Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Driving an LED with PWM signal for higher light intensity

Status
Not open for further replies.

SimonTHK

Member
Hello electro tech online

I am working with LED and trying to make them as bright as possible.

What I have read, I can provide higher current through the LED with pulses. I am now running a 30 ma LED with 90 ma and pulses. 1khz with 1/10 duty cycle.

I was trying to measure the light intensity, but there is no difference between a normal none wave setup wtih 30 ma, compared to my PWM setup.

The measurements was done with a photodiode and I measured the voltage after. Can this be a bad way to measure the light and therefore I cant trust the outcome, cause maybe it shows more the average light intensity/voltage.
 
Hello,

You can put higher current through the LED with pulses that's true, but the current only lasts for the pulse period unlike a constant current. That means the average current stays the same. The problem is, with higher current the LED becomes more inefficient, so pulsing an LED with an average current the same as a constant current actually does not appear as bright.

I proved this some time ago by making measurements like you are doing. The better way to measure the light output difference (if any) is to compare it to an LED that is driven with a constant current, since this is what we want to test anyway. You can use your eye to see any change in light output with the pulsed compared to the constant current driven LED. It helps to construct a comparative light brightness device that would help see the difference, but you can even see the difference by shining the pulsed LED on a white wall and the non pulsed LED on the same wall in another place.
Of course you want to make sure you start out with LEDs that are similar in character, so you test them both first with constant current. Then, to make sure the dynamics of both LED are the same after you do the first test you swap LEDs and repeat the test again and you had better get the same results.

My conclusion was that it's not easy to make the test but the results came out pretty much like the data sheet indicates...when the LED is driven harder the efficiency drops.

There are other reasons to pulse however which you may wish to look into.
 
I wish to pulse because they have to be placed outdoors and have to be brighter than the sun (I will not work with UV light).

So the point is to give more light to my photodiode for a short time instead of constant current. This is possible and is what my teachers have told me. What I understand from what you are saying is, that it doesnt change the brightness effect. But is this brightness effect you are talking about for the eye that sees or the photodiode? If the photodiode is able to see this very bright light that happens for 1/10 duty cycle at 1khz, but your eyes cant, then I can use this for my outdoor lightgate (passing through and interrupting light), but not for more light in a room.
 
lets say I apply 3 times as much current, then I wont get 3 times brightness. Though if I can get 2 or maybe 2,5 times the brightness for just a short time (to measure on) then it is enough for me.
 
Last edited:
The eye has persistance that can make a pulsed light look brighter. On the down side, the LED light output efficiency drops at higher drive current. These two factors counteract. There may be an optimum point but you will have to use your eye to judge.
 
I use PWM to dim LEDs, and it does not make them brighter.
Persistence is a visual occurance for light durations that are fairly long since our vision is not a peak-detector. So short duration pulses of light appear dimmed.
 
Thank you people :)

With some testing on the osciulloscope we could see that an increased duty cycle would make the LED appear more bright for the eye, though the measured lumen (with photodiode) didnt change at all, and the LED would eventually burn since we have high current pulses.

The short pulses seems to lower the brightness, though measured over a photodiode shows that our single pulses are very strong. This makes us able to use it in an application outdoors and not interfer with the sun, which is what we want.

The link you send ronv is great.

Im sure this is what you guys also have been mentioning :)

RCinFLA There is alot to know about such a simple component as an led :) your two sentences makes a great point thanks.
 
Last edited:
Hello again,

Im sorry i thought you were talking about using the LED as a normal source of light for a human being, not for a photo detector of any kind. In that case the LED will appear *MUCH* brighter to the photo detector but only for a short time period as you noted. This of course assumes that the type of photo detector can respond fast enough to detect the light pulse properly. The average light falling on the detector will still be the same, but the peak light will be much higher and so should develop a much higher output signal from the detector for the pulse period.

The efficiency drops off as current increases, but you do get brighter light for that shorter time period. If you are starting with a 20ma LED and you pulse it at 40ma for 50 percent of the time or less (50 percent duty cycle) then the light output will be somewhat less than 2 times the brightness. If you pulse it at 80ma for 25 percent of the total time period then you will get maybe 3 times the light output.

In any case, the pulse time has to be at least that which does not cause the LED to see any more than the average current rating (such as 20ma) and also the peak current must be limited to the manufacturers recommendations unless you want to take a chance and try a higher peak current just to see how it would work out in the long run.
To keep the LED at the max average current rating you use a duty cycle that is proportional to the average current. For example a 20ma LED pulsed at 80ma you would use a duty cycle (expressed as a fraction) of 20/80=0.25, which is 25 percent.

If you are using a high power LED then you have to watch the peak current more closely as those LEDs are not rated as high as the smaller LEDs are proportionally.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top