I want to build some emergency lighting equipment using LEDs and 12V. A
friend told me that it is possible to get more light per Watt by pulsing the
LED to instantaneous levels well above its average ratings.

Is this true, if you include power consumed in the pulsing circuit?

If it is, can anyone suggest a simple and cheap circuit and the appropriate
operating values?

The reason for the 12V requirement is that I'm also working on a solar
charger for my transceiver batteries and want to use the same charger for
the lighting batteries.

Suggestions would be appreciated.

Tom, N3IJ

"Tom Coates" bravely wrote to "All" (12 Oct 05 19:28:03)
--- on the heady topic of "Pulsing LEDs for higher efficiency"

TC From: "Tom Coates"
TC Xref: core-easynews rec.radio.amateur.homebrew:88233

TC I want to build some emergency lighting equipment using LEDs and 12V.
TC A friend told me that it is possible to get more light per Watt by
TC pulsing the LED to instantaneous levels well above its average ratings.

TC Is this true, if you include power consumed in the pulsing circuit?

TC If it is, can anyone suggest a simple and cheap circuit and the
TC appropriate operating values?

TC The reason for the 12V requirement is that I'm also working on a solar
TC charger for my transceiver batteries and want to use the same charger
TC for the lighting batteries.

TC Suggestions would be appreciated.

TC Tom, N3IJ

This pulsed drive technique is supposed to take advantage of the human
vision's persistance. If the led is driven brighter for a long enough
time the eye will perceive it as being brighter than if using the
equivalent average continuous current. Visual persistance is
approximately 10 milli-seconds, and if the repetition rate is faster
than this then the led will seem to be on all the time. The same
technique is used with multiplexed digital displays.

One problem with pulsed led drive is that it generates rfi due to the
rapidly switching voltage levels. Unless care is taken with risetime,
wire lengths, decoupling, and shielding, then the gain in efficiency
is a doubtful tradeoff for the simplicity of direct dc drive. It
depends on the application. If you want really bright flashes then use
a xenon strobe tube.

A*s*i*m*o*v

.... Neurotoxin Lite! Tastes great... Less drooling...

RFI isn't going to be a problem if the switch is close to the application.
You can use an LM3524, NE555 etc. to drive the gate of a MOSFET.

The tradeoff is between LED life and pulse current. See www.agilent.com for
pulsed LED application notes.

"Asimov" wrote in message
...
"Tom Coates" bravely wrote to "All" (12 Oct 05 19:28:03)
--- on the heady topic of "Pulsing LEDs for higher efficiency"

TC From: "Tom Coates"
TC Xref: core-easynews rec.radio.amateur.homebrew:88233

TC I want to build some emergency lighting equipment using LEDs and 12V.
TC A friend told me that it is possible to get more light per Watt by
TC pulsing the LED to instantaneous levels well above its average ratings.

TC Is this true, if you include power consumed in the pulsing circuit?

TC If it is, can anyone suggest a simple and cheap circuit and the
TC appropriate operating values?

TC The reason for the 12V requirement is that I'm also working on a solar
TC charger for my transceiver batteries and want to use the same charger
TC for the lighting batteries.

TC Suggestions would be appreciated.

TC Tom, N3IJ

This pulsed drive technique is supposed to take advantage of the human
vision's persistance. If the led is driven brighter for a long enough
time the eye will perceive it as being brighter than if using the
equivalent average continuous current. Visual persistance is
approximately 10 milli-seconds, and if the repetition rate is faster
than this then the led will seem to be on all the time. The same
technique is used with multiplexed digital displays.

One problem with pulsed led drive is that it generates rfi due to the
rapidly switching voltage levels. Unless care is taken with risetime,
wire lengths, decoupling, and shielding, then the gain in efficiency
is a doubtful tradeoff for the simplicity of direct dc drive. It
depends on the application. If you want really bright flashes then use
a xenon strobe tube.

A*s*i*m*o*v

.... Neurotoxin Lite! Tastes great... Less drooling...

In article ,
Tom Coates wrote:
I want to build some emergency lighting equipment using LEDs and 12V. A
friend told me that it is possible to get more light per Watt by pulsing the
LED to instantaneous levels well above its average ratings.

