# LED Supply Volatges

Discussion in 'The Projects Forum' started by jj_alukkas, Jan 13, 2009.

1. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
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?

Apr 5, 2008
15,803
2,388
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.

Greetings,
Bertus

3. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
A simple test with a multimeter?? And a power supply?

Apr 5, 2008
15,803
2,388
5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
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...

6. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
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..

7. ### Solar1 Member

Jan 12, 2009
15
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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?

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
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.

9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
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...

10. ### Solar1 Member

Jan 12, 2009
15
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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

11. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
What abt the volatge control for that above circuit?? What will come out?? We can set current limit to some 15mA.

12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
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.

13. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
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%.

Last edited: Jan 13, 2009
14. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
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??

15. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
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.

16. ### italo New Member

Nov 20, 2005
205
1
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 .

17. ### jj_alukkas Thread Starter Distinguished Member

Jan 8, 2009
751
5
Ya I know.. But current wont be stable for such a setup.. I used to test that way.. When these guys opened my eyes..