Nav Lights using Luxeon LEDs

[Andrew Sarangan]

Recently I designed and built a nav light system for my Europa aircraft
using the Luxeon LEDs with great results.

I also measured the radiation pattern (after fitting the LEDs to the
aircraft) and compared it to the FAR specs. It exceeds the requirements
by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
use as a landing light.

Actually, I am surprised that most manufacturers do not provide a
measured radiation pattern. They just say that it complies with the
FAR. Well, meeting the specs is not the same as exceeding it. Since
LEDs lose power with age, this could be an important consideration.

Just in case someone might find this useful, I wrote it up as an
article:
http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf

Let me know what you think.

[JKimmel]

I think you are a great guy.

Andrew Sarangan wrote:
> Recently I designed and built a nav light system for my Europa aircraft
> using the Luxeon LEDs with great results.
>
> I also measured the radiation pattern (after fitting the LEDs to the
> aircraft) and compared it to the FAR specs. It exceeds the requirements
> by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> use as a landing light.
>
> Actually, I am surprised that most manufacturers do not provide a
> measured radiation pattern. They just say that it complies with the
> FAR. Well, meeting the specs is not the same as exceeding it. Since
> LEDs lose power with age, this could be an important consideration.
>
> Just in case someone might find this useful, I wrote it up as an
> article:
> http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf
>
> Let me know what you think.
>


--
J Kimmel

www.metalinnovations.com

"Cuius testiculos habes, habeas cardia et cerebellum." - When you have
their full attention in your grip, their hearts and minds will follow.

[Rob Turk]

"Andrew Sarangan" > wrote in message
oups.com...
> Recently I designed and built a nav light system for my Europa aircraft
> using the Luxeon LEDs with great results.
>
> I also measured the radiation pattern (after fitting the LEDs to the
> aircraft) and compared it to the FAR specs. It exceeds the requirements
> by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> use as a landing light.
>

Splendit! Great work, Andrew!

One note... When looking at the schematics it appears you're building two
current sources. Without digging too deep it seems the serial resistors are
switched between the two drawings (0.69 Ohm for 1400mA, 1.3 Ohm for 700mA)??

Rob

[Dan]

Andrew Sarangan wrote:
> Recently I designed and built a nav light system for my Europa aircraft
> using the Luxeon LEDs with great results.
>
> I also measured the radiation pattern (after fitting the LEDs to the
> aircraft) and compared it to the FAR specs. It exceeds the requirements
> by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> use as a landing light.
>
> Actually, I am surprised that most manufacturers do not provide a
> measured radiation pattern. They just say that it complies with the
> FAR. Well, meeting the specs is not the same as exceeding it. Since
> LEDs lose power with age, this could be an important consideration.
>
> Just in case someone might find this useful, I wrote it up as an
> article:
> http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf
>
> Let me know what you think.
>

3 questions:

1) are the electronics going to survive vibration? I don't see any
isolation.

2) won't the reflector tend to oxidize fairly rapidly?

3) why use a fuse? Jim Weir did a rather nice article in Kitplanes®
awhile back discussing a solid state circuit breaker (I forget what it's
called) that might be a good idea.

I rather like the set up. Very good write up :)

Dan, U.S. Air Force, retired

[Andrew Sarangan]

At first I did not like the use of a fan either. However, the
alternative is to install heftier heatsinks (more weight and more
space). This is also why I installed a thermistor. If the fan fails,
the temperature will rise quite dramatically, and you will know
something is wrong. But you have a good point. The thermistor could
have been easily incorporated with the circuit to automatically scale
down the current with tempertaure rise.

