really don't know the answers to your questions. The info is probably out there on the web somewhere.

 

[Optikon]

I have about 115 of them, so I can afford to lose a few. how would the brightness increase from 20mA to 30mA? would there be a large increase? Is there any sort of formula for the brightness output? Also, would running the LED's at greater than 20mA cause them to fail quicker?
I'll probably test the max current and brightness this thursday in one of my EE labs.

Thanks for the replies.

The brightness may not increase if the part becomes stressed and begins to burn out. I would not go above 25mA (which is typical max for LEDs).

You will certainly reduce the life at higher currents. The brightness will generally increase up to the point where the die heating becomes excessive and then it will not function properly. Most LEDs do not have a good way for heat to escape from the die and so power dissipation is a real concern.

When you do the blow-out test keep somehting in mind. If you discover that 40mA blows them out, that doesnt necessarily mean 30mA is safe. I would go with the mfg recommendation to ensure maximum life (they have already done extensive testing).

Considering the hypothetical example, if 40mA blows em out right away, 30mA may kill it after 1000 hours etc... if it is rated for 100,000 hours then that will only be accomplished by not exceeding max spec (usually 25mA)

Also blue LED's usually cannot withstand much reverse voltage (some of them like 3-5V) so make sure your circuit doesnt cause brief high reverse voltage conditions as that too will reduce the life drastically.

 

[Oznog]

Life span calculations usually don't involve the part dying, but rather the light output decays below an acceptable limit.

Using the LEDs at higher than rated current will make them lose intensity much sooner as the heat degrades the die. Actually, if it's a flashlight, this may not be significant due to the low number of hours it's used, many flashlights use LEDs way past their ratings.

 

[sardineta]

Define the current, and work, too much current will stress the led

you need to limit the current with a resistor (calculate and connect it in series with the led)

for example
your diode can work at 3.5 volts @ 15mA then
voltage source 9V

(9-3.5)/0.015 = R = 366 ohms, but thi value it just in calculation, the nearest value is 330 or 680 ohm's
(Vs-Vled)/(LEDcurrent0)=R

so you will need to take into account spending more current with 330 or make an array of resistors.

 

[John Sorensen]

Is there a reason you need a blinding output? This is probably for your car, huh? Nevertheless, I usually set my LEDs to the minimum current needed for a decent output. Discernable output increase per mA falls off rapidly.

j.

 

[crust]

Most LEDs can tolerate higher currents if the duty cycle (i.e. mean current) is reduced. For instance, if switched with high enough frequency you can run an LED at 50mA but for 1/2 the time and get about equivalent power dissipation and light output to 25mA for full time. This is used often in multiplexing displays.

 

[theboss]

upto my understanding and observation of running led's i see that led merely heat up so i conculde that LED are designed to run with almost negligable heat dissipation..

so pump up the led as long as it doesnt generate any considerable heat. As soon as u feel the heat, u will know that the max limit has been crossed. Step down and fix in a virgin led. The point where brightnest feel optimum with negligible heat dissipation is approxmiate power u must pump into the led..