Why are LED pulsed?

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Bingo!

;-) Rich

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Not true. If you have 10 LEDs in the array and each one is on for
10% of the time and you want the apparent brightness of each LED to

That is most likely the correct answer. It is often cheaper to use multiplexing circuit to drive large number of LEDs (LCD and VFD display as well) than to control each single segment individually.

Another factor is the current source. If it's just driving LED and nothing else, cheap design may not have AC to DC conversion and just feed AC straight into LED which results in 50hz or 60hz pulses.

--
When you hear the toilet flush, and hear the words "uh oh", it\'s already
too late.    - by anonymous Mother in Austin, TX

----- Original Message ----- From: "Mr. C" Newsgroups: sci.electronics.basics Sent: Friday, January 13, 2006 1:19 PM Subject: Why are LED pulsed?

Many people believe that the eye does act as a peak detector, and therefore a pulsed LED will use less power than a continuously lighted one off the same apparent brightness.

Not always so. They are pulsed because the total curent demand at one time is lower. If you have 10 leds drawing 10 ma each, that is 100 ma you have to design the power supply for. If you pulse them so only one is on at a time , you desing the supply for 10 ma.

If we start listing the things that many people believe (in the absence of demonstrated facts), it is going to make a long list.

As with most stuff in consumer electronics it's to save money.

something like that happens but the reason for the pulsing is that the way the LEDS are wired up they can only light a few of them at a time.

Exactly. And those lines are often the most expensive part of the integrated circuits driving the LEDs.

Bye. Jasen

Because whoever designed it wants the LED to get attention, and they know that our "lizard brain" naturally focuses on things that change brightness about once a second.

It's called the "Ooh, shiney!" effect. ;>)

Mark L. Fergerson .

Right on!

Brian

Not having any personal experience in the field, I'll stick to the "many people believe" line. But the idea does have some credence among the experts.

The following is an excerpt from On Semi ap note AND8067D

"There are two main reasons why LEDs are brighter when pulsed. First, the human eye functions as both a peak detector and an integrator; therefore, the eye perceives a pulsed LED's brightness somewhere between the peak and the average brightness [4]. Thus, an LED driven by a high intensity low duty cycle light looks brighter in a pulsed circuit compared to a DC drive circuit that is equal to the average of the pulsed signal. The second factor controlling the improved brightness is shown in the relative efficiency versus peak current curves of an LED."

Actually, I have done this experiment in a casual way. Just a bit of fun at lunch hour, and I won't make any claims about the validity of my results. The result was that I was unable to verify any improvement in efficiency with the pulsed driver.

But do I misunderstand the OP's question? I think he asked why some designers pulse LEDs. My answer is that some designers pulse LEDs because they believe it will improve efficiency. Is this not a correct answer to the question?

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Not true.

For the same apparent brightness when the LED isn\'t driven CW, the

How so? Show the math.

Assume a typical 3v 20mA LED:

continuously on = 0.06 watts per hour

50% duty cycle (such as direct off AC source) = 0.03 watts per hour

10% duty cycle = 0.006 watts per hour

Simple math. Unlike incascedant buld, LED doesn't suck extra power on initial illumination.

--
When you hear the toilet flush, and hear the words "uh oh", it\'s already
too late.    - by anonymous Mother in Austin, TX

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It would have been more helpful to the discussion if you provided the=20 link:

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I would have to read the referenced document, "4. Smith, George,=20 =93Multiplexing LED Displays: Appnote 3,=94 Siemens Semiconductor." befor= e=20 I was willing to give this any credence. I think they are overusing=20 the reference.

Here is the closest I could find to this reference:

(an excerpt) "The luminous intensity, or the luminance of GaAsP LEDs, is=20 essentially proportional to forward current over a wide range, but=20 certain phenomena modify this condition. At low currents, the presence=20 of nonradiative recombination processes results in less light output=20 than the linear relationship would predict. This effect is noticeable=20 just below 5 mA per segment (for 1/ 4 inch characters). The result is=20 that noticeable difference in luminance from segment to segment can=20 occur at low currents. At high currents, the power dissipation in the=20 chip causes substantial temperature rise, and this reduces the=20 dissipation efficiency Figure 4.

As a result, the light output versus forward current curve falls below=20 the straight line, at high currents (Figure 5). It should be=20 emphasized that this latter effect is entirely due to self heating. If=20 the power dissipation is limited, by running short pulses at low duty=20 cycle, the output follows the straight line up to very high current=20 densities. Whereas 100 A/cm2 may be used in DC operation, as much as=20

104 A/cm2 can be used under pulsed conditions, with a proportionate=20 increase in peak intensity. (If this did not occur, GaAsP lasers could=20 not be built.) Gallium Phosphide, however, has an inherent saturation=20 mechanism that causes a drastic reduction in efficiency at high=20 current densities even if the junction temperature remains constant.=20 This effect is due to competing non-radiative recombination mechanisms=20 at high current density. As a first approximation the brightness of a=20 pulsed LED will be similar to being operated at a DC forward current=20 equal to the average pulsed current."

I think such peak response effect of the eye is weak to the point of=20 nonexistence for pulse frequencies that produce no visible flicker.=20 Sight is a photochemical process that produces output proportional to=20 the rate of photon arrival.

actor=20

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The curve shown is incorrect, if efficiency refers to light produced=20 per watt of electrical power. The curves show light output versus LED=20 current, not efficiency. Since all curves roll off to the right,=20 actual efficiency goes down above some current not much above the=20 normal 20 mA for all examples. This is science by salesman.

I suggest you set up the experiment. Design a circuit that alternates=20 a second at a fixed current with a second at a pulsed drive, with=20 the period, duty cycle and peak current settable. For many=20 combinations of period and duty cycle, visually find the pulse drive=20 current that produces no visible brightness changes on alternate=20 seconds. The pulse current setting would be unknown till you conclude=20 the no flicker condition, to reduce experimenter bias. I would like=20 very much to see your experimental results.

Then we can refer such discussions to your paper.

0.06 watts, not watts per hour. Watts are equivalent to joules per second.

(snip)

And apparently, half as bright as the DC case.

You could, instead, lower the current to 10 ma, and get the same light output, but the LED voltage would go down a little below 3 volts, so the power would go a little lower than 0.03 watts, beating your 50% duty cycle case.

I guess it is possible that some designers do believe that pulsing LEDs makes raises the luminous efficiency and some apnote writers are, apparently, trying to convince designers that it is the case.

I think they are mistaken.

Do you mean that the eye perceives a continuous brightness equal to the pulsed brightness? If so, how do you know this to be true?

The limited ability of the eye to perceive pulsations above some frequency as separate events, is what allows a flashing sequence of images to be perceived as a smooth variation in time is what makes motion pictures possible, but that doesn't mean that the eye sees the continuous time approximation at the brightness of the individual flashes.

This discussion has tweaked my memory. I remember the one and only time I ever saw a suggestion that pulsing an LED was advantageous was in the HP Journal article that discussed the design of the original HP35 calculator. They said that pulsing the 7-segment displays had a bightness advantage, IIRC, but I don't remember the details. I'll see if I still have that around, or if someone can find it on the web, then I won't have to.