karel wrote:
> "Andrew Sarangan" > schreef in bericht
> oups.com...
> > Recently I designed and built a nav light system for my Europa aircraft
> > using the Luxeon LEDs with great results.
> >
> > I also measured the radiation pattern (after fitting the LEDs to the
> > aircraft) and compared it to the FAR specs. It exceeds the requirements
> > by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> > use as a landing light.
> >
> > Actually, I am surprised that most manufacturers do not provide a
> > measured radiation pattern. They just say that it complies with the
> > FAR. Well, meeting the specs is not the same as exceeding it. Since
> > LEDs lose power with age, this could be an important consideration.
> >
> > Just in case someone might find this useful, I wrote it up as an
> > article:
> > http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf
> >
> > Let me know what you think.
>
> Nice! The one thing I do not like about it
> is the dependence on mechanical ventilators,
> and nothing to check they are working OK.
> A nice addition would be a temp sensor that
> reduces LED current if things get hot.
>
> The documentation you supply might serve
> as an example to many: complete and clear.
> My heartfelt compliment!
> KA
>
> >

[Andrew Sarangan]

Actually, those are current sensing resistors, not current limiting.
The voltage across the resistor is compared with a fixed reference
voltage, and it is the FET which limits the current. So, a higher
resistor does not necessarily mean lower current.

Rob Turk wrote:
> "Andrew Sarangan" > wrote in message
> oups.com...
> > Recently I designed and built a nav light system for my Europa aircraft
> > using the Luxeon LEDs with great results.
> >
> > I also measured the radiation pattern (after fitting the LEDs to the
> > aircraft) and compared it to the FAR specs. It exceeds the requirements
> > by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> > use as a landing light.
> >
>
> Splendit! Great work, Andrew!
>
> One note... When looking at the schematics it appears you're building two
> current sources. Without digging too deep it seems the serial resistors are
> switched between the two drawings (0.69 Ohm for 1400mA, 1.3 Ohm for 700mA)??
>
> Rob

[Andrew Sarangan]

Very good point about the reflector oxidation. I guess I should paint
it right away with reflective paint.

I looked at the solid state breakers you refer to. They are made of a
polymer, which opens the circuit when its temperature gets above a
preset limit. The reason I decided not to use those is because it is
strongly dependent on the ambient temperature. ie it will trip at
different currents when it is cold outside vs hot outside. This could
be a problem since the aircraft is likely to see large swings in
temperature from winter to summer, and at different altitudes. The
filament fuses are in a sealed glass tube, which is relatively
independent of temperature.

Mechanical vibration... The LEDs don't care about mechanical vibration
as much as regular lamps do. The electronics are all solid state, so
they won't care either. The solder joints might care, but if that were
the case, all other electronics in the cockpit will also be vulnerable.
I've built other electronics for the cockpit, and never had vibration
related problems. I could be wrong. I guess I will find out.

Thanks for your pointers. I will have to take them into
consideration...



Dan wrote:
> Andrew Sarangan wrote:
> > Recently I designed and built a nav light system for my Europa aircraft
> > using the Luxeon LEDs with great results.
> >
> > I also measured the radiation pattern (after fitting the LEDs to the
> > aircraft) and compared it to the FAR specs. It exceeds the requirements
> > by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> > use as a landing light.
> >
> > Actually, I am surprised that most manufacturers do not provide a
> > measured radiation pattern. They just say that it complies with the
> > FAR. Well, meeting the specs is not the same as exceeding it. Since
> > LEDs lose power with age, this could be an important consideration.
> >
> > Just in case someone might find this useful, I wrote it up as an
> > article:
> > http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf
> >
> > Let me know what you think.
> >
>
> 3 questions:
>
> 1) are the electronics going to survive vibration? I don't see any
> isolation.
>
> 2) won't the reflector tend to oxidize fairly rapidly?
>
> 3) why use a fuse? Jim Weir did a rather nice article in Kitplanes®
> awhile back discussing a solid state circuit breaker (I forget what it's
> called) that might be a good idea.
>
> I rather like the set up. Very good write up :)
>
> Dan, U.S. Air Force, retired

[Rob Turk]

"Andrew Sarangan" > wrote in message
oups.com...
> Actually, those are current sensing resistors, not current limiting.
> The voltage across the resistor is compared with a fixed reference
> voltage, and it is the FET which limits the current. So, a higher
> resistor does not necessarily mean lower current.
>

Ahh yes. I saw the current sensing resistors but I overlooked the resistor
values for the divider that creates the fixed reference. I'm still wondering
about a few things though, hope you can elaborate a bit.

- You have the fuse placed in the ground wire. However, the largest
'touchable' area on your PCB and heatsinks are all attached to the area
behind the fuse. A simple protruding screw or rivet would be enough to turn
the circuit into an unfused system. Why not fuse the 12V line?

- The LM7805 really likes having a small decoupling capacitor at it's output
to prevent oscillation (and even self-destrucion on cheap non-brand 7805's).
How about adding 100nF between the 5V output and ground?

- The current sense resistor in the 1400mA version will dissipate about (1.4
x 1.4 x 1.3) 2.6W, so a 3W resistor is OK. The 700mA version would only
dissipate (0.7 x 0.7 x 0.69) about 0.36 W. You could do with a smaller
version (0.5W) there??

Mind you, I'm not trying to be negative about your design, but just curious
about some of your design decisions. The Luxeon's are really nice gadgets
8-)

Rob

[Rob Turk]

"Andrew Sarangan" > wrote in message
oups.com...
> At first I did not like the use of a fan either. However, the
> alternative is to install heftier heatsinks (more weight and more
> space). This is also why I installed a thermistor. If the fan fails,
> the temperature will rise quite dramatically, and you will know
> something is wrong. But you have a good point. The thermistor could
> have been easily incorporated with the circuit to automatically scale
> down the current with tempertaure rise.
>

Have you considered using a PWM circuit to drive the LED's? Those can get
you much higher efficiency than a series current regulator. That should
allow you to eliminate the fan entirely.

Maybe check out http://www.leddynamics.com/LuxDrive/drivers.php , they offer
a small solid-state PWM module called the Micropuck, specifically designed
for automotive purpose. Reasonably cheap, light and small.

Rob

[Don W]

Impressive. Nice fab job. I'm also impressed that you measured
the radiation pattern and compared it to the FAR. Most people
wouldn't have done that.

Someone else mentioned about oxidation of the reflector, and that
is a potential problem, but easily solveable. I have seen metalized
plastic film that is stick on and highly reflective.

Someone else mentioned the possibility of changing your circuitry
to throttle back the LED current based on a temperature sensor.
Of course, if you do that you'll have to remeasure the radiation
patterns at the lowest current based on the highest ambient temp
that you expect to be operating at. The good thing is that your
nav lights are only required to be operating at night.

It seems to me that you could dispense with the cooling fan by
incorporating a large enough passive heat sink. My suggestion
would be to think along the lines of a big piece of really thin
aluminum. You could also use passive air cooling by ducting
some of the high pressure air from the lower side of the wing
over the heat sink, and out the back of the airfoil. Or, you
could just keep checking that the fan is working and replace it
when it craps out ;-)

Good work!

Don W.

Andrew Sarangan wrote:
> Recently I designed and built a nav light system for my Europa aircraft
> using the Luxeon LEDs with great results.
>
> I also measured the radiation pattern (after fitting the LEDs to the
> aircraft) and compared it to the FAR specs. It exceeds the requirements
> by 3-5 times, and draw only 2 Amps total. It is bright enough to almost
> use as a landing light.
>
> Actually, I am surprised that most manufacturers do not provide a
> measured radiation pattern. They just say that it complies with the
> FAR. Well, meeting the specs is not the same as exceeding it. Since
> LEDs lose power with age, this could be an important consideration.
>
> Just in case someone might find this useful, I wrote it up as an
> article:
> http://www.sarangan.org/aviation/europa/wing/Wing-tip-Lights.pdf
>
> Let me know what you think.
>

[Andrew Sarangan]

Why not fuse the 12V line - that's what I wanted to do, but due to the
way I had done the board layout, it was simpler to fuse the ground
line. Besides, this is a fiberglass airplane, so it is unlikely to come
in contact with dangling ground wires. But your point is valid, and it
might be worth changing in the future.

I had not thought of the decoupling capacitor for the 7805. Again, a
good point.

I knew about the low power dissipation in the 0.69Ohm resistor. The
only reason I used a 3W resistor was because I wanted to use the same
board for both circuits. A smaller resistor in a big slot won't look as
pretty (I know, lame reason..)

Someone else brought up the point about switching regulators for
greater efficiency. Actually, the efficiency here is not all that bad.
The red-side transfers 50% power to the LED, and the green-side
transfers 80% power to the LED. Most switching regulators are around
80-90% efficient, so I concluded that switching regulators were not
worth the extra complexity and the possible RF interference to gain a
little bit of extra efficiency. In addition, most off-the-shelf
switching regulator chips put out constant voltages, so they would
still need a circuitry to convert to a constant current application (so
we are back to the same problem). Also, most off-the-shelf switching
regulators put out standard PC motherboard voltages (3.3V, 5.0V..), and
non-standard voltages would be costly components. I once tried to
design my own switching regulator, but I could not entirely isolate the
noise. I could hear a hiss on my FM radio.




Rob Turk wrote:
> "Andrew Sarangan" > wrote in message
> oups.com...
> > Actually, those are current sensing resistors, not current limiting.
> > The voltage across the resistor is compared with a fixed reference
> > voltage, and it is the FET which limits the current. So, a higher
> > resistor does not necessarily mean lower current.
> >
>
> Ahh yes. I saw the current sensing resistors but I overlooked the resistor
> values for the divider that creates the fixed reference. I'm still wondering
> about a few things though, hope you can elaborate a bit.
>
> - You have the fuse placed in the ground wire. However, the largest
> 'touchable' area on your PCB and heatsinks are all attached to the area
> behind the fuse. A simple protruding screw or rivet would be enough to turn
> the circuit into an unfused system. Why not fuse the 12V line?
>
> - The LM7805 really likes having a small decoupling capacitor at it's output
> to prevent oscillation (and even self-destrucion on cheap non-brand 7805's).
> How about adding 100nF between the 5V output and ground?
>
> - The current sense resistor in the 1400mA version will dissipate about (1.4
> x 1.4 x 1.3) 2.6W, so a 3W resistor is OK. The 700mA version would only
> dissipate (0.7 x 0.7 x 0.69) about 0.36 W. You could do with a smaller
> version (0.5W) there??
>
> Mind you, I'm not trying to be negative about your design, but just curious
> about some of your design decisions. The Luxeon's are really nice gadgets
> 8-)
>
> Rob

[Rob Turk]

"Andrew Sarangan" > wrote in message
oups.com...
> Why not fuse the 12V line - that's what I wanted to do, but due to the
> way I had done the board layout, it was simpler to fuse the ground
> line. Besides, this is a fiberglass airplane, so it is unlikely to come
> in contact with dangling ground wires. But your point is valid, and it
> might be worth changing in the future.
>
> I had not thought of the decoupling capacitor for the 7805. Again, a
> good point.
>
> I knew about the low power dissipation in the 0.69Ohm resistor. The
> only reason I used a 3W resistor was because I wanted to use the same
> board for both circuits. A smaller resistor in a big slot won't look as
> pretty (I know, lame reason..)
>
> Someone else brought up the point about switching regulators for
> greater efficiency. Actually, the efficiency here is not all that bad.
> The red-side transfers 50% power to the LED, and the green-side
> transfers 80% power to the LED. Most switching regulators are around
> 80-90% efficient, so I concluded that switching regulators were not
> worth the extra complexity and the possible RF interference to gain a
> little bit of extra efficiency. In addition, most off-the-shelf
> switching regulator chips put out constant voltages, so they would
> still need a circuitry to convert to a constant current application (so
> we are back to the same problem). Also, most off-the-shelf switching
> regulators put out standard PC motherboard voltages (3.3V, 5.0V..), and
> non-standard voltages would be costly components. I once tried to
> design my own switching regulator, but I could not entirely isolate the
> noise. I could hear a hiss on my FM radio.
>

The switching circuits I was referring to are not the 78xx equivalents of
standard voltage regulators. These Micropuck drivers are specifically
designed to drive Luxeon LED's, they are producing the correct drive current
for them and can be used with a wide input range (8-32V, making them
suitable for both 14V and 28V installations).

The RF interference is definitely something to worry about. With a metal
plane it should be easier to get things working OK but in a fibreglass plane
it may require extra decoupling attention.

For your own plane I can imagine you're not too worried about having
dangling ground wires, but note that because you published the design,
others may be encouraged to reproduce the design for their own plane.
Especially because you did such a great and professional looking job
documenting it, others may not think twice when applying it to their kit.
Safety first...

Rob

[Andrew Sarangan]

Rob

The fan is for cooling the LED; not for cooling the regulator (although
it can do that too). I don't think a switching regulator will solve
the LED heating problem.

The LED junction temperature has to be kept below 130C, which is not
easy to do without active cooling or a hefty heatsink. I did the
calculations a while back (posted it to sci.engr.lighting). The best
and worst case scenarios were 100C and 140C. When I turn the fan off,
the heat sink temperature rises quite dramatically (taken from the
thermistor).

I had not looked at the LED Dyamics parts. Good pointer. Thanks.


Rob Turk wrote:
> "Andrew Sarangan" > wrote in message
> oups.com...
> > At first I did not like the use of a fan either. However, the
> > alternative is to install heftier heatsinks (more weight and more
> > space). This is also why I installed a thermistor. If the fan fails,
> > the temperature will rise quite dramatically, and you will know
> > something is wrong. But you have a good point. The thermistor could
> > have been easily incorporated with the circuit to automatically scale
> > down the current with tempertaure rise.
> >
>
> Have you considered using a PWM circuit to drive the LED's? Those can get
> you much higher efficiency than a series current regulator. That should
> allow you to eliminate the fan entirely.
>
> Maybe check out http://www.leddynamics.com/LuxDrive/drivers.php , they offer
> a small solid-state PWM module called the Micropuck, specifically designed
> for automotive purpose. Reasonably cheap, light and small.
>
> Rob

[[email protected]]

Won't pulse modulation take care of the heat problem.

Andrew Sarangan wrote:
> Rob
>
> The fan is for cooling the LED; not for cooling the regulator (although
> it can do that too). I don't think a switching regulator will solve
> the LED heating problem.
>
> The LED junction temperature has to be kept below 130C, which is not
> easy to do without active cooling or a hefty heatsink. I did the
> calculations a while back (posted it to sci.engr.lighting). The best
> and worst case scenarios were 100C and 140C. When I turn the fan off,
> the heat sink temperature rises quite dramatically (taken from the
> thermistor).
>
> I had not looked at the LED Dyamics parts. Good pointer. Thanks.
>
>
> Rob Turk wrote:
>
>>"Andrew Sarangan" > wrote in message
oups.com...
>>
>>>At first I did not like the use of a fan either. However, the
>>>alternative is to install heftier heatsinks (more weight and more
>>>space). This is also why I installed a thermistor. If the fan fails,
>>>the temperature will rise quite dramatically, and you will know
>>>something is wrong. But you have a good point. The thermistor could
>>>have been easily incorporated with the circuit to automatically scale
>>>down the current with tempertaure rise.
>>>
>>
>>Have you considered using a PWM circuit to drive the LED's? Those can get
>>you much higher efficiency than a series current regulator. That should
>>allow you to eliminate the fan entirely.
>>
>>Maybe check out http://www.leddynamics.com/LuxDrive/drivers.php , they offer
>>a small solid-state PWM module called the Micropuck, specifically designed
>>for automotive purpose. Reasonably cheap, light and small.
>>
>>Rob
>
>

[Andrew Sarangan]

No, it won't. PWM can be used as a dimmer, but it does not change the
LED efficiency. Running an LED at 1Amp half-duty PWM is the same as
running DC at 500mA. Your eye integrates the signal. The only time when
PWM can increase efficiency is in nonlinear devices. LED's power-out vs
current-in is very linear. PWM is used where a digital signal is used
to control the LED brightness. It is often easier to turn the LED on
and off at a fixed current, rather than changing the current.




