How About A LED Primer?

Here's a site I found for determining a multiple LED circuit:

LED WIZARD

A Light Emitting Diode (LED) is a diode (obviously) that emits light of a single specific frequency (color) when forward-biased. This means that the diode is passing current (in the direction of the arrow in the symbol) as opposed to blocking it. The brightness is typically a function of current, but is not linear to current - in other words if 300 mA produces full brightness, 150 mA will produce less than half brightness. Also notice that LEDs being diodes only pass current in one direction, so we typically want to use DC to power them. You can use AC in certain scenarios (easily producing yellow from a R/G diode), but that's for post #2.

Just like a regular diode, an LED will always have a voltage drop across it when forward biased. This is a known value that does not change (at least as far as we're concerned) with respect to current. It is known by many names, including "forward bias voltage", "built-in voltage", etc., and is typically in the range of 1.5 to 2.0 volts (CORRECTED 11/8/11 @ 18:33).

That being said, what we care about is current through the LED. Based on our supply voltage and the LED's built-in voltage, we need to select a resistor to put in series with the LED such that the current flowing through the LED is as close to BUT NOT EXCEEDING the current rating of the LED. Too much will fry it, too little will produce a dim light.

To calculate the ideal resistor value, use the following formula where Vs is the supply voltage, Vd is the LED's built-in voltage (drop, in Volts), i is the LED's maximum rated current (in Amperes or "Amps"), and R is the resistor value (in Ohms).

R = (Vs - Vd) / i

Once you calculate that, you need to find the next HIGHEST standard resistor value as found below.

http://ecee.colorado.edu/~mccl...resistorsandcaps.pdf

To determine what power rating of resistor you need (1/8 W, 1/4 W, etc.) you need the value of the standard resistor you've chosen above and the two voltages (Vd and Vs) to plug into the following formula.

P = ((Vs - Vd)^2) / R

Yes, that's a "square" - one way to calculate electrical power dissipated by a resistor is the voltage across the resistor (equal to "Vs - Vd") squared, divided by the resistance. See this for more iterations of Ohm's Law. Anyway - just like when sizing the resistor, you need to find the next highest power rating for the resistor. If it works out to .23 Watts, then a 1/4 (0.25) Watt resistor will work just fine.

Those are the basic tools you need - how to calculate the desired resistor value based on your supply voltage, and how to calculate the resistor power rating after that.

quote:

Does that help, or muddy the water?

With the background I have that type of info is not a problem, but I think we need more of a "How do I put together a circuit to light a 3-story building?".

Folks don't need to know how an internal combustion engine works to drive a car type of thing, although it's not a bad idea

Let's take this example:

Say someone wants to light a 3-story building with a 12vdc wallwart and a single LED on each floor.

They can go to the link I provided and:

1) Plug in the value of their voltage source = 12
2) Plug in the forward voltage (a chart comes up for the color of LED) of the LEDs they want to use (white) = 3.3
3) Plug in the forward current = 20
4) Plug in how many LEDs for the project = 3
5) Pick wiring diagram

The result will show what the wiring diagram will look like showing the LEDs and resistor value.

Now I'm not sure if this plug-in formula will work for every situation, but that's an easy way of getting a 3-story building to light up.

I used the link and plugged in various numbers of LEDs, up to 60 (enough for 20 3-story buildings with single LED on each floor).

The only change I saw in the parameters was the current requirements of the power source (wallwart). 60 LEDs requires a wallwart of 400ma. 3 LEDs require a wallwart of only 20ma.

That would help folks determine what wallwart or other power source they would need.

That seems too easy though, is this program leaving something out?

quote:

Vs is the supply voltage

Is Vs in the formula AC or DC? Does it matter?

Folks, an excellent LED and Electrical site in general, available form the ElectricRR site page: http://electricrr.com/links.htm

And click on:
"Kelsey Park School Electronics Club"
URL: Kelsey Electronics Club

These folks were kind enough to allow me to link to their site. Worth a look.

DC, and yes it does matter.

