Maker Pro
Maker Pro

LED forward voltage and brightness

Rajinder

Jan 30, 2016
568
Joined
Jan 30, 2016
Messages
568
Hi
I wanted to ask a question on LEDs.
If i have a 3V3 supply and using a LED of 2.1V forward voltage and IF of 20mA.the mcd is 200. I tend not to run st full 20mA but give some head room so 15mA is what i normally use.

To calculate the required resistor is
R = 3V3-2.1V/15mA

How is this current related to luminous intensity i.e. mcd? Is it the more current the closer it gets to 200mcd?

Also if i have a pure green LED with VF of 3V, then i believe it is not worth trying to use with a 3V3 supply. Am i correct in this?
Thanks in advance.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
The luminous output of an led is roughly proportional to the current.

You are right that the current will not be well controlled when the led Vf.is 3.0V and the supply is 3.3V.

Bob
 

Rajinder

Jan 30, 2016
568
Joined
Jan 30, 2016
Messages
568
Hi Bob
Thanks for your reply. So am.i correct in saying that if i increase the current the LED will be brighter and get closer to the mcd value of 200?
Thanks in advance.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
Yes, but your eye will barely distinguish the difference between 15 and 20 mA.

Bob
 

kellys_eye

Jun 25, 2010
6,514
Joined
Jun 25, 2010
Messages
6,514
The MCD values are usually ignored in the types of application you're using. You (generally) set the current so the LED can be seen under the expected ambient conditions of use - whatever that turns out to be.

Same as a car might be able to rev to 6000rpm but you don't go driving with the engine screaming like that all the time!

If circuit power consumption is critical then use LEDs that have specific high mcd/mA (high efficiency) where you can actually 'see' the LED with only a single mA (or a couple of mA). If current consumption is 'irrelevant' then run them 'just under' their specified parameters to ensure longevity.
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
And be aware that mcd is NOT a measure of how much light an LED outputs and mcd / power is NOT a measure of efficiency. The viewing angle is also necessary to determine these. You might have one LED that is rated at 1000 mcd and another that is rated at 100 mcd. But if the first one as a viewing angle of 10 degrees and the second has a viewing angle of 120 degrees, the second is putting out more light.

Bob
 

Rajinder

Jan 30, 2016
568
Joined
Jan 30, 2016
Messages
568
Thanks for the info. I really thought that mcd was how much the led was outputtcloseing. I thought it was the more current through the led the closer it gets to the mcd stated on the datasheet.
I need a led to light through some plastics. 35mcd was not good. But i found a 220mcd and with the right selection of resistor it seems fine. So am i doing something wrong?
 
Top