Could Somebody figure out the normal and maximum supply voltages for normal red, green,yellow,blue and white LED's..

Calculating the resistance values is a mess without them.
Also different brands have slightly different voltages.How can we find out the correct value without damaging it?

 

Hello,

There are so called led-testers, they contain one or more constant current sources.
There are types for 10 mA 20 mA and 50 mA.
When the current is flowing through the led the voltage can be measured.
See here an example for sale in holland.
http://www.leds-buy.nl/led-tester-p-476.html

Greetings,
Bertus

 

A simple test with a multimeter?? And a power supply?

 

Hello,

You do not know what current is used by the multimeter.
There is an diode test in most multimeters, but that is most times limited to 1.999 Volts.
Some leds need upto 4 Volts.
Here is a nice chart with some voltages by color.
http://www.theledlight.com/color_chart.html

The link comes from the EDUCYPEDIA pege on led's.
http://www.educypedia.be/electronics/composemiconductorsled.htm

Greetings,
Bertus

 

Constant current sources are super easy to build. The LM317 is a great example, it and one resistor will do exactly that.

As with transistors the Vf of a LED is an average. An even rougher average can be...

White, Blue = 3.5V
Green, Yellow = 3.0
Red = 2.5

Older LEDs were built different, anything built 15 years ago or so is more like...

Green, Yellow = 2.0V
Red = 1.5V

The newer LEDs are much brighter, so bright it is dangerous to put them directly next to your eye when fully lit.

The constant current source as a tester is cheap (I've bought several over time) and easy.

http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=11647

Here is the LM317 version, I've recorded this one in my album since we use it so often...

 

I also used to test led's with LM317.. It shows around 17 ma on the multimeter.. And yes a multimeter can be used in series to find the current used by the led.. voltage is trail and error.. I found the safe volatge for normally bright blue as 2.8v and works well till 3v.. at 3.1v, it starts flickering and current shoots to 28mA after that in few secs, it blew off.. It was not a high intensity led..

 

Shouldn't they come labeled from the manufacturer?

That seems a natrual piece of info the customer needs up front when buying/using the LED?

Am I way off here in my thinking?

 

The term is constant current source, not a voltage regulator. If you had used a constant current source that diode would be alive today, illuminating the world, instead of dead and dark.

LEDs are current devices that happen to drop a voltage. It is up to the designer to limit that current.

 

Shouldn't they come labeled from the manufacturer?

That seems a natrual piece of info the customer needs up front when buying/using the LED?

Am I way off here in my thinking?

Uh, yes and know. They don't come with a label, but a set of specs. It's up to the user to know and look up those specs. There is a reason we have this section...

http://forum.allaboutcircuits.com/showthread.php?t=3246

 

Thanks for the info. That will definitely come in handy - especially when using components stripped from old devices.

Say Bill, do you have any suggestions for me on my 10.5v, 160mA Solar thread?

Thanks,

Solar1

 

What abt the volatge control for that above circuit?? What will come out?? We can set current limit to some 15mA.

 

LEDs need current. A constant current source is like a constant voltage source, it will take any voltage above the the minimum and it will regulate the same current. If you have a constant voltage (as in power supply) a resistor is simplier.

The LM317 does either job quite well. If you feed a constant current of 20ma you can measure a diodes Vf, which is what the testers are good for.

 

If you want a nominal 20mA current from an LM317, connect a 62 Ohm resistor from the OUT terminal to the ADJ terminal, and take the current from the ADJ terminal.

R1 = 1.25/DesiredCurrent, where 10mA <= DesiredCurrent <= 1.5A
For example:
R1 = 1.25/15mA = 1.25/0.015 = 83.3... Ohms.
The closest standard value is 82 Ohms. Calculating what the resulting current will be:
I = 1.25/82 = 15.244mA

An LM317 will drop about 3v across itself when used as a constant current source. So, to be useful in testing most types of LEDs, you will need a power supply capable of 8v to 12v output.

Note that even LEDs in the same lot won't have the same Vf. 75-85% of them will fall in a fairly narrow range, but the remaining 15-25% will measure significantly higher or lower, perhaps as much as 10%.

 

Ok ok lets start it clearly.. I want to drive a couple of leds on an automobile supply of a single colour. The voltage varies a lot. So can I use LM317 on constant current mode taking into account the total current by all the leds and calculating the resistance? What should be the current I should take for 1 led for maximum brightness without compromising its life??

 

The proper current for the LED is specified by the manufacturer.

At the very least, the LED should come with a typical Vf @ current specification. Most LEDs (75%-85%) in a given batch will be within a few percent of that Vf at the specified current. Some may be off by as much as 10%.

Most LEDs you get nowadays will handle 20mA with no problem.
Some are rated higher; 25mA, 30mA, 50mA, 70mA - it all depends on what the manufacturer specifies.

If you have a batch of unknown LEDs, you can get an idea of what their current rating is by slowly increasing the current through the LED until the color changes very slightly, then back off several mA's. The test LED will have a much shorter life due to the excessive current.

 

Just get an lm317 make variable for 1.2 to 5v. That should cover every ncolor size of LED. be carefull to start from 1.2v to higher voltage. there is no set precise voltage for LED depends on color size .

 

Ya I know.. But current wont be stable for such a setup.. I used to test that way.. When these guys opened my eyes..