I get my LED's from Christmas lights.We all mostly use 3MM Led's because they're the right size for engines and do a pretty good job in buildings.In the Christmas light world your going to find 2 different kinds of LED's . The round top which is best for our use and a flat top which usually comes in the 5MM range anyway.

I look for the GE brand lights in the Warm White not cool white unless you want everything to look blue.If you look you'll find a set from GE that have the twinkle light look of the incandescent bulbs.The twinkle light cap pulls off and inside is a 3MM Warm White round top 3.5V .75ma LED.After Christmas you can get these sets at Wal Mart or Lowes for about a buck.
I did buy some multi color sets but I've yet to find a use for them.

I use a 1/4 watt 750 ohm resistor with them and it gives it a pretty good voltage range or you can feed them without a resistor using a Lemax 1000ma 3.5 volt power supply(Lowes)

Now this is just me but I like the one LED- one resistor way of wiring and I'll tell you why .I had LED's in all my N scale buildings and I was using one resistor for the whole lot about 100 .Well the resistor failed and I lost every LED .You talk about a pain in the butt re wiring all those buildings

Once you get going it doesn't take long to make one.I buy a 25 foot roll of telephone wire at lowes .It has a red,green black and yellow wire in it.If you cut it in about 16 inch lengths you don't have to skin it to get the wire out ,just reach in there with needle nose and pull on the wires they'll come right out of the sheath.I use the red and yellow for hot wires and the green and black for grounds so your actually getting 50 feet of wire .16 inches makes a nice pig tail to drop down under the layout

The solid wire is stiff enough so you can position it just right with a little tape.

David

Thought I'd give a few picks for the uses for these things.

Heres my scratch built modern coal tower and my Atlas switch tower at night. "

here's my Atlas station.Note the phone booth try that with an incandescent bulb and you'd have a blob of plastic in about 2 minutes.



And last I take back that I didn't find any use for the multi color sets as I remember where I did use 2 green ones



Headlight, ditch lights, number boards and class lights all LED's from Christmas lights


David

David:

What do you as a power supply voltage for this set up? Can you please post some pictures of how you wire the individual LEDS with the resistors, etc. Thanks!

I have written an LED primer as a chapter in a e-book that I intend to write about electronic projects for model railroaders. This chapter on LEDs is unfinished, but for what there is, I've published it to my blog for you guys. You can find it here:

Fun with LEDs

Coments are welcome.

TT

This reminds me of why I switched out of EE before I flunked out.

Gerry

David:

Thanks, that was pretty straight forward and simple to do. Appreciate it!

Blytovski, I suggest you rework your earlier posting.

quote:

It is known by many names, including "forward bias voltage", "built-in voltage", etc., and is typically in the range of 0.4 to 0.7 volts (0.6 is typical).

This is incorrect. LEDs typically have a forward drop of 1.8-2.0 volts for most colors, and about 3 volts for white.

Also, 350 mA is way too much current for a normal small LED like we use. A 20 mA current (.020 amps) is more appropriate. That is why the resistance and wattage in your example is so extreme.

These posting seem to stick around for a long time, and false information just makes for lots of confused users.

I agree that I was incorrect about the typical forward voltage of an LED and that has been fixed above. I agree that the resistances I came up with in my example are extreme as a result of my low forward voltage.

However, unless I was able to foresee the applications of all the people reading this in the future and posted all of those answers, any single answer would be "false". Hence why I gave the formula and showed them how to calculate it themselves - in my opinion that's the important part, not the example.

quote:

I agree that the resistances I came up with in my example are extreme as a result of my low forward voltage.

Your resistance is so low because of your assumed current, not the voltage drop. Despite variations in applications, you example current is off by over an order of magnitude from typical train usage. This gives people the impression that they will need high-power resistors rather than common 1/4 watt and 1/2 watt varieties.

My goal is to inform, not confuse. I've taken the liberty to remove the example from my original post which should remedy all your concerns. Now back to our regularly scheduled programming...

quote:

I didn't want to complicate this for those just getting started as you don't need stereo instructions just a way to get it done.

Don't know why those of you that have a lot of expertise just can't figure this out.I don't know who your trying to impress other than yourself by posting alot of electronic theory and mathematical equations.All these folks need is an easy way to start using LED's on their layout.


