# Why are LED pulsed?

Discussion in 'Electronic Basics' started by Mr. C, Jan 13, 2006.

1. ### Mr. CGuest

I am curious. I wonder why LED are pulsed on many of the consumer
products I see. I have seen several devices pulse the LEDs
and I can actually see the blinking, especially if my eyes scan across
the display or a panel with discrete LEDs. Could it be that a super
high intensity captures the eye and the eye retains the recognition
for a while even when the LED is off? If so, then the LED would have
to be pulsed often enough for the eye to think it is still on. But
why is a shorter high current pulse better than a longer (or even
continuous) duty cycle at a lower current? Maybe its related to a
multiplexed drive arrangement where fewer lines are used if pulsed.
Just curious.

2. ### Rich GriseGuest

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bingo!

;-)
Rich

3. ### Ralph MoweryGuest

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.

4. ### ImpmonGuest

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.

5. ### John FieldsGuest

---
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
remain constant, then you'll need to drive each one at ten times its
normal CW rating for 10% of the time and drive the array fast enough
so that persistence of vision won't see the flicker.

That means that if you have ten LEDs which are being driven at 100mA
each, for a tenth of a second each, and you're driving ten LEDs that
way, the average current out of the supply will be 100mA.

6. ### BlakeGuest

----- 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.

7. ### John PopelishGuest

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

8. ### Jasen BettsGuest

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

9. ### Mark FergersonGuest

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

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

Mark L. Fergerson

Right on!

Brian

11. ### BlakeGuest

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."

12. ### Homer J SimpsonGuest

And as an aside, pulsing or strobing reduces the power supply demands.

13. ### John PopelishGuest

How so? Show the math.

14. ### John FieldsGuest

---
Not true.

For the same apparent brightness when the LED isn't driven CW, the
power supply will be called upon to deliver high peak currents which
it may not be capable of delivering. That won't reduce the power
supply demands, it'll aggravate them and require the power supply to
be designed to supply those peak loads.

15. ### ImpmonGuest

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.

16. ### John PopelishGuest

It would have been more helpful to the discussion if you provided the
http://www.onsemi.com/pub/Collateral/AND8067-D.PDF
I would have to read the referenced document, "4. Smith, George,
“Multiplexing LED Displays: Appnote 3,” Siemens Semiconductor." before
I was willing to give this any credence. I think they are overusing
the reference.

Here is the closest I could find to this reference:
http://digchip.com/datasheets/parts/datasheet/000/APP03.php

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

As a result, the light output versus forward current curve falls below
the straight line, at high currents (Figure 5). It should be
emphasized that this latter effect is entirely due to self heating. If
the power dissipation is limited, by running short pulses at low duty
cycle, the output follows the straight line up to very high current
densities. Whereas 100 A/cm2 may be used in DC operation, as much as
104 A/cm2 can be used under pulsed conditions, with a proportionate
increase in peak intensity. (If this did not occur, GaAsP lasers could
not be built.) Gallium Phosphide, however, has an inherent saturation
mechanism that causes a drastic reduction in efficiency at high
current densities even if the junction temperature remains constant.
This effect is due to competing non-radiative recombination mechanisms
at high current density. As a first approximation the brightness of a
pulsed LED will be similar to being operated at a DC forward current
equal to the average pulsed current."

I think such peak response effect of the eye is weak to the point of
nonexistence for pulse frequencies that produce no visible flicker.
Sight is a photochemical process that produces output proportional to
the rate of photon arrival.
The curve shown is incorrect, if efficiency refers to light produced
per watt of electrical power. The curves show light output versus LED
current, not efficiency. Since all curves roll off to the right,
actual efficiency goes down above some current not much above the
normal 20 mA for all examples. This is science by salesman.

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

Then we can refer such discussions to your paper.

17. ### Sjouke BurryGuest

At 50 percent you have to doublee the current to
get the same brightness,so where is the advantage??
Or do you accept halve the lightlevel??
Because thats wat you get .

18. ### John PopelishGuest

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.

19. ### BlakeGuest

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?

20. ### John PopelishGuest

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.