As I understand it, this is not true --- it was true in a way for older
LEDs, but isn't true for modern LEDs. Pulsing a diode above its rated
current could get it to emit some of its light at shorter wavelengths, which
were closer to the middle of the visible spectrum and so seemed brighter.

But if you're not taking advantage of that effect, then I don't
think pulsing gets you anything. The instantaneous output of the LED is
proportional to the instantaneous current, and the percieved brightness
of the pulsed light source is the same as the average brightness, if it's
pulsing fast enough ( 10 Hz or so ). So it works out that the percieved
brightness is the same as it would be if you'd driven the LEDs with
a constant current.

If resistive losses are significant, then pulsing will actually reduce the
overall efficiency, since resistive losses are proportional to the
average of I^2.

On the other hand, if your battery is at a higher voltage than you want
to drive the LEDs, then pulsing them might be more efficient than using
a linear regulator --- that has more to do with the inefficiency of the
linear regulator than anything to do with the LEDs, though :-)


I did some googling to refresh my memory about pulsed LEDs and found
a short article on the subject:
http://www.caves.org.uk/led/foot4.pdf

On the other hand, I also found some articles on pulsed LEDs saying
the opposite --- that the percieved brightness of a pulsed source
is closer to the peak brightness than to the average brightness.
It might be worthwhile tracking down some actual experimental data,
or doing an experiment yourself...

--
Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1

"Wim Lewis" wrote in message
...
In article ,
Tom Coates wrote:
I want to build some emergency lighting equipment using LEDs and 12V. A
friend told me that it is possible to get more light per Watt by pulsing
the
LED to instantaneous levels well above its average ratings.

As I understand it, this is not true --- it was true in a way for older
LEDs, but isn't true for modern LEDs. Pulsing a diode above its rated
current could get it to emit some of its light at shorter wavelengths,
which
were closer to the middle of the visible spectrum and so seemed brighter.

But if you're not taking advantage of that effect, then I don't
think pulsing gets you anything. The instantaneous output of the LED is
proportional to the instantaneous current, and the percieved brightness
of the pulsed light source is the same as the average brightness, if it's
pulsing fast enough ( 10 Hz or so ). So it works out that the percieved
brightness is the same as it would be if you'd driven the LEDs with
a constant current.

If resistive losses are significant, then pulsing will actually reduce the
overall efficiency, since resistive losses are proportional to the
average of I^2.

On the other hand, if your battery is at a higher voltage than you want
to drive the LEDs, then pulsing them might be more efficient than using
a linear regulator --- that has more to do with the inefficiency of the
linear regulator than anything to do with the LEDs, though :-)


I did some googling to refresh my memory about pulsed LEDs and found
a short article on the subject:
http://www.caves.org.uk/led/foot4.pdf

On the other hand, I also found some articles on pulsed LEDs saying
the opposite --- that the percieved brightness of a pulsed source
is closer to the peak brightness than to the average brightness.
It might be worthwhile tracking down some actual experimental data,
or doing an experiment yourself...

--
Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1

If the brightness of an LED is a 1:1 linear relationship with the power
supplied, that suggests that perceived brightness of a pulsed LED would be
neither at the arithmetic mean (average) nor at the peak but rather at rms
(root mean square). Comments?

"Roger Conroy" bravely wrote to "All" (14 Oct 05 09:06:17)
--- on the heady topic of " Pulsing LEDs for higher efficiency"

RC From: "Roger Conroy"
RC Xref: core-easynews rec.radio.amateur.homebrew:88243

RC If the brightness of an LED is a 1:1 linear relationship with the
RC power supplied, that suggests that perceived brightness of a pulsed
RC LED would be neither at the arithmetic mean (average) nor at the peak
RC but rather at rms (root mean square). Comments?


I'm pretty certain it is not a simple rms function because the
duration of the visual persistance effect must be taken into account.
If the peak brightness duration is long enough it will be perceived as
brighter than the average. Consider that the eye's retina cone signal
fades off say to 80% in 10 milliseconds after exposure to light. As
long as the light pulse repetition rate is high enough such that it
results in a drop of optic nerve signal less than the minimum light
change that can be perceived then the light will be seen as
continuously bright. You can always run a subjective experiment on
your workbench with an led and a 555 timer used as an astable.

A*s*i*m*o*v

.... Forcast for tonight: Dark.