Even for the experienced the KISS system should always be used when trying to explain how to do something to a person for the first time. KISS stands for Keep It Simple Stupid and that last word doesn't always apply to the person trying to learn.

David

I am sure everyone here respects your knowledge, but I think info in layman terms would be more helpful for the majority of us

quote:

Originally posted by Blystovski:
A Light Emitting Diode (LED) is a diode (obviously) that emits light of a single specific frequency (color) when forward-biased. This means that the diode is passing current (in the direction of the arrow in the symbol) as opposed to blocking it. The brightness is typically a function of current, but is not linear to current - in other words if 300 mA produces full brightness, 150 mA will produce less than half brightness. Also notice that LEDs being diodes only pass current in one direction, so we typically want to use DC to power them. You can use AC in certain scenarios (easily producing yellow from a R/G diode), but that's for post #2.

Just like a regular diode, an LED will always have a voltage drop across it when forward biased. This is a known value that does not change (at least as far as we're concerned) with respect to current. It is known by many names, including "forward bias voltage", "built-in voltage", etc., and is typically in the range of 1.5 to 2.0 volts (CORRECTED 11/8/11 @ 18:33).

That being said, what we care about is current through the LED. Based on our supply voltage and the LED's built-in voltage, we need to select a resistor to put in series with the LED such that the current flowing through the LED is as close to BUT NOT EXCEEDING the current rating of the LED. Too much will fry it, too little will produce a dim light.

To calculate the ideal resistor value, use the following formula where Vs is the supply voltage, Vd is the LED's built-in voltage (drop, in Volts), i is the LED's maximum rated current (in Amperes or "Amps"), and R is the resistor value (in Ohms).

R = (Vs - Vd) / i

Once you calculate that, you need to find the next HIGHEST standard resistor value as found below.

http://ecee.colorado.edu/~mccl...resistorsandcaps.pdf

To determine what power rating of resistor you need (1/8 W, 1/4 W, etc.) you need the value of the standard resistor you've chosen above and the two voltages (Vd and Vs) to plug into the following formula.

P = ((Vs - Vd)^2) / R

Yes, that's a "square" - one way to calculate electrical power dissipated by a resistor is the voltage across the resistor (equal to "Vs - Vd") squared, divided by the resistance. See this for more iterations of Ohm's Law. Anyway - just like when sizing the resistor, you need to find the next highest power rating for the resistor. If it works out to .23 Watts, then a 1/4 (0.25) Watt resistor will work just fine.

Those are the basic tools you need - how to calculate the desired resistor value based on your supply voltage, and how to calculate the resistor power rating after that.

quote:

Originally posted by Railking:
I am sure everyone here respects your knowledge, but I think info in layman terms would be more helpful for the majority of us

Whilst I completely agree with you Alan lets try not to bite the hand that's attempting to feed us!

I went to the two JCS studio links and that was my immediate impression, that the information whilst no doubt invaluable, is of limited use if one doesn't understand what's being said.

A perfect example being the 'how to run two trains on the same loop' I'm sorry but I couldn't make heads nor tails of the description or the diagram. So what we need is someone to clarify and simplify the info, I think. Experts are usually the worst because they take so much for granted - 'well of course everyone will know what forward bias is' - that they are unable to see it from the students point of view.

Also having the gurus argue with each other over the details of this, that or the other example isn't at all helpful because it tends to drive away the poor novice who is probably hanging on by a thread to what he was just told and is now being disputed.

EE's need to come to grips with the fact that what turns a lot of people off electronics is the darn math, we're not all happy with formulas. However inaccurate it may be, its more use to say things like 'use a 470k resistor to protect the LED from getting too much current' (Thats NOT a real example) even though there is much more to it than that.

Well its early days yet, lets hope that together we can work out some means of bridging the gap between the gurus and the novices.

Here's another link I found that has quite a bit of info on LEDs:

FIBER OPTICS

Maybe I should rename this thread:

"How About A LED And Fiber Optic Primer?".

