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Driving LEDs - high-power flash

J

Jonathan Kirwan


I was a little interested in some of the details, so I looked further.

The complete symposium is copyrighted as a whole, I think, and one must pay a
fair fee for it. But the list of papers presented will help find individual
articles. The list is found here:

http://knt186.knt.vein.hu/cie2002/data/aaTimeTable-Final.pdf

As an example, one of those articles can be found here:

http://cie2.nist.gov/TCs/Open_documents/TC2-49/Ohno1_CIE Sympo 2002.pdf

A more detailed overview of various articles are here:

http://knt186.knt.vein.hu/cie2002/Abst_data/

Jon
 
R

R.Lewis

Jonathan Kirwan said:
I was a little interested in some of the details, so I looked further.

The complete symposium is copyrighted as a whole, I think, and one must pay a
fair fee for it. But the list of papers presented will help find individual
articles. The list is found here:

http://knt186.knt.vein.hu/cie2002/data/aaTimeTable-Final.pdf

As an example, one of those articles can be found here:

http://cie2.nist.gov/TCs/Open_documents/TC2-49/Ohno1_CIE Sympo 2002.pdf

A more detailed overview of various articles are here:

http://knt186.knt.vein.hu/cie2002/Abst_data/

Jon

If you are interested in the subject see Schmidt-Clausens 'Historical
overview of Flashing Light Photometry' in the Report on the CIE
workshop:photometry of Flashing Lights from here

http://physics.nist.gov/Divisions/Div844/facilities/photo/Publications/CIE_F
lash_Workshop.pdf

and follow the references through from the early days.

Blondel-Rey is still widely used/specified but it is not all that simple!
 
On Sun, 26 Sep 2004 10:49:36 +0100, "R.Lewis" <h.lewis-not this
If you bother to learn something about the subject you would not need to ask
such silly questions.
The statement above leads me to think that I did not explain myself
clearly. Here goes again:

Intent: We are trying to achieve maximum perceived brightness using a
plulsed LED, but using the least power consumption.

1) If we leave the LED on solid, without pulsing it, then we will use
up too much energy and we will not achieve a high brightness because
the LED can not handle hundreds of milliamps for a long time.

2) If we pulse the LED using a 1ms pulse, we will have to drive so
much current into the LED in order to achieve any brightness, that it
will be destroyed.

3) If we give the LED a strong 20ms pulse then we will achieve a
blinding flash without destroying the LED. Note however, that dumping
the spike from a fully charged inductor into an LED directly, puts
tremendous thermal stress on the die; so we should strive to shape the
waveform such that thermal cycling can take place more gently. Some
kind of pulse forming LC network would be good. I thought a radar
modulator type "simulated transmission line" energy storage network
might be interesting to try because it would avoid the spike, but
since we are not talking about huge power levels here, perhaps slowly
switching a transistor on and off might do the trick. Here is a simple
PWM soft start circuit for doing that:

In---1M--+--100K--1uF--+
| |
| |
+--|>o--------+--Out to driver stage
| 4584
|
1nF
|
GND

4) When I did a little personal experiment in my home lab and
delivered a 2 millijoule pulse of "energy" to a red LED, I quickly
found that a 10ms pulse appears much brighter than a 1ms pulse or a 1
second pulse. Furthermore, I found that if I operate the LED just
below the threshold of destruction then the 15ms pulse gave me the
most percieved brightness for the least ammount of energy.
 
J

Jonathan Kirwan

On Sun, 26 Sep 2004 22:31:36 +0100, "R.Lewis" <h.lewis-not this
If you are interested in the subject see Schmidt-Clausens 'Historical
overview of Flashing Light Photometry' in the Report on the CIE
workshop:photometry of Flashing Lights from here

http://physics.nist.gov/Divisions/Div844/facilities/photo/Publications/CIE_F
lash_Workshop.pdf

and follow the references through from the early days.

Blondel-Rey is still widely used/specified but it is not all that simple!

Thanks. Some time ago, I had to develop something akin to CIE color and
intensity separation for binning parts from a major manufacturer. That was my
start into this area and I enjoyed it, particularly reading through some of the
work by Edwin Land in the late 1970's and early 1980's. Read about a lot more
than I needed for the work, but it was fun. This looks like more to add to the
mix.

Jon
 
K

Ken S. Tucker

John Woodgate said:
I read in sci.electronics.design that Ken S. Tucker
oogle.com>) about 'Driving LEDs - high-power flash', on Sun, 26 Sep
2004:


People don't take any notice of bells and sirens, and they don't 'see'
signs. But a **voice announcement** gets immediate results and can
impart far more information.

'BING-BONG. Emergency announcement. There is an incident at the front of
the building. Please leave the building immediately in an orderly manner
by the side and rear exits.'

Repeated in other languages if necessary. Try that with a bell!

Then: 'BING-BONG. Muster at the far end of the rear car-park for roll-
call. Please assist any disabled or confused people.'

Right, we're are talking about 100 American's killed because
of inadequate annuciation, audio, visual or some other way!

The OP mentioned an aggravating Hertz Flash that is bound to
get attention by annoyance, I just looked for how that could be
useful, apart from annoying.
Ken
 
Right, we're are talking about 100 American's killed because
of inadequate annuciation, audio, visual or some other way!
For instance one could signall everyone to leave a building through
the use of a bad smell. Natural Gas leaks use such signalling.
 
D

Don Klipstein

White LEDs become inefficient at high currents. This is because the
fluorescent material saturates. Use non-white LEDs to avoid that.

I have noticed that white LEDs are less efficient at higher currents,
but I believe the phosphor is not the explanation. I have found blue LEDs
to also be less efficient at higher currents, and white LEDs are blue
ones with phosphor.

- Don Klipstein ([email protected])
 
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