LED driver to illuminate 300+ LEDs efficiently help

[Kristopher keller]

I've just been getting back into electrical circuits and understand
the basic concept (I'm a newbie). I'm looking at making some large
bright colorful LED signs for effects at parties. This will require
about 300 - 500 LEDs. I know i could just pump the correct amount of
voltage to run them, however I've heard by running a "duty cycle" to
leds, you can use 1/2 the energy with the same if not more brightness.
I think I see this on newer bus tailights, when you pan your eyes
real quick, you see pulses of light (about 60 per second, I counted
them) I've also made a few smaller circuits which used the 555 timer
to accomplish this cool technique, in fact, ran from 2 AA batteries
for a month straight. I'd like to get the same energy type savings,
but powering 300 leds. Since the 555 timer controlled about 10 leds,
I dont want to have to create 30 separate 555 circuits - it'd get
expensive, this is just a hobby type project, and I'm sure 1 555 timer
chip would not be able to handle the wattage to run 300 of them. So
i'm looking for a little assistance in designing something to drive
300+ LEDs from a standard 110-115AC. I know how to convert AC into DC
and such like that. I'm guessing the power consumption shouldn't be
more than 10-20 watts.

Thanks in advance.
-kris

 

[Jeroen Vriesman]

Well, 1/2 the energy, from the specs you can see that effectivity increases
with current, at some current you just need to flash them in order not to
blow them up, but my experience is an increase of about 20-30 % (still a
lot).
Also, a 50% duty cycle at 2*X lumen looks brighter than continious X lumen.

You are talking about using the correct voltage, but you should start
thinking in current to drive your leds, because of tolerance in the led
voltage, driving them in parallel with a voltage would make one led
brighter than the other.

So put them in series. Depending on how much voltage drop the leds have,
and what your max V+ supply is, you use the maximum number of leds in
series.

The output stage after the 555 can be a simple current source:

V+
|
series of leds
|
|
|/
out 555 O-------| BC547
|\
|
| | R (e.g. 150 ohm for 30mA at 5V ne555 supply)
| |
|
|
GND


For example, you make a V+ of 50V, and the led voltage is about 1.8V, you
can put (50-5)/1.8 = 25 leds in series, so you need the output stage above
12 times for 300 leds. The 555 has no problem driving ( 150 / 12) *hFe
load.

by the way, I saw some nice cheap blue and white leds at
http://www.goldmine-elec.com/ , bought some myself for my bicycle.

cheers,
Jeroen.

 

[Frank Bemelman]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

 

[Don Pearce]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

No reason it shouldn't be true - simply a factor of the Gamma
coefficient of the retina.

d

_____________________________

http://www.pearce.uk.com

 

[John Popelish]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

No reason it shouldn't be true - simply a factor of the Gamma
coefficient of the retina.

You might test your assumptions. Or read:
http://members.misty.com/don/ledp.html

 

[Don Pearce]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X
lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

No reason it shouldn't be true - simply a factor of the Gamma
coefficient of the retina.

You might test your assumptions. Or read:
http://members.misty.com/don/ledp.html

OK, Klipstein found non-linearities up to 10%. Non-linearity of the
LED is ruled out here, because the has specified the power in Lumens,
not milliwatts. So the difference in brightness has to come from
somewhere - and that somewhere can really only be the response of the
eye.

d

_____________________________

http://www.pearce.uk.com

 

[James (Jim) Meyer]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

The key word here is "looks". An eye's response is not linear. The
best way to answer your question is to perform a few experiments. Get some LEDs
and power some with DC and some with various duty cycles of current. Compare
the visual effects. Measure the power applied to the LEDs. Try different
colors of LEDs. Try different amounts of background ambient light.

You could quickly become an LED expert and settle the question once and
for all.

Remember, a peek at the answer is worth a thousand guesses. Or ten
thousand news group "theories".

Jim

 

[John Popelish]

OK, Klipstein found non-linearities up to 10%. Non-linearity of the
LED is ruled out here, because the has specified the power in Lumens,
not milliwatts. So the difference in brightness has to come from
somewhere - and that somewhere can really only be the response of the
eye.

LEDs produce photons in rough proportion to the current passing
through them. They may be nonlinear by producing more photons per
milliamp at low current or by producing more photons per milliamp at
higher current. I have seen examples of both effects in different LED
designs, but I agree with Don that the nonlinearities are in the 10%
(photons proportional to the current raised to the power in the range
of .9 power to the 1.1). Most modern designs are more linear than
this. But this is the efficiency of producing light in proportional
to current, not to electrical power. When you take the power losses
in internal resistance drop of the LEDs into account, I have not seen
any examples where the electrical efficiency rose as the duty cycle
went down. So unless the extra voltage is free, then you are only
loosing brightness by pulsing LEDs, if any change at all.

 

[R.Lewis]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

You are correct Frank.
Decently above the 'flicker threshold', 2*X lumen for 1/2 the time is
exactly the same X lumen although every couple of months a discussion is
started by declaring this is not so.
(If you think about it, is inherent in the definition of a lumen)

I suppose we must be enlightened again by such nonsense as the eye being a
non-linear lumen sensor and leds having magical properties when driven by
other than DC.
(Note however that it is possible to improve upon the DC performance for
some leds at very low currents and hence very low lumen outputs but for all
practical purposes 'pulsing' leds offers from a small to a large diminution
of performance)

 

[R.Lewis]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

The key word here is "looks". An eye's response is not linear. The
best way to answer your question is to perform a few experiments. Get some LEDs
and power some with DC and some with various duty cycles of current. Compare
the visual effects. Measure the power applied to the LEDs. Try different
colors of LEDs. Try different amounts of background ambient light.

You could quickly become an LED expert and settle the question once and
for all.

Yes, why not re-invent the wheel.

 

[Kevin McMurtrie]

I've just been getting back into electrical circuits and understand
the basic concept (I'm a newbie). I'm looking at making some large
bright colorful LED signs for effects at parties. This will require
about 300 - 500 LEDs. I know i could just pump the correct amount of
voltage to run them, however I've heard by running a "duty cycle" to
leds, you can use 1/2 the energy with the same if not more brightness.
I think I see this on newer bus tailights, when you pan your eyes
real quick, you see pulses of light (about 60 per second, I counted
them) I've also made a few smaller circuits which used the 555 timer
to accomplish this cool technique, in fact, ran from 2 AA batteries
for a month straight. I'd like to get the same energy type savings,
but powering 300 leds. Since the 555 timer controlled about 10 leds,
I dont want to have to create 30 separate 555 circuits - it'd get
expensive, this is just a hobby type project, and I'm sure 1 555 timer
chip would not be able to handle the wattage to run 300 of them. So
i'm looking for a little assistance in designing something to drive
300+ LEDs from a standard 110-115AC. I know how to convert AC into DC
and such like that. I'm guessing the power consumption shouldn't be
more than 10-20 watts.

Thanks in advance.
-kris

If live wires aren't a problem, then running right off rectified 115VAC
is the easiest way to go.

--------------+-------+---/\/\/\/\/-- to LEDs
/ \ | +---/\/\/\/\/-- to LEDs
AC / \ | +---/\/\/\/\/-- to LEDs
----/\/\/\/\/-----/ \--+ --- +---/\/\/\/\/-- to LEDs
----------------- \ --- / -| ---
bridged \ / |
rectifier \ / |
--------------+--------- common

The first resistor and the capacitor are to absorb power surges.
Something in the area of 10 Ohms 5W and 200µF 250V should work. Then
you'll want to use the rectified power (about 160V) to drive strings of
LEDs in series. Start out with a high value resistor, like 100K, while
you're figuring out how many LEDs to put in series. Once you have that
figured out you can substitute in the proper resistor for 20mA or
whatever. Don't forget to discharge the capacitor before working on
this!

Don't bother with pulsing the LEDs. Back in the old days, some LEDs had
a peak efficiency higher than the desired operating current. You could
therefore make them more efficient by pulsing them. This trick will
have the opposite effect for most modern LEDs. In the pulsed tail
lights you've seen, the pulsing is a way to vary the intensity using a
very cheap switching MOSFET. It's a matter of component cost.

 

[Frank Bemelman]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

The key word here is "looks". An eye's response is not linear. The
best way to answer your question is to perform a few experiments. Get some LEDs
and power some with DC and some with various duty cycles of current. Compare
the visual effects. Measure the power applied to the LEDs. Try different
colors of LEDs. Try different amounts of background ambient light.

You could quickly become an LED expert and settle the question once and
for all.

Remember, a peek at the answer is worth a thousand guesses. Or ten
thousand news group "theories".

I think I already know the answer, but often 'compose' my replies in a
diplomatic manner...

Notice the 'I think'....

;-)

 

[Active8]

On Sun, 12 Oct 2003 16:22:41 GMT, John Popelish, said...

LEDs produce photons in rough proportion to the current passing
through them. They may be nonlinear by producing more photons per
milliamp at low current or by producing more photons per milliamp at
higher current. I have seen examples of both effects in different LED
designs, but I agree with Don that the nonlinearities are in the 10%
(photons proportional to the current raised to the power in the range
of .9 power to the 1.1). Most modern designs are more linear than
this. But this is the efficiency of producing light in proportional
to current, not to electrical power. When you take the power losses
in internal resistance drop of the LEDs into account, I have not seen
any examples where the electrical efficiency rose as the duty cycle
went down. So unless the extra voltage is free, then you are only
loosing brightness by pulsing LEDs, if any change at all.

what extra voltage? what did i miss here?

what i got out of the article was that 20mA and up LEDs, when pulsed (at
a rate that won't cause them to flicker - 60Hz or so, you know,
persistance and all) will appear as bright as those driven at DC.

now what's going to use more energy, an LED pulsed at 1.8V 60Hz 50% DC,
or an LED operating at 1.8VDC.

now consider the ballast resistor. will it dissipate more power at 50%
DC or 100%?

brs,
mike

 

[John Popelish]

On Sun, 12 Oct 2003 16:22:41 GMT, John Popelish, said...


what extra voltage? what did i miss here?