Mike,

How right you are.I spent 26 years at the power company trying to explain every day and sometimes 10 times a day, why this happened or that to people that had no knowledge of electricity.I found that very simplified explanations are the best.

That's why in my post I gave a list of materials needed , how to put them together and a way to check the work when your done.

LED's are fun to use. They have so many applications for our hobby.They don't generate any heat so they won't melt plastic.They have a 100,000 hour life span so you'll probably expire before they do , and if you buy them right you can make a working LED light for less than 10 cents each.

Simple explanations are whats going to make this a great new forum.I could explain how the A,B,and C curve of a 19.9KV Oil Circuit Recloser works(If you'd like about 20 pages to sift thru) or I can just say it's designed to reset the power so someone doesn't have to come out in the middle of the night to do it.

KISS


David

David,

IMO you are dead on with the idea that the simple explanations are the best. I'm sure there are folks who would like to know the gory details of what makes it all work - and good for them to look for that. For me, I just want to understand the basics, a little theory and to know enough to be safe and enjoy the trains. I am also sure that if I did want to get a more detailed explanation of somethings, there are more than a few highly competent folks here who'd be more than willing to share their expertise. I suspect your one of those folks as well.

Thanks,
Ed

quote:

Is Vs in the formula AC or DC? Does it matter?

John,
LEDs need DC. That being said, if someone wanted to use an AC power source they could, but they'd have to convert the output to DC by using a bridge rectifier to convert AC to DC.

I just wanted to make it clear to folks that an AC transformer can be the source of power.

quote:

Originally posted by Bob Delbridge:

quote:

Is Vs in the formula AC or DC? Does it matter?

John,
LEDs need DC. That being said, if someone wanted to use an AC power source they could, but they'd have to convert the output to DC by using a bridge rectifier to convert AC to DC.

I just wanted to make it clear to folks that an AC transformer can be the source of power.

Bob,

Your right.
You can even use your LED's without a resistor.Lemax makes a little 1000 ma 3 Volt transformer It's 120 Volts A/C in and 3 Volts D/C out.If you keep the polarity right like I said earlier ,you can wire this directly to your LED because the Christmas light LED's are 3.5 volt so you don't need the resistor.

You can get the Lemax transformer at Lowes and they should be out now or in the next couple weeks.The LED's are just .75 ma so you could easily power several hundred.

It would save an extra step because you could wire it just like a regular light bulb and wouldn't have to buy the resistors.

David

I was not biting the hand, just stating exactly what I meant.. There was nothing derogatory or malicious in my statement.. if you perceive it as such, then there is nothing more I can say..

quote:

Originally posted by mike dench:

quote:

Originally posted by Railking:
I am sure everyone here respects your knowledge, but I think info in layman terms would be more helpful for the majority of us

Whilst I completely agree with you Alan lets try not to bite the hand that's attempting to feed us!

I went to the two JCS studio links and that was my immediate impression, that the information whilst no doubt invaluable, is of limited use if one doesn't understand what's being said.

A perfect example being the 'how to run two trains on the same loop' I'm sorry but I couldn't make heads nor tails of the description or the diagram. So what we need is someone to clarify and simplify the info, I think. Experts are usually the worst because they take so much for granted - 'well of course everyone will know what forward bias is' - that they are unable to see it from the students point of view.

Also having the gurus argue with each other over the details of this, that or the other example isn't at all helpful because it tends to drive away the poor novice who is probably hanging on by a thread to what he was just told and is now being disputed.

EE's need to come to grips with the fact that what turns a lot of people off electronics is the darn math, we're not all happy with formulas. However inaccurate it may be, its more use to say things like 'use a 470k resistor to protect the LED from getting too much current' (Thats NOT a real example) even though there is much more to it than that.

Well its early days yet, lets hope that together we can work out some means of bridging the gap between the gurus and the novices.

Alan I agree, that's why I selected those links, it (the guesswork) is mostly already done for you.

About the hardest thing I see with LEDs is telling the polarity (easy if you know what to look for) and type (if you happen to have some loose ones laying around and don't know where they came from).

I had put 3 LEDs in an Ameritowne 3-story building a while back. I wired them and slid them inside a length of clear plastic tubing so each floor would have a bulb. I drilled holes in the interior floors I made and slid the tubing up from the bottom. I then connected the LED string to a wallwart.

It worked fine, but was a PITA to get in and out of the building. Next time I'll drill holes in the rear walls for the LEDs and run the wiring up the back,maybe put the wires in something to keep them covered.

Are there pop-on covers that can be placed on LEDs so the light is diffused? If I put them in from the back of the building anyone looking into it will be blinded by the intense light they emit, not to mention how unreal it would look. A diffuser, even a small plastic shield, would eliminate the spot light effect you get when looking at them from the top.

I will post this here but it could go in most of the threads. This is a good example of the problem of trying to explain something that is simple to somebody that doesn't understand simple. Its like somebody saying "My car will not run. What do I fix?" Where do you start? Do you ask if he has gas in the tank or did he try the key in the ignition? He may feel insulted if you start that simple or he may not have known what kind of gas to put in the tank.

One gentlemen tried to explain that an LED is not a light bulb but a diode that emits light. The next question, What is a diode??? Another tried to explain that LED's run on DC but you can use AC if you rectify it. Now I know somebody out there will think, "What is DC and what is AC?" Don't Laugh!! Finally somebody tried to explain how to determine the size of the series resistor. Unfortunately he picked bad values for his example. The math was right but the results were very untypical. He was upset when this was pointed out. The problem is that somebody who does not understand the math or anything about LED,s won't know that those results are not typical and will think they are. Then when the error gets pointed out he gets all confused.

If it has not been stated already somebody will soon try to claim that you do not need a resistor if you run a 3 volt LED off of a 3 volt supply. The correct answer is Yes, No, and Maybe. See, if the "forward voltage drop" of the LED (An LED does not have an operating voltage like a light bulb but a specific voltage drop accross it when current is flowing through it.) is exactly 3 volts and the output of the power supply is exactly 3 Volts then the answer is yes. But this is never the case. Good, safe, practice says use a higher voltage supply and a dropping resistor. Some LED,s will say 3 volts but be 2.7 to 3.3 volts and the power supply may be as high as 4 volts or more. Your LED may light but probably will not last as long as it should. Now I know there is somebody out there that will read this and say that he has been doing this for years and has had no trouble. There are exceptions but he will proclaim loud and long that I am wrong. Will your car do 110 MPH, Maybe but should you? It is in this environment of what is and what should be that we are trying to explain to those who do not understand what is.

Please, if you are asking a question give as much information as possible. We cannot read minds. If you are trying to answer a question be as correct as possible and as typical as possible. If the answer you get seems so basic as to be insulting remember, we do not know you. Be patient, we are trying to help. If you do not understand the answer ask again. Consider the possibility that the proper fix may be beyond your capability. Not all doctors make good plumbers.

Al

quote:

He was upset when this was pointed out.

I'm not upset. Dale had a valid point that my values were atypical and misleading, which I agreed, so I remedied the problem the easiest way I knew how - removing the example.

I agree with a lot of your post, Al. Thanks for the suggestions. That last paragraph should be made into an "Intro" sticky on this part of the forum.

Bob and David,

Thank you for your replies. I always wondered what a bridge rectifier did, especially when you use LED's to replace the old "christmas" lights in the K-line heavyweights.

I'm looking forward to bouncing some ideas off you guys in the future!

thanks DPC, I may now have the courage to try with your simple explanation. There are some real experts here on the forum, but I always get lost reading the very knowledgeable posts.

Gentlemen:

In the link that I posted above "Fun With LEDs" I anticipated most of the objections that have been raised here and I tried to address them. I explain what an LED does in the simplest terms; I point out the necessity to hook up the plus and minus leads properly, and I identify the leads with a picture. I do give the equations for computing the value of the resistor, with an explanation; but I also provide a simple look up table so that no math is necessary. Finally I give a "one size fits all" choice for the resistor value.

I go on to illustrate, with pictures, test set-ups that many people should be able to duplicate.

Does this address most of the concerns with keeping it simple? Do you find this helpful? What should be done to improve it?

TT

TT,

I'm still looking at your site but what you have is pretty easy to understand if a person has limited knowledge of how electronics works, especially with the diagrams and photos.

For a person with no electronics knowledge, it might be a little more involved, but easy enough to comprehend if they put their mind to work.

Folks who do not want to learn anything about electronics yet want to use them (we can call them the "Plug-N-Play" crowd) may need a little more help in understanding a few things (or should stay away from electronics projects).

Here's the way I see it...ANYTHING in the way of help someone offers is good enough for me. You, Dale, and Blystovski please continue to post. Not to listen to knowledgeable folks talk about electronics would be like not listening to Warren Buffett talk about making money.

quote:

Originally posted by Bob Delbridge:
TT,

I'm still looking at your site but what you have is pretty easy to understand if a person has limited knowledge of how electronics works, especially with the diagrams and photos.

For a person with no electronics knowledge, it might be a little more involved, but easy enough to comprehend if they put their mind to work.

Folks who do not want to learn anything about electronics yet want to use them (we can call them the "Plug-N-Play" crowd) may need a little more help in understanding a few things (or should stay away from electronics projects).

Here's the way I see it...ANYTHING in the way of help someone offers is good enough for me. You, Dale, and Blystovski please continue to post. Not to listen to knowledgeable folks talk about electronics would be like not listening to Warren Buffett talk about making money.

Like Bob I think what you have there is pretty good but a close up of a completed model without the alligator clips would be better.
It was simple to understand and should work even for the plug and play crowd

I also like the posting of pic's it sort of gets a persons creative juices flowing and maybe will jump start them into trying this themselves.

Since these thing don't generate heat you can make a permanent mount for them just by drilling 2 - 3/32 holes in a piece of styrene and gluing it in place

They also look good as street lights or the old time industrial fixtures.

Here's a pic of that same coal tower in the daytime.The lights are made from a brass disc.To make those I cut a disc then paired it up to a drill bit the same size .
I found a 1/4 thick piece of steel and drilled it just far enough so the cutting edge of the bit wasn't visible now I had a round indentation in the steel.I lay the disc on top of the indentation and using that same drill bit I tap it lightly with a hammer rotating the bit a few times as I go.

Now I have a nice little cone shape.I drill the center and you can see the 3MM warm white LED just under the shade.




David

quote:

In the link that I posted above "Fun With LEDs"

TT,

I went to your site and just read the above. I can honestly say the water is clearing up. What are you writing next?

I haven't rewired my buildings since I moved the layout upstairs. When I do I want to use mini-plugs on them and plug them up so it looks something like an electrical connection box and not stick out like a sore thumb. I have enough mini-plugs from all the computers and peripherals I've torn apart to do all my buildings.

David, that's a fine looking coaling tower by the way

Whoa! That thing must be 3 feet tall

I bought a string of LEDs at Walmart this morning. It's 30 LEDs and runs off 3 "C" batteries. The LEDs are just the right distance apart so when I cut them off they'll fit nicely from floor to floor in my Ameritowne 3-story buildings. I have some wallwarts laying around that I'll use for power. Once I finish ballasting the industrial sidings of my layout I'll start putting back the buildings, with lights

These LEDs appear to be 3mm, maybe smaller. I'm going to drill holes in the rear wall of each building and stick the LEDs in them with a piece of Plastruct covering the wiring.

It may be a while but I'll post photos of what I end up making.

Bob,

I think it measures right at 30 inches.It's modeled after the 400 ton coal towers that could coal 3 engines at once.I'm working now on my own first 4 Ameritowne buildings.3 of the buildings I'm building as designed with window coverings and LED's on all floors but the 4th I'm building as "Mary's Confectionery" with a detailed interior with donuts and candy,tables and chairs and a counter.

Marysville is growing one building at a time.My wife Mary as you can guess can't see why it takes so long to go from kit to finish.Woman are very impatient but each time I finish one with her name on it I get a kiss on the cheek so it's worth it
I'll be looking for Pic's of your finished buildings .It'll be nice to see how they all turn out

David

quote:

wallwarts

Ok, I give....what are wallwarts?