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Flashing a high brightness LED

M

Michael

Jan 1, 1970
0
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can see)....followed
by a somewhat dimmer consistent light....all in a matter of a few
seconds....it would be rather annoying if they were flashed, I imagine.

Perhaps ALL led's operate this way and because these are the high bright
white variety I can finally SEE it?

The power supply is constant current and more than ample....I did have about
470uf capacitance before the regulator that I took out but little
difference.

Maybe I could do something to create a "soft start" something?

Thanks for any comments.
 
L

Luhan

Jan 1, 1970
0
Michael said:
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can see)....followed
by a somewhat dimmer consistent light....all in a matter of a few
seconds....it would be rather annoying if they were flashed, I imagine.

Perhaps ALL led's operate this way and because these are the high bright
white variety I can finally SEE it?

The power supply is constant current and more than ample....I did have about
470uf capacitance before the regulator that I took out but little
difference.

Maybe I could do something to create a "soft start" something?

Thanks for any comments.

The effect you are describing happens when you send way to much current
into an LED. These devices go from normal current to excessive current
with only a few tenths of a volt change.

What is your voltage and what is the value of your series resistor?

Luhan
 
F

Fred Bartoli

Jan 1, 1970
0
Luhan a écrit :
The effect you are describing happens when you send way to much current
into an LED. These devices go from normal current to excessive current
with only a few tenths of a volt change.

What is your voltage and what is the value of your series resistor?

Is it this (depends on the 'circuit' but it's said to be constant
current) or simply normal eye reaction to a too bright light?
 
N

Nico Coesel

Jan 1, 1970
0
Michael said:
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can see)....followed
by a somewhat dimmer consistent light....all in a matter of a few
seconds....it would be rather annoying if they were flashed, I imagine.

Eyes are not suitable instruments to measure brightness. What you need
is a flash-meter like: http://www.lmt.de/en/sf100.html
and a simple storage oscilloscope.
 
C

Charles Schuler

Jan 1, 1970
0
Nico Coesel said:
Eyes are not suitable instruments to measure brightness. What you need
is a flash-meter like: http://www.lmt.de/en/sf100.html
and a simple storage oscilloscope.

You are correct (human vision quickly adapts to brightness). But, for hobby
use, there are simple methods to measure brightness that might work here.
 
L

Luhan

Jan 1, 1970
0
Fred said:
Luhan a écrit :

Is it this (depends on the 'circuit' but it's said to be constant
current) or simply normal eye reaction to a too bright light?

Or maybe slow response from the supply providing the 'constant current'
?

Luhan
 
T

Tam/WB2TT

Jan 1, 1970
0
Michael said:
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can
see)....followed by a somewhat dimmer consistent light....all in a matter
of a few seconds....it would be rather annoying if they were flashed, I
imagine.

Perhaps ALL led's operate this way and because these are the high bright
white variety I can finally SEE it?

The power supply is constant current and more than ample....I did have
about 470uf capacitance before the regulator that I took out but little
difference.

Maybe I could do something to create a "soft start" something?

Thanks for any comments.
Are you sure the constant current source is not changing? You might want to
put a 'scope across the LED and see if the voltage changes. Also, I don't
understand about the constant current source being "ample". It's either
right, or it's not.

Tam
 
Nico Coesel ha escrito:
Eyes are not suitable instruments to measure brightness. What you need
is a flash-meter like: http://www.lmt.de/en/sf100.html
and a simple storage oscilloscope.

I don`t see how a flash meter would help in the design, LEDs are meant
to be seen by the eye, unless you are going for a home-made optical
circuit. The person that sees the LED flash doesn`t give a damn what
the flash meter says, if the naked eye detects that the light is
inconsistent (or whatever you would call it) that's all that matters
and that's what the design should revolve around.
 
M

Michael

Jan 1, 1970
0
I'm using an LM317 in constant current mode as per a data sheet....where
output is drawn from the adjust pin of the regulator.

It calls for an input voltage that provides (3.45 x 2) the voltage drop of
the LED's (2 in series) plus an additional 3 volts for the regulator.

I have 11 volts at 1 amp going into the LM317 input pin and show 300ma going
through both the LED's.

Like one responder said, could it just be my eye sensitivity at such a
brightness and color?

