What Is the Maximum Power Level Should I Run this infrared LED? Its published power rating?

Thread Starter


Joined Nov 4, 2018

My circuit is running at 109mw(See below). How much closer to its published power rating 150mw can I run on continuous use? As a caveat my ballast resistors total at 3 watts. I know its spec sheet says 150mw and I am under that but how close to max should I go -- is there a rule of thumb?

All direct measurements from circuit.

3 IR-LEDs in series
3 x 47ohm(1 watt each) ballast->141 ohms/3 watts

Vf LED = 1.47volts (measured)
measured Ia =
LED1 = .074a
LED2 = .074a
LED3 = . 075a

power = Vf x Iaf
= (1.47v) x (.074a)
= .109watts
= 109mw


Joined Sep 9, 2010
The answer depends on how well you are moving the heat away, and how carefully regulated it is. The closer you get to an edge, the more important it becomes to not have excursions that might take you over that edge. The published power rating might be exaggerated to assume a perfect and infinite heat sink. You probably don't have that.

It's also a sad fact of life that LED life is inversely proportional to brightness. The published rating assumes some acceptable minimum life, maybe 2000 hours. A lower power level might last 5000 hours. So it's up to the designer to consider what matters more, life or brightness. Tastes great or less filling.

Thread Starter


Joined Nov 4, 2018
Thanks for the reply. Is there any "rule of thumb" or something equivalent that is a guide to -let say- drive the LED at 80% its published power or something like that? That factors in inflation and exaggeration, etc. I understand I may be grasping at straws.


Joined Jun 5, 2013
Normally, the datasheet gives a rating for continuous use and a higher rating for pulsed use. Can you link to the datasheet.


Thread Starter


Joined Nov 4, 2018
I am remiss to say I don't have anything but the sale specs on Ebay:

※1xPCS 50Pcs 5mm Infrared IR LED Night Vision 940nm Luminous Diode LED infrared Lamp.

Features:※ Small and light appearance, it is convenient to carry.※ Small volume, low power consumption, excellent directivity.※ With IR function, it is easy to use.※ It can be used in the field of monitoring, remote control, photoelectric control, target tracking etc.

Specifications:※ Quantity: 50pcs※ Diameter: 5mm / 0.2"※ Material: GaAsP※ Emitted color: Infrared ray※ Forward voltage: 1.4--1.65V※ Backward voltage: 5V※ Forward current: 100mA※ Wavelength: 940nm※ Angle: 90/60/45/30 degree※ Luminous power: 150mw※ Frequency characteristic: Low frequency.

The default angle is 90 degree,if you need other angle,pls kindly contact us when you place order!


Joined Nov 16, 2007
My "rule of thumb" for buying from ebay or other questionable sources is to stay clear of maximums, especially if you want to make the device last. I'd take 10%-20% off the "rated" value.