On Fri, 14 Oct 2005 16:11:05 GMT, Asimov wrote:
"Roger Conroy" bravely wrote to "All" (14 Oct 05 09:06:17)
--- on the heady topic of " Pulsing LEDs for higher efficiency"

RC From: "Roger Conroy"
RC Xref: core-easynews rec.radio.amateur.homebrew:88243

RC If the brightness of an LED is a 1:1 linear relationship with the
RC power supplied, that suggests that perceived brightness of a pulsed
RC LED would be neither at the arithmetic mean (average) nor at the peak
RC but rather at rms (root mean square). Comments?

I'm pretty certain it is not a simple rms function because the
duration of the visual persistance effect must be taken into account.
If the peak brightness duration is long enough it will be perceived as
brighter than the average.

Perhaps. But, will it _illuminate_ any better?
The OP wasn't interested in _staring_ at a bank of leds. :-)

Maybe better discussed in rec.bio-tronics HI!HI!

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK

While I understand the Persistance comment, I don't think this is it. A
couple of things. The eye has to convert the light energy to chemical
reaction/change and this takes energy. The perception, therefore, would
also be a factor of energy and a short pulse has less energy than a long
pulse, so this would bring you back into the average vs, RMS discussion and
favor the RMS idea. In other words, One would think power is what is being
received, just like our ears, so the RMS idea seems to have merrit.
However, I recall reading that the eye is a "somewhat" peak detecting device
(my paraphrasing of part peak / part RMS) and that was the reason (or part
of the reason) that the pulsed LEDs looked brighter.

BTW, shining the LED onto a surface would have the same argument, so
"_staring_ at a bank of leds" isn't a factor.

73, Steve, K,9.D;C'I

"Allodoxaphobia" wrote in message
...
On Fri, 14 Oct 2005 16:11:05 GMT, Asimov wrote:
"Roger Conroy" bravely wrote to "All" (14 Oct 05 09:06:17)
--- on the heady topic of " Pulsing LEDs for higher efficiency"

RC From: "Roger Conroy"
RC Xref: core-easynews rec.radio.amateur.homebrew:88243

RC If the brightness of an LED is a 1:1 linear relationship with the
RC power supplied, that suggests that perceived brightness of a
pulsed
RC LED would be neither at the arithmetic mean (average) nor at the
peak
RC but rather at rms (root mean square). Comments?

I'm pretty certain it is not a simple rms function because the
duration of the visual persistance effect must be taken into account.
If the peak brightness duration is long enough it will be perceived as
brighter than the average.

Perhaps. But, will it _illuminate_ any better?
The OP wasn't interested in _staring_ at a bank of leds. :-)

Maybe better discussed in rec.bio-tronics HI!HI!

Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
Pueblo, Colorado | @ | Jonesy | OS/2 __
38.24N 104.55W | config.com | DM78rf | SK

Thanks, everyone, for the insights. I think I'll get it working without the
pulsing and then reconsider.

It appears that what was described to me as a standard technique is much
more experimental.

Tom, N3IJ

"Tom Coates" wrote in message
...
I want to build some emergency lighting equipment using LEDs and 12V. A
friend told me that it is possible to get more light per Watt by pulsing
the
LED to instantaneous levels well above its average ratings.

Is this true, if you include power consumed in the pulsing circuit?

If it is, can anyone suggest a simple and cheap circuit and the
appropriate
operating values?

The reason for the 12V requirement is that I'm also working on a solar
charger for my transceiver batteries and want to use the same charger for
the lighting batteries.

Suggestions would be appreciated.

Tom, N3IJ

Wim Lewis writes:

On the other hand, I also found some articles on pulsed LEDs saying
the opposite --- that the percieved brightness of a pulsed source
is closer to the peak brightness than to the average brightness.
It might be worthwhile tracking down some actual experimental data,
or doing an experiment yourself...

--
Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1

HP used to have a nice app note on this. The main trick is that the eye
responds most nearly to intensity, which is close to being proportional
to the square of the brightness (analogous to power vs voltage). Just
like "pulsey" currents have higher RMS values, "pulsey" light can have
a higher intensity. The eye acts as a smoothing filter. This is a well
known trick in lots of lighting and display areas. If you really care,
you can use LEDs as "catch" diodes in switching regulator circuits to
get some pretty amazing "lumens per watt."

Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA