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LED power.......

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avidsid(at)yahoo.com
Guest

Posted: Sun Apr 16, 2006 7:46 am Post subject: LED power.......

Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

  anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

  Sid

  -----------------------



Sid Hausding <avidsid(at)yahoo.com> wrote:

  



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

		

---------------------------------

klehman(at)albedo.net
Guest

Posted: Sun Apr 16, 2006 8:32 am Post subject: LED power.......

Hi Sid

The vast majority of LED's require a current limiting resistor. A few 

may have one included for example a panel indicator that specifically 

says 12 volts. The operating voltage of an LED is around 1.7 volts or so 

depending on the color. I usually limit the current to 8 to 15 ma. for 

general use. 1000 ohms might be in the ballpark for an indicator but 

play around with it. Radio shack used to have cheap paper back books 

(written by Forrest Mims ?) that are great for learning things like 

this. There are also a number of web sites on using LED's.

Ken



Sid Hausding wrote:



 	  Quote: 	
		  



Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

  anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

  Sid

  -----------------------



Sid Hausding <avidsid(at)yahoo.com> wrote:

  



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

		

---------------------------------

Mark Phillips in TN

Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN

Posted: Sun Apr 16, 2006 8:35 am Post subject: LED power.......

Hi Sid-



To properly power an LED or even several in series, you simple provide an 

appropriate amount of resistance in series with the correctly-biased LED(s) that 

results in an actual current flow of about 20 mA.  You can also reduce their 

brightness by using more resistance, or less resistance, which results in 

overdriving the LEDs somewhat.  This will likely shorten their lifespan, but what 

is 50% of maybe 100K hrs in the life of an airplane? When I first got 

interested in playing with these things for my RV, I ordered a bunch of different types 

and colors from Marlin P Jones, got a resistor assortment from JapShack, and 

did a bunch of playing with them.  The parts are extremely cheap.  See:



http://mpja.com/listitems.asp?dept=116&main=122



Radio Shack carries this resistor assortment- you can make virtually any 

resistance by combining resistors:



http://www.radioshack.com/product/index.jsp?productId=2062304&cp=&origkw=resis

tor&kw=resistor&kwCatId=2032058&parentPage=search



If that ugly URL doesn't work, go to Radioshack.com and search for 271-308.



If you can do Excel, I've got a SS that lists required resistances for 

various types and series strings of LEDs. Will send copy direct & if anyone else 

interested will send also...



Mark Phillips

_________________
From The PossumWorks...

avidsid(at)yahoo.com
Guest

Posted: Sun Apr 16, 2006 9:26 am Post subject: LED power.......

Free cheap beer all day for Mark...........yay, and thank you profusely for getting me started.  Just too lazy to review all the old manuals and relearn what I had forgotten years ago.  You have provided me with the fast step to getting me back up to speed..........Happy Easter to you and yours and this list.

   

  Sid

  Avid Speedwing

  ----------------------------------------

  



Fiveonepw(at)aol.com wrote:

  



Hi Sid-



To properly power an LED or even several in series, you simple provide an 

appropriate amount of resistance in series with the correctly-biased LED(s) that 

results in an actual current flow of about 20 mA. You can also reduce their 

brightness by using more resistance, or less resistance, which results in 

overdriving the LEDs somewhat. This will likely shorten their lifespan, but what 

is 50% of maybe 100K hrs in the life of an airplane? When I first got 

interested in playing with these things for my RV, I ordered a bunch of different types 

and colors from Marlin P Jones, got a resistor assortment from JapShack, and 

did a bunch of playing with them. The parts are extremely cheap. See:



http://mpja.com/listitems.asp?dept=116&main=122



Radio Shack carries this resistor assortment- you can make virtually any 

resistance by combining resistors:



http://www.radioshack.com/product/index.jsp?productId=2062304&cp=&origkw=resis

tor&kw=resistor&kwCatId=2032058&parentPage=search



If that ugly URL doesn't work, go to Radioshack.com and search for 271-308.



If you can do Excel, I've got a SS that lists required resistances for 

various types and series strings of LEDs. Will send copy direct & if anyone else 

interested will send also...



Mark Phillips





"Why can't we all just get along?"

		

---------------------------------

retasker(at)optonline.net
Guest

Posted: Sun Apr 16, 2006 9:27 am Post subject: LED power.......

Essentially all LEDs require some sort of current limiter.  That is, 

LEDs are current driven devices (rather than voltage driven like a 

normal incandescent lights) and putting a voltage higher than required 

to light them will cause almost instant self destruction.  But it looks 

like you already knew that...



That said, the LED assemblies offered on the referenced site may very 

well have internal limiters, especially where they come attached to a 

threaded plug that is designed to thread into an incandescent lamp socket.



Most single (unassembled) LEDs do NOT have an internal limiter and are 

designed to work with something between 10-30 mA through them.  This can 

be accomplished in a number of ways, but the simplest is just to wire a 

resistor in series with the LED.  When you purchase an LED it should 

come with specifications that tell you what the operating voltage is 

(the voltage that it lights at) and the rated maximum current.  Let's 

take a sample red LED with an operating voltage of 1.7V and a maximum 

current of 30 mA.  Let's further assume that you want to operate this at 

something less than the maximum current to allow for high temperatures 

in the cockpit and choose 20 mA (a little less than maximum brightness, 

but probably still very bright).  If we are operating this from the 

alternator/battery voltage we can assume about 14V available (actually 

12-14.5V - see below).  If we just hooked this to the LED it would be 

very bright (for a millisecond or two), then very dark   .  We need 

to limit the current to the aforementioned 20 mA.



So...  We take the available voltage (14V) and subtract what the LED 

needs (1.7V) to get the voltage we need to "discard" (12.3V).  We then 

calculate the resistor we need to allow 20 mA with this voltage 

(12.3V).  Using Ohm's law (V=I*R) and rearranging (R=V/I) we plug the 

numbers into the equation to get 12.3V/0.02A=615 ohms.  That is not one 

of the standard available values of resistor so we choose a value that 

is close (620 ohms for 5% resistors or 619 ohms for 1% resistors).  Now 

we have a value but we still have to calculate the power "discarded" in 

the resistor so we can get one that will not burn up as soon as we turn 

things on.  Power in watts is (W=V*I).  Plugging the numbers in we get 

12.3V * 0.02A = 0.246 W or 246 mW.  Resistors come is various power 

ratings and we have to choose one that is at least what we just 

calculated.  That means we should use 1/4W at absolute minimum.  

However, if we choose the 1/4W resistor it will run very hot to 

dissipate 0.246W of power - potentially shortening it's life and 

reliability.  So we really want to choose at least a 1/2W resistor.  

That will give it some "elbow room" especially important if it will be 

operating in a warm environment.



Use the same idea for any LED you may buy (different colors have 

different operating voltages).  Additionally, you can connect two LEDs 

in series for whatever reason (lighting, etc.) and just do the same 

calculations but use 3.4V (in this case 1.7V times the two LEDs) instead 

of 1.7V.  You can do this with several LEDs but you must allow some 

series voltage to drop across the resistor (remembering that the voltage 

on the aircraft bus varies from about 14.5V max. to about 12V min.).  As 

the voltage varies the LEDs will vary in brightness.  If you have one 

LED the "leftover" voltage will vary between 12.8V to 10.3V - a change 

of approximately 20-25%.  If you series several LEDs (say four for a 

drop of 6.8V), the voltage will vary between 7.7V to 5.2V - a change of 

30-50%.  So do not add too many in series that you have too low a 

voltage across the resistor.



Do not connect LEDs in parallel!!!  If you do, the one that has a 

slightly lower operating voltage will "hog" all the current - causing it 

to use too much current (overheat) and be much brighter than the 

other(s).  If you want to wire them in parallel for some reason, use a 

series resistor for each LED.



If you have more questions, please ask.  



Dick Tasker



Sid Hausding wrote:



 	  Quote: 	
		  



Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

  anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

  Sid

  -----------------------



Sid Hausding <avidsid(at)yahoo.com> wrote:

  



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

		

---------------------------------

 

 

 





  




	


-- 

Please Note: 

No trees were destroyed in the sending of this message.  We do concede, however,

that a significant number of electrons may have been temporarily inconvenienced.

--

avidsid(at)yahoo.com
Guest

Posted: Sun Apr 16, 2006 9:44 am Post subject: LED power.......

Okay Ken, 

  I'm relearning some of this from a very long time ago.  Its was always interesting and still is to me, just wanted a short cut to get these dang things in for indicator lights..........will search around and bring myself up to speed on some of this "old memory stuff".

  Happy Holiday, 

   

  Sid

  --------------------------------



Ken <klehman(at) wrote:

  



Hi Sid

The vast majority of LED's require a current limiting resistor. A few 

may have one included for example a panel indicator that specifically 

says 12 volts. The operating voltage of an LED is around 1.7 volts or so 

depending on the color. I usually limit the current to 8 to 15 ma. for 

general use. 1000 ohms might be in the ballpark for an indicator but 

play around with it. Radio shack used to have cheap paper back books 

(written by Forrest Mims ?) that are great for learning things like 

this. There are also a number of web sites on using LED's.

Ken



Sid Hausding wrote:



 	  Quote: 	
		  



Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

 anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

 Sid

 -----------------------



Sid Hausding wrote:

 



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

 

---------------------------------

 

 

 





 




	






"Why can't we all just get along?"

		

---------------------------------

rhdudley(at)att.net
Guest

Posted: Sun Apr 16, 2006 9:44 am Post subject: LED power.......

Hi Sid,

Bob Nucolls has piece on LEDs. I think that it answers many of your 

questions.

Try this link:       http://www.aeroelectric.com/articles/leds3.pdf



Regards,



Richard Dudley

-6A flying



Sid Hausding wrote:



 	  Quote: 	
		  



Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

  anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

  Sid

  -----------------------



Sid Hausding <avidsid(at)yahoo.com> wrote:

  



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

		

---------------------------------

avidsid(at)yahoo.com
Guest

Posted: Sun Apr 16, 2006 10:00 am Post subject: LED power.......

you're going to be a great instructor in you "other" life.........and thanks a million, now I don't even have to read anymore..........just follow your little examples and "getter' done" as the cable guy says.............and a Happy Holiday to you and the family also.  Great info this morning........

   

  Sid

  I even used to know Ohm's law..........    

  ------------------

   

  



"Richard E. Tasker" <retasker(at)> wrote:

  



Essentially all LEDs require some sort of current limiter. That is, 

LEDs are current driven devices (rather than voltage driven like a 

normal incandescent lights) and putting a voltage higher than required 

to light them will cause almost instant self destruction. But it looks 

like you already knew that...



That said, the LED assemblies offered on the referenced site may very 

well have internal limiters, especially where they come attached to a 

threaded plug that is designed to thread into an incandescent lamp socket.



Most single (unassembled) LEDs do NOT have an internal limiter and are 

designed to work with something between 10-30 mA through them. This can 

be accomplished in a number of ways, but the simplest is just to wire a 

resistor in series with the LED. When you purchase an LED it should 

come with specifications that tell you what the operating voltage is 

(the voltage that it lights at) and the rated maximum current. Let's 

take a sample red LED with an operating voltage of 1.7V and a maximum 

current of 30 mA. Let's further assume that you want to operate this at 

something less than the maximum current to allow for high temperatures 

in the cockpit and choose 20 mA (a little less than maximum brightness, 

but probably still very bright). If we are operating this from the 

alternator/battery voltage we can assume about 14V available (actually 

12-14.5V - see below). If we just hooked this to the LED it would be 

very bright (for a millisecond or two), then very dark  . We need 

to limit the current to the aforementioned 20 mA.



So... We take the available voltage (14V) and subtract what the LED 

needs (1.7V) to get the voltage we need to "discard" (12.3V). We then 

calculate the resistor we need to allow 20 mA with this voltage 

(12.3V). Using Ohm's law (V=I*R) and rearranging (R=V/I) we plug the 

numbers into the equation to get 12.3V/0.02A=615 ohms. That is not one 

of the standard available values of resistor so we choose a value that 

is close (620 ohms for 5% resistors or 619 ohms for 1% resistors). Now 

we have a value but we still have to calculate the power "discarded" in 

the resistor so we can get one that will not burn up as soon as we turn 

things on. Power in watts is (W=V*I). Plugging the numbers in we get 

12.3V * 0.02A = 0.246 W or 246 mW. Resistors come is various power 

ratings and we have to choose one that is at least what we just 

calculated. That means we should use 1/4W at absolute minimum. 

However, if we choose the 1/4W resistor it will run very hot to 

dissipate 0.246W of power - potentially shortening it's life and 

reliability. So we really want to choose at least a 1/2W resistor. 

That will give it some "elbow room" especially important if it will be 

operating in a warm environment.



Use the same idea for any LED you may buy (different colors have 

different operating voltages). Additionally, you can connect two LEDs 

in series for whatever reason (lighting, etc.) and just do the same 

calculations but use 3.4V (in this case 1.7V times the two LEDs) instead 

of 1.7V. You can do this with several LEDs but you must allow some 

series voltage to drop across the resistor (remembering that the voltage 

on the aircraft bus varies from about 14.5V max. to about 12V min.). As 

the voltage varies the LEDs will vary in brightness. If you have one 

LED the "leftover" voltage will vary between 12.8V to 10.3V - a change 

of approximately 20-25%. If you series several LEDs (say four for a 

drop of 6.8V), the voltage will vary between 7.7V to 5.2V - a change of 

30-50%. So do not add too many in series that you have too low a 

voltage across the resistor.



Do not connect LEDs in parallel!!! If you do, the one that has a 

slightly lower operating voltage will "hog" all the current - causing it 

to use too much current (overheat) and be much brighter than the 

other(s). If you want to wire them in parallel for some reason, use a 

series resistor for each LED.



If you have more questions, please ask. 



Dick Tasker



Sid Hausding wrote:



 	  Quote: 	
		  



Yes, nice site with lots of lighting ideas...........but, no tech ideas for powering up the little buggers.....

 anyone done this for their panels or other electric/electronic devices and can tell me the needed requirements to keep the LED's from burning out......?

 Sid

 -----------------------



Sid Hausding wrote:

 



Wondering if all LED's come with a built in current limiting device or if most will need to have a resistor wired in line to keep the small power requirements they use from burning out.........any info posted somewhere on wiring in a 15mamp, 12dcv device to show power to the system out there?

Sid

--------------------



"Alan K. Adamson" wrote:




Opps, helps if you post the link...



http://www.ledtronics.com/



Don't know if this has been posted before or not, but this place has just

about every LED imaginable and then some.



"Why can't we all just get along?"



---------------------------------



"Why can't we all just get along?"

 

---------------------------------

 

 

 





 




	


-- 

Please Note: 

No trees were destroyed in the sending of this message. We do concede, however,

that a significant number of electrons may have been temporarily inconvenienced.

--





"Why can't we all just get along?"

		

---------------------------------

jschroeder(at)perigee.net
Guest

Posted: Sun Apr 16, 2006 11:42 am Post subject: LED power.......

Sid -



Most LEDs you get do not have the resistors needed to bias them. Sooo, you  

need to do this. Most fixtures using LED's these days come with the  

biasing already installed for that particular fixture/light. All of the  

replacement bulbs for cars have the bias installed.



Try the following for some of the how-to stuff:



http://www.eaa724.org/TechnicalStuff%20.html

http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

http://members.misty.com/don/ledx.html

http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng



This should get you going.



John Schroeder

On Sun, 16 Apr 2006 08:44:40 -0700 (PDT), Sid Hausding <avidsid(at)yahoo.com>  

wrote:



 	  Quote: 	
		   



 Yes, nice site with lots of lighting ideas...........but, no tech ideas  

 for powering up the little buggers.....

   anyone done this for their panels or other electric/electronic devices  

 and can tell me the needed requirements to keep the LED's from burning  

 out......?

   Sid

Peter Laurence

Joined: 10 Jan 2006
Posts: 50

Posted: Mon Apr 17, 2006 6:32 am Post subject: LED power.......

Dick

Nice explanation.

Would you give us a scenarion using a voltage regulator in a current 

limiting mode?

Peter Laurence

 	  Quote: 	
		   Essentially all LEDs require some sort of current limiter.  That is,

 LEDs are current driven devices (rather than voltage driven like a

 normal incandescent lights) and putting a voltage higher than required

 to light them will cause almost instant self destruction.  But it looks

 like you already knew that...

 That said, the LED assemblies offered on the referenced site may very

 well have internal limiters, especially where they come attached to a

 threaded plug that is designed to thread into an incandescent lamp socket.

 Most single (unassembled) LEDs do NOT have an internal limiter and are

 designed to work with something between 10-30 mA through them.  This can

 be accomplished in a number of ways, but the simplest is just to wire a

 resistor in series with the LED.  When you purchase an LED it should

 come with specifications that tell you what the operating voltage is

 (the voltage that it lights at) and the rated maximum current.  Let's

 take a sample red LED with an operating voltage of 1.7V and a maximum

 current of 30 mA.  Let's further assume that you want to operate this at

 something less than the maximum current to allow for high temperatures

 in the cockpit and choose 20 mA (a little less than maximum brightness,

 but probably still very bright).  If we are operating this from the

 alternator/battery voltage we can assume about 14V available (actually

 12-14.5V - see below).  If we just hooked this to the LED it would be

 very bright (for a millisecond or two), then very dark   .  We need

 to limit the current to the aforementioned 20 mA.

 So...  We take the available voltage (14V) and subtract what the LED

 needs (1.7V) to get the voltage we need to "discard" (12.3V).  We then

 calculate the resistor we need to allow 20 mA with this voltage

 (12.3V).  Using Ohm's law (V=I*R) and rearranging (R=V/I) we plug the

 numbers into the equation to get 12.3V/0.02A=615 ohms.  That is not one

 of the standard available values of resistor so we choose a value that

 is close (620 ohms for 5% resistors or 619 ohms for 1% resistors).  Now

 we have a value but we still have to calculate the power "discarded" in

 the resistor so we can get one that will not burn up as soon as we turn

 things on.  Power in watts is (W=V*I).  Plugging the numbers in we get

 12.3V * 0.02A = 0.246 W or 246 mW.  Resistors come is various power

 ratings and we have to choose one that is at least what we just

 calculated.  That means we should use 1/4W at absolute minimum.

 However, if we choose the 1/4W resistor it will run very hot to

 dissipate 0.246W of power - potentially shortening it's life and

 reliability.  So we really want to choose at least a 1/2W resistor.

 That will give it some "elbow room" especially important if it will be

 operating in a warm environment.

 Use the same idea for any LED you may buy (different colors have

 different operating voltages).  Additionally, you can connect two LEDs

 in series for whatever reason (lighting, etc.) and just do the same

 calculations but use 3.4V (in this case 1.7V times the two LEDs) instead

 of 1.7V.  You can do this with several LEDs but you must allow some

 series voltage to drop across the resistor (remembering that the voltage

 on the aircraft bus varies from about 14.5V max. to about 12V min.).  As

 the voltage varies the LEDs will vary in brightness.  If you have one

 LED the "leftover" voltage will vary between 12.8V to 10.3V - a change

 of approximately 20-25%.  If you series several LEDs (say four for a

 drop of 6.8V), the voltage will vary between 7.7V to 5.2V - a change of

 30-50%.  So do not add too many in series that you have too low a

 voltage across the resistor.

 Do not connect LEDs in parallel!!!  If you do, the one that has a

 slightly lower operating voltage will "hog" all the current - causing it

 to use too much current (overheat) and be much brighter than the

 other(s).  If you want to wire them in parallel for some reason, use a

 series resistor for each LED.

 If you have more questions, please ask.  

 Dick Tasker

 Sid Hausding wrote:

>

>

>Yes, nice site with lots of lighting ideas...........but, no tech ideas 

>for powering up the little buggers.....

>  anyone done this for their panels or other electric/electronic devices 

> and can tell me the needed requirements to keep the LED's from burning 

> out......?

>  Sid

>  -----------------------

>

>Sid Hausding <avidsid(at)yahoo.com> wrote:

>  

>

>Wondering if all LED's come with a built in current limiting device or if 

>most will need to have a resistor wired in line to keep the small power 

>requirements they use from burning out.........any info posted somewhere 

>on wiring in a 15mamp, 12dcv device to show power to the system out there?

>Sid

>--------------------

>

>"Alan K. Adamson" wrote:

>

>

>Opps, helps if you post the link...

>

>http://www.ledtronics.com/

>

>Don't know if this has been posted before or not, but this place has just

>about every LED imaginable and then some.

>

>"Why can't we all just get along?"

>

>---------------------------------

>

>"Why can't we all just get along?"

>

>---------------------------------

>

 -- 

 Please Note:

 No trees were destroyed in the sending of this message.  We do concede, 

 however,

 that a significant number of electrons may have been temporarily 

 inconvenienced.

 --

retasker(at)optonline.net
Guest

Posted: Mon Apr 17, 2006 1:13 pm Post subject: LED power.......

In concept it is really simple   .



The quickest way (since you can probably get the parts at Radshack) is 

to use a three terminal regulator (in, out, ground) like the LM7805 

series, although any voltage regulator will work.  Connect a resistor 

between the output of the regulator and the ground pin of the regulator 

(do NOT connect the regulator ground pin to aircraft ground).  Connect 

the LED and the regulator assembly in series between +14V and ground - 

it doesn't matter if you have the regulator connected to +14V to the LED 

and then to ground or vice versa.  For example, connect the input of the 

regulator to +14V, the ground pin of the regulator/resistor to the 

positive terminal of the LED and the negative terminal of the LED to 

aircraft ground.  You also must add appropriate capacitors per the 

manufacturers datasheet recommendation - I really can't make any 

recommendations here as each regulator is different.



Choose the resistor value based on how much current you want as 

follows:  Let's assume you want 20 mA and you have chosen a 5V 

regulator.  Calculate the resistor value (using ohms law) R = 5V / 0.2 

mA = 250 ohms.  The power dissipated in the resistor is P = 5V * 0.02mA 

= 0.1W.  In this case a 1/4 W resistor is fine.



Depending on how much current you choose and how many LEDs you use you 

may have to add a heat sink to the regulator (unlikely).  Bur just in 

case, you calculate the power dissipated in the regulator like so 

(assuming use of a 5V regulator):  Find the maximum voltage across the 

regulator +14.5V- 5V - 1.7V = 7.8V.  Multiply that by the current, which 

is essentially whatever you chose earlier (20 mA in this case) 7.8V * 

0.02mA = 0.156W - a number that all but the smallest regulators can 

handle without a heatsink.



As with a resistor limiter, you can use more than one LED in series.  

You have to have enough voltage when you add all the voltage drops to 

still be lower than the minimum expected aircraft supply voltage 

(typically about 12V).  The voltages that you must add together are: the 

5V due to the regulator, the LED drop * the number of LEDs and the 

minimum voltage required across the regulator itself (not the 5V, but a 

voltage called the dropout voltage which is anywhere from several volts 

to less than a volt - depending on the actual regulator used).  As an 

example, using a LM7805 with 2V dropout, you would have 12V - 5V - 2V = 

5V, meaning you could drive three red LEDs down to about the 12V.  You 

can also find regulators that allow a smaller output voltage (than 5V) 

which means that you have more for the LEDs - for instance, the LP2950 

is available in a 3V version and this also has a lower dropout voltage 

of about 0.5V.



The advantage of using an active regulator is that the current (and 

therefore brightness) is essentially constant over the full aircraft 

supply voltage (12V to 14.5V).  The disadvantage is that it is a little 

more complicated than a simple series resistor.



Dick Tasker



Peter Laurence wrote:



 	  Quote: 	
		  



Dick



Nice explanation.



Would you give us a scenarion using a voltage regulator in a current 

limiting mode?

Peter Laurence

  



>Essentially all LEDs require some sort of current limiter.  That is,

>LEDs are current driven devices (rather than voltage driven like a

>normal incandescent lights) and putting a voltage higher than required

>to light them will cause almost instant self destruction.  But it looks

>like you already knew that...

>

>That said, the LED assemblies offered on the referenced site may very

>well have internal limiters, especially where they come attached to a

>threaded plug that is designed to thread into an incandescent lamp socket.

>

>Most single (unassembled) LEDs do NOT have an internal limiter and are

>designed to work with something between 10-30 mA through them.  This can

>be accomplished in a number of ways, but the simplest is just to wire a

>resistor in series with the LED.  When you purchase an LED it should

>come with specifications that tell you what the operating voltage is

>(the voltage that it lights at) and the rated maximum current.  Let's

>take a sample red LED with an operating voltage of 1.7V and a maximum

>current of 30 mA.  Let's further assume that you want to operate this at

>something less than the maximum current to allow for high temperatures

>in the cockpit and choose 20 mA (a little less than maximum brightness,

>but probably still very bright).  If we are operating this from the

>alternator/battery voltage we can assume about 14V available (actually

>12-14.5V - see below).  If we just hooked this to the LED it would be

>very bright (for a millisecond or two), then very dark   .  We need

>to limit the current to the aforementioned 20 mA.

>

>So...  We take the available voltage (14V) and subtract what the LED

>needs (1.7V) to get the voltage we need to "discard" (12.3V).  We then

>calculate the resistor we need to allow 20 mA with this voltage

>(12.3V).  Using Ohm's law (V=I*R) and rearranging (R=V/I) we plug the

>numbers into the equation to get 12.3V/0.02A=615 ohms.  That is not one

>of the standard available values of resistor so we choose a value that

>is close (620 ohms for 5% resistors or 619 ohms for 1% resistors).  Now

>we have a value but we still have to calculate the power "discarded" in

>the resistor so we can get one that will not burn up as soon as we turn

>things on.  Power in watts is (W=V*I).  Plugging the numbers in we get

>12.3V * 0.02A = 0.246 W or 246 mW.  Resistors come is various power

>ratings and we have to choose one that is at least what we just

>calculated.  That means we should use 1/4W at absolute minimum.

>However, if we choose the 1/4W resistor it will run very hot to

>dissipate 0.246W of power - potentially shortening it's life and

>reliability.  So we really want to choose at least a 1/2W resistor.

>That will give it some "elbow room" especially important if it will be

>operating in a warm environment.

>

>Use the same idea for any LED you may buy (different colors have

>different operating voltages).  Additionally, you can connect two LEDs

>in series for whatever reason (lighting, etc.) and just do the same

>calculations but use 3.4V (in this case 1.7V times the two LEDs) instead

>of 1.7V.  You can do this with several LEDs but you must allow some

>series voltage to drop across the resistor (remembering that the voltage

>on the aircraft bus varies from about 14.5V max. to about 12V min.).  As

>the voltage varies the LEDs will vary in brightness.  If you have one

>LED the "leftover" voltage will vary between 12.8V to 10.3V - a change

>of approximately 20-25%.  If you series several LEDs (say four for a

>drop of 6.8V), the voltage will vary between 7.7V to 5.2V - a change of

>30-50%.  So do not add too many in series that you have too low a

>voltage across the resistor.

>

>Do not connect LEDs in parallel!!!  If you do, the one that has a

>slightly lower operating voltage will "hog" all the current - causing it

>to use too much current (overheat) and be much brighter than the

>other(s).  If you want to wire them in parallel for some reason, use a

>series resistor for each LED.

>

>If you have more questions, please ask.  

>

>Dick Tasker

>

>Sid Hausding wrote:

>

>    

>

>>

>>

>>Yes, nice site with lots of lighting ideas...........but, no tech ideas 

>>for powering up the little buggers.....

>> anyone done this for their panels or other electric/electronic devices 

>>and can tell me the needed requirements to keep the LED's from burning 

>>out......?

>> Sid

>> -----------------------

>>

>>Sid Hausding <avidsid(at)yahoo.com> wrote:

>> 

>>

>>Wondering if all LED's come with a built in current limiting device or if 

>>most will need to have a resistor wired in line to keep the small power 

>>requirements they use from burning out.........any info posted somewhere 

>>on wiring in a 15mamp, 12dcv device to show power to the system out there?

>>Sid

>>--------------------

>>

>>"Alan K. Adamson" wrote:

>>

>>

>>Opps, helps if you post the link...

>>

>>http://www.ledtronics.com/

>>

>>

>>

>>Don't know if this has been posted before or not, but this place has just

>>about every LED imaginable and then some.

>>

>>

>>

>>

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>>

>>

>>"Why can't we all just get along?"

>>

>>---------------------------------

>>

>>

>>

>>

>>

>>

>>

>>

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>>

>>

>>

>>

>>

>>"Why can't we all just get along?"

>>

>>---------------------------------

>>

>>

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>>

>>      

>>

>-- 

>Please Note:

>No trees were destroyed in the sending of this message.  We do concede, 

>however,

>that a significant number of electrons may have been temporarily 

>inconvenienced.

>--

>

>

>    

>

william.p.dube(at)noaa.go
Guest

Posted: Mon Apr 17, 2006 3:41 pm Post subject: LED power.......

If you are going to go through the effort to use a regulator, use an 

LM317 instead. You can easily set it up as a current regulator and it 

will put exactly 20 mA (or whatever you choose) through the LED. 

Attached is a wiring diagram.



    Here is a nice web page that gives all the basic "how to" for 

running LEDs.



http://www.trainweb.org/girr/tips/tips7/white_led_tips.html



    Use a 60 ohm resistor with the lm317 to get exactly 20 mA. A 62 ohm 

will give slightly less, but you will never notice and it is a standard 

value.



Here is the digikey order page for the style you would want to get. 

Notice that thy cost less than $0.50.



http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=357151&Row=74749&Site=US



Get the TO-92 package as you don't need the large current that the 

larger package will supply. You also don't want surface mount.



Bill Dube'

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