wrote:
> Won't pulse modulation take care of the heat problem.
>

[GeorgeB]

On 26 Feb 2006 20:59:02 -0800, "Andrew Sarangan" >
wrote:

>No, it won't. PWM can be used as a dimmer, but it does not change the
>LED efficiency. Running an LED at 1Amp half-duty PWM is the same as
>running DC at 500mA. Your eye integrates the signal. The only time when
>PWM can increase efficiency is in nonlinear devices. LED's power-out vs
>current-in is very linear. PWM is used where a digital signal is used
>to control the LED brightness. It is often easier to turn the LED on
>and off at a fixed current, rather than changing the current.

Andrew, I've heard that the eye responds to the peak; indeed, long
ago, I was told that the eye's response allowed far lower average
powers via pwm than DC in the old LED calculator displays. If true
(and I'm not claiming it is, only that I THINK I remember it), is it
only in the lower brightness range (viewing vs illumination)? You
seem very well versed in optics.

wrote:
>> Won't pulse modulation take care of the heat problem.
>>

[Don W]

Hi Karel,

Actually, I'll bet you could make a passive heat sink that wouldn't
weigh more than the fan. It would _not_ be one of those extruded
aluminum heat sinks. At any rate, it doesn't matter because he's
got the fan in there now.

I agree that using a switching DC:DC converter would use a lot
less power than the linear regulator.

Don W.

karel wrote:
>
> Don,
> Andrew rightly preferred a fan over a passive heatsink for
> weight reasons and I think he was right. However the
> real solution was suggested by Rob Turk (dag, Rob!):
> a switching regulator is light and incorporates no mechanics.
>
> Karel

[[email protected]]

Hi Andrew,

The way i understand it is that you get the same brightness with PWM but
in bursts. Your still pulsing at 500ma which gives the same brightness.

The eye/brain won't see the pulses, but the led is only on "X" percent
of full duty, but appears to be constant.
Kind of like florescent light which flashes at 60 hertz.

Are you increasing efficiency? Maybe not, but the duty cycle of the LED
is less which may allow the use of a heat sink without the use of a fan.

Is my understanding correct?

It's a very nice arrangement anyways!

Thanks for the ideas.

Andrew Sarangan wrote:
> No, it won't. PWM can be used as a dimmer, but it does not change the
> LED efficiency. Running an LED at 1Amp half-duty PWM is the same as
> running DC at 500mA. Your eye integrates the signal. The only time when
> PWM can increase efficiency is in nonlinear devices. LED's power-out vs
> current-in is very linear. PWM is used where a digital signal is used
> to control the LED brightness. It is often easier to turn the LED on
> and off at a fixed current, rather than changing the current.
>
>
>
>
> wrote:
>
>>Won't pulse modulation take care of the heat problem.
>>
>
>

[Smitty Two]

> somebody talked about PWM

Flashing an LED on and off (pulse width modulation) is a great way to
conserve battery life in some applications, primarily where the light is
used as an indicator. True, persistence of vision will make it appear
that the LED remains on. But, it will dim the light, proportionally to
the ON/OFF ratio, which may not be desirable for airplane navigation
lights. So in this case, I don't think it's a practical way to reduce
heat.

[[email protected]]

Andrew,

Your powersupply is way too inefficient... use a National Semiconductor
simple switcher LM2576 and you'll be much better off. Don't worry, it
won't emit a bunch of RF with the toroidal inductor it uses...

Dean Wilkinson

[Andrew Sarangan]

A flickering light will appear to be less bright than if it were
steady. This is because the eye takes the average between the on and
the off cycles. There is an opposing theory that says this averaging is
not perfectly linear (ie the on-cycle is not given the same weighting
as the off-cycle), but for most practical purposes we can take it to
be linear. So, at 50% duty cycle you will get 50% brightness, 50% power
consumption and 50% thermal heating. Everything scales by the same
ratio. You don't gain anything.

In highly nonlinear devices, where the light vs current is not a
straight line, such as laser diodes, PWM can actually increase the
brightness. But LED's are fairly linear devices so this effect does not
come into play.

In addition, pulsing a high-current LED may introduce other problems. 1
Amp turning on and off will generate all kinds of noise you may not
want in an aircraft.



wrote:
> Hi Andrew,
>
> The way i understand it is that you get the same brightness with PWM but
> in bursts. Your still pulsing at 500ma which gives the same brightness.
>
> The eye/brain won't see the pulses, but the led is only on "X" percent
> of full duty, but appears to be constant.
> Kind of like florescent light which flashes at 60 hertz.
>
> Are you increasing efficiency? Maybe not, but the duty cycle of the LED
> is less which may allow the use of a heat sink without the use of a fan.
>
> Is my understanding correct?
>
> It's a very nice arrangement anyways!
>
> Thanks for the ideas.
>
> Andrew Sarangan wrote:
> > No, it won't. PWM can be used as a dimmer, but it does not change the
> > LED efficiency. Running an LED at 1Amp half-duty PWM is the same as
> > running DC at 500mA. Your eye integrates the signal. The only time when
> > PWM can increase efficiency is in nonlinear devices. LED's power-out vs
> > current-in is very linear. PWM is used where a digital signal is used
> > to control the LED brightness. It is often easier to turn the LED on
> > and off at a fixed current, rather than changing the current.
> >
> >
> >
> >
> > wrote:
> >
> >>Won't pulse modulation take care of the heat problem.
> >>
> >
> >

[Andrew Sarangan]

I have done the efficiency analysis, and had concluded that a switching
regulator did not perform much better than a straight DC current
regulator. Firstly, the LM2576 is a voltage converter. You would still
need to follow this with a constant current source to drive the LED. A
more efficient approach is to use a current-mode converter, such as the
ADP1864, which has a seperate current-sense pin. In either case, the
benefits are not that overwhelming.

The green LEDs need 700mA at 10V. There is an additional 0.5V drop
across the wiring (cockpit to wingtip) that is unrecoverable. The
conversion efficiency of ADP1864 chip is around 85% for 700mA at 10V.
The overall efficiency of the system will be 82%, which is almost
exactly the same as the simple DC current source. So there is no
advantage in this case.

The red LEDs need 1.4A at 6V. Doing the same analysis, the overall
efficiency with the switching circuit will be 81%. The straight DC
source has an efficiency of only 50%, so there is some advantage here.
However, in real numbers, this difference amounts to only 250mA. I
didn't think that was enough to justifying installing a switching
system. The potential of a noise problem made the decision even easier.




wrote:
> Andrew,
>
> Your powersupply is way too inefficient... use a National Semiconductor
> simple switcher LM2576 and you'll be much better off. Don't worry, it
> won't emit a bunch of RF with the toroidal inductor it uses...
>
> Dean Wilkinson

[Andrew Sarangan]

Hello

It was my impression that mounting the LED directly to the reflector
(flashing aluminum) will not provide enough heat sinking. A large
surface area is not the only determining factor because the thermal
conductivity of the metal will limit how far the heat can spread. The
thinner the metal, the worse the thermal conductivity. Flashing
aluminum is only 1/100" thick so its conductivity is not very high. One
could construct fins on the back side to increase the surface area, but
commercial heat sinks do that much better.




wrote:
> Great write up, and well documented and good composite stuff along with
> the electronics.
>
> I'll throw in my 2 cents, on top of an already good design.
>
> 1) Sometimes less is more, especially in aircraft. If one LED hits
> your numbers, go that way. With a single LED, it increases the
> benefits if a switching arrangement versus the linear one deployed.
> The hockey pucks mentioned elsewhere might be an easy way to go. As
> far as extra margin for degradation over time, this is really an issue
> for applications that run 24/7, but for an aircraft and then only the
> hours its used at night, these babies aren't going to see over 1000
> hours in their life.
> 2) I hate fans/Heat sinking- airplanes and heat sinks are made out of
> the same stuff. Use your reflector as the heat sink by epoxying the
> LED to the reflector.
>
> So if you went that way you'd have an off the shelf circuit, one LED on
> each side, and no fans.

[Montblack]

("karel" wrote)
> PS Andrew, would you mind answering _below_ ?
> It really makes sense, you know...


Besides, June is Top Posting Month at rec.aviation*.


Montblack :-)

[Ernest Christley]

Andrew Sarangan wrote:
> Hello
>
> It was my impression that mounting the LED directly to the reflector
> (flashing aluminum) will not provide enough heat sinking. A large
> surface area is not the only determining factor because the thermal
> conductivity of the metal will limit how far the heat can spread. The
> thinner the metal, the worse the thermal conductivity. Flashing
> aluminum is only 1/100" thick so its conductivity is not very high. One
> could construct fins on the back side to increase the surface area, but
> commercial heat sinks do that much better.
>

I think you have a beautiful design with good tradeoffs, Andrew.
However, if one were inclined to go fanless, there are heatsinks now
that are much lighter will provide much greater surface area. It uses a
much greater number of fins, and each is very thin.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

[GeorgeB]

On Sun, 05 Mar 2006 05:16:51 GMT, Ernest Christley
> wrote:

>Andrew Sarangan wrote:
>> Hello
>>
>> It was my impression that mounting the LED directly to the reflector
>> (flashing aluminum) will not provide enough heat sinking. A large
>> surface area is not the only determining factor because the thermal
>> conductivity of the metal will limit how far the heat can spread. The
>> thinner the metal, the worse the thermal conductivity. Flashing
>> aluminum is only 1/100" thick so its conductivity is not very high. One
>> could construct fins on the back side to increase the surface area, but
>> commercial heat sinks do that much better.
>>
>
>I think you have a beautiful design with good tradeoffs, Andrew.
>However, if one were inclined to go fanless, there are heatsinks now
>that are much lighter will provide much greater surface area. It uses a
>much greater number of fins, and each is very thin.

Ernest, it takes cross sectional area to conduct heat; if you look at
the TRULY designed heatsinks to maximize heat transfer to air, you
will note a tapered base with tapered fins, larger and thicker fins
close, smaller and thinner further away.

I am weak in describing this, but I think you will get the picture;
think about holding a piece of aluminum foil 2 inches long in a candle
flame, then a piece of 1/4" aluminum rod in the same flame. Your
fingers won't get warm with the foil, but will burn with the rod.

[[email protected]]

Hi Andrew,

Here is what you do:

Put a 0.1 ohm 1 Watt resistor in series with your LEDs between them and
ground.

Use an LM2904 op amp to amplify the voltage across the 0.1 ohm resistor
such that the LM2904 outputs 1.2 VDC when the desired current is
flowing through the resistor. The gain resistors you need is something
you should be able to figure out.

The output of the LM2905 op amp goes to the Feedback input of the
LM2576.

Best Regards,

Dean Wilkinson

[Andrew Sarangan]

wrote:
> Hi Andrew,
>
> Here is what you do:
>
> Put a 0.1 ohm 1 Watt resistor in series with your LEDs between them and
> ground.
>
> Use an LM2904 op amp to amplify the voltage across the 0.1 ohm resistor
> such that the LM2904 outputs 1.2 VDC when the desired current is
> flowing through the resistor. The gain resistors you need is something
> you should be able to figure out.
>
> The output of the LM2905 op amp goes to the Feedback input of the
> LM2576.
>
> Best Regards,
>
> Dean Wilkinson


Thanks for that explanation. It makes sense now. I am still a bit
hesitant to put a switching regulator next to my VHF antennas, but I
will give it a try.

[[email protected]]

For some reason I thought you'd mentioned that it was .1". If its that
thin (.01") you might need to either use thicker stock or epoxy (with
special high heat transfer epoxy) that heat sink behind the LED.