It takes more voltage to push 40 ma through an LED than it does to
push 20 ma through it. This means that it takes more than twice the
power (when the LED is on) to run it at 40 ma than it does to run it
at 20. So if you run it at 40 ma half the time it takes more average
power than it takes to run the same LED at 20 ma DC.

 

[Active8]

On Sun, 12 Oct 2003 16:22:41 GMT, John Popelish, said...


what extra voltage? what did i miss here?

It takes more voltage to push 40 ma through an LED than it does to
push 20 ma through it.[/QUOTE]

of course. twice as much if it were linear.

This means that it takes more than twice the
power (when the LED is on) to run it at 40 ma than it does to run it
at 20. So if you run it at 40 ma half the time it takes more average
power than it takes to run the same LED at 20 ma DC.

that would be the non-linearity you've been discussing. i wasn't sure if
you were talking linearity of the lumens to electrical power curve or
the voltage to current curve.

what about, as i said, pulsing the thing at 20ma as opposed to running
it at 20 ma DC. persistance of vision should make it appear just as
bright and then you *would* be saving energy. and *that* is what *i* got
out of the article

brs,
mike

 

[James (Jim) Meyer]

"Jeroen Vriesman" <[email protected]> schreef in bericht

[snip]

Also, a 50% duty cycle at 2*X lumen looks brighter than continious X

lumen.

Is that really true? I've read/heard that many times, but I also
seem to recall that this is an urban legend.

[snip]

The key word here is "looks". An eye's response is not linear. The
best way to answer your question is to perform a few experiments. Get some LEDs
and power some with DC and some with various duty cycles of current. Compare
the visual effects. Measure the power applied to the LEDs. Try different
colors of LEDs. Try different amounts of background ambient light.

You could quickly become an LED expert and settle the question once and
for all.

Remember, a peek at the answer is worth a thousand guesses. Or ten
thousand news group "theories".

Jim

 

[John Popelish]

of course. twice as much if it were linear.

As in ohmic, yes. But even if it is a good diode with a minimum of
additional series resistance, doubling the current requires some extra
voltage drop.

that would be the non-linearity you've been discussing. i wasn't sure if
you were talking linearity of the lumens to electrical power curve or
the voltage to current curve.

They both have to be taken into account to answer this question.

what about, as i said, pulsing the thing at 20ma as opposed to running
it at 20 ma DC. persistance of vision should make it appear just as
bright and then you *would* be saving energy. and *that* is what *i* got
out of the article

If the flicker frequency is high enough to be undetectable, then the
eye averages the illumination power, so persistence of vision makes
the 20 ma 50% diode look just about as bright as one running 10 ma DC,
assuming the LED is linear in its current to photon output.
Persistence of vision is not a peak follower effect.

 

[Robert Monsen]

what about, as i said, pulsing the thing at 20ma as opposed to running
it at 20 ma DC. persistance of vision should make it appear just as
bright and then you *would* be saving energy. and *that* is what *i* got
out of the article

I just tried this with my signal generator:

GEN ---->|----WWW--0V

With a DC voltage of 5V, a small red LED is about twice as bright as it is
with a 60Hz square wave. Too bad, think of the energy we could save...

Regards,
Bob Monsen

 

[Spehro Pefhany]

On Sun, 12 Oct 2003 16:22:41 GMT, John Popelish, said...


what extra voltage? what did i miss here?

Voltage drop of the LED goes up with current... this increases
power, which could be considered to have a proportional to I
component and a proportional to I^2 component. Sometimes
even if the voltage is free, the derating to keep the die
temperature in spec will kill any small improvement.

what i got out of the article was that 20mA and up LEDs, when pulsed (at
a rate that won't cause them to flicker - 60Hz or so, you know,
persistance and all) will appear as bright as those driven at DC.

now what's going to use more energy, an LED pulsed at 1.8V 60Hz 50% DC,
or an LED operating at 1.8VDC.

It will use even less energy if you leave it off entirely, but that
will tend to reduce the brightness.

now consider the ballast resistor. will it dissipate more power at 50%
DC or 100%?

If the average current is the same, and the supply voltage fixed, the
resistor will run cooler and the LED hotter for pulsed operation.

Best regards,
Spehro Pefhany

 

[Robert Monsen]

Don't bother with pulsing the LEDs. Back in the old days, some LEDs had
a peak efficiency higher than the desired operating current. You could
therefore make them more efficient by pulsing them. This trick will
have the opposite effect for most modern LEDs. In the pulsed tail
lights you've seen, the pulsing is a way to vary the intensity using a
very cheap switching MOSFET. It's a matter of component cost.

I'm building an LED clock with 5" display numbers. The LED manufacturer
(lite-on) seems to suggest a 10% duty cycle at 1kHz. (At least, thats what
they test max current at.)

http://rocky.digikey.com/WebLib/Lite-on/Web Data/LTS-50801HRB.pdf

Its a bit of a pain (well, not too painful) to flash them, so I'm quite
interested in the group take on this datasheet.

If I don't have to do it, I can save a pin on my PIC, and use a cheaper
part.

Regards,
Bob Monsen