Keeping your LED cool with PWM

SO I need to use some 1W and 3W leds for the brightnes, but my application will not allow the start heat sink. I have come across a few articles talking about using PWM to drive LEDs Does pulsing an LED at higher current yield greater apparent brightness?

What got me in the link above is

as you reduce your duty cycle you can increase your current because there will be less time for heating affects to kick in. I have done this with a very very short duty cycle using a pulsed laser power supply at 2kV on a normal 1.9V diode. This was pulsing so often that they eye cannot tell. the brightness of the diode increased significantly. My professor and I built a power supply that had a short duty cycle and only overvoltaged the diode to a little over an amp of current and successfully made a normal diode look brighter then a 1W LED when it was only a cheap LED. – Kortuk Jul 28 '11 at 16:09 Blockquote

What I'm needing to achieve is an LED that can be seen in day. I'm putting them on a RC Airplane.

• And your actual question is ...? Aug 31 '18 at 20:24
• How are you going to power 3W LEDs in an RC airplane? How heavy of a load can it carry? Sometimes PWM is more efficient than a current limiting resistor. Depends on power supply i.e. battery voltage and discharge curve. I do not believe that article to be true. Aug 31 '18 at 20:30
• @Bimpelrekkie I'm looking for the brightest LED I can get that will run cool. Sep 1 '18 at 4:13
• I don't know if that is really true. If it is, it is due to a visual effect of the human eye (maybe your eye acts like a peak detector rather than an average brightness detector). From a physics and measurement perspective, it is definitely not true that pulsing the LED makes it brighter. Sep 1 '18 at 18:50

You can indeed achieve higher brightness by PWMing an LED. You can also (sometimes) achieve better power efficiency. But those things are usually mutually exclusive.

If you turn an LED on for 50% of the time, and off for the other 50%, you will decrease its perceived brightness by about half. This creates a lower brightness with less power consumption (provided you are providing it the same current).

The same goes for the other way. You can provide a higher current in a short PWM cycle to achieve higher brightness. Keep in mind that this usually uses more power and can shorten the life of the LED. The extent of these drawbacks depends on how much current you are applying to the LED and for how long (depends on the PWM cycle you decide to use).

If you look at a data sheet for a 1 Watt LED you will see there is a DC forward current and a Pulsed Current.

Your specifications will likely differ but for this LED:

• DC Forward Current: 350ma
• Pulsed Forward Current: 700ma

The manufacturer also specifies a Typical Period (TP) of 100uS and a duty cycle of 0.005. This means that 1 cycle takes 100 microseconds and the LED is on for 0.5% (0.005 x 100). That means that if implemented as the specification says the LED will be on for 0.5us then off for 99.5us.

The most useful thing is that it keeps the LED cool. The LED is turned on for a small period and off for a larger period while it cools down.

To achieve what you want, higher brightness, you can certainly over power the LED at small duty cycle. Just be aware of the consequences of more heat and potentially less life time.

To tell how much current you should apply to achieve your desired results, you should be able to look at your LED Spec sheet. It should have a graph of current (ma) against luminosity (lumens). In general you can see what the apparent brightness is with: Apparent Brightness = Luminosity x Duty Cycle

• Interesting (and upvoted), seems like 'over-currenting' a LED. Sep 20 '18 at 10:47