Or just the nature of the device?

It could also be my lens material which is acrylic (amplifying) the effect,
I suppose.....it's does reduce as I move the lens farther away.

But considering the lens is a passive device?....it's just showing me what I
normally wouldn't be able to see?
 
M

Michael Black

Jan 1, 1970
0
Nico Coesel ha escrito:


I don`t see how a flash meter would help in the design, LEDs are meant
to be seen by the eye, unless you are going for a home-made optical
circuit. The person that sees the LED flash doesn`t give a damn what
the flash meter says, if the naked eye detects that the light is
inconsistent (or whatever you would call it) that's all that matters
and that's what the design should revolve around.
The eye has a response time. Be outside in bright light, and when you go in,
you have to adjust to the lesser light. Be in a place with little light,
and when you go out into the sun, you'll have to adjust.

The previous poster is suggesting that's what's going on. That it may
not be a circuit issue, but that the original poster's eyes see a dimming
because the eyes have adjusted to the light. The only way to rule
that out is a light meter of some sort.

Mihcael
 
G

GregS

Jan 1, 1970
0
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can see)....followed
by a somewhat dimmer consistent light....all in a matter of a few
seconds....it would be rather annoying if they were flashed, I imagine.

Perhaps ALL led's operate this way and because these are the high bright
white variety I can finally SEE it?

The power supply is constant current and more than ample....I did have about
470uf capacitance before the regulator that I took out but little
difference.

Maybe I could do something to create a "soft start" something?

I have not "seen" this, but one can measure it, using the Luxeons.
I think all LED's will dim as it heats up. The more heat, you get less light.
You need to keep the sink below 25C
to max out. The brightness shold remain fairly stable below ratings.

greg
 
T

Tam/WB2TT

Jan 1, 1970
0
Michael said:
I'm using an LM317 in constant current mode as per a data sheet....where
output is drawn from the adjust pin of the regulator.

It calls for an input voltage that provides (3.45 x 2) the voltage drop of
the LED's (2 in series) plus an additional 3 volts for the regulator.

I have 11 volts at 1 amp going into the LM317 input pin and show 300ma
going through both the LED's.

Like one responder said, could it just be my eye sensitivity at such a
brightness and color?

Or just the nature of the device?

It could also be my lens material which is acrylic (amplifying) the
effect, I suppose.....it's does reduce as I move the lens farther away.

But considering the lens is a passive device?....it's just showing me what
I normally wouldn't be able to see?
Here are some simple experiments you could try:

1. Bypass the 317 and connect a current limiting resistor directly to the
11V. You would need at least a 2W resistor.

2. Turn the LED ON/OFF by switching the 11V instead of the load. This
should also lend itself to some soft start scheme. If you are switching the
load, I don't know what your current source is doing during the times it is
running at no load.

Tam
 
Q

qrk

Jan 1, 1970
0
I have been experimenting with the super high brightness Luxeon white LEDs.

I had hoped to be able to flash on/off a couple of the Luxeon 1 stars
behind a lens but there is an obvious "settling in" after the LED's are
powered up.

What I mean is an initial surge of brightness (that you can see)....followed
by a somewhat dimmer consistent light....all in a matter of a few
seconds....it would be rather annoying if they were flashed, I imagine.

Perhaps ALL led's operate this way and because these are the high bright
white variety I can finally SEE it?

The power supply is constant current and more than ample....I did have about
470uf capacitance before the regulator that I took out but little
difference.

Maybe I could do something to create a "soft start" something?

Thanks for any comments.

All mentioned before...

1. The emitter could be dimming due to heating. When you apply
current, this will heat the device causing the brightness to decrease.
Are you properly heatsinking the device? Some high brightness emitters
have a slug on the back for this purpose.

2. Your eye response. Use an instrument to monitor light output. Use a
small emitter as a photodiode. Reverse bias it for wider dynamic range
and faster response. Circuit is: series string of +power (5 to 10
volts), into reversed biased emitter, into resistor to ground.
Resistor can be 50 Ohms to 1k Ohms depending on your response time
needs. Connect oscilloscope across the resistor.

3. Your constant current supply isn't working as expected. Monitor the
LED current with an oscilloscope.
 
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