# LED Refresher jan 2010



## T-Man

I always go crazy with LED applications. I wish to update some facts on their use. 
I do use a 3v flat battery to test the LED. POS being the top of the battery.
If using a diode I have always gone with 1N4001. For one diode you can go with a smaller one, 1N914. RS 276-1620. They are rated at 75ma so they can handle two LED's on DC and FOur on AC if they are back to back. They are a better deal than the conservative 1n4001. In the first picture I have a 3mm yellow common from Radio Shack with the small diode and a 1/4 watt 475 ohm resistor.
Two changes are here. First the diode is across the LED leeds not inline off the negative end. The resistance
is 470 not 750. 750 ohms is used for 18v DC. This is 18 v AC, but because of the RMS voltage it is only 70 per cent of the DC . It just means I can reduce the resistance to 70 per cent and still be safe. I hav ealways been conservative so now I know I can use 1/4 watt resistors at a lower value. Also the smaller diode.

The second picture shows the 1N4001 diode with 330 ohms at 1/2 watt. Here I reduce resistance further because my track voltage is closer to 12 or 14 volts than 18v. This is a blue 5mm with 8000mcd brightness.
The last is white at 13000 mcd
I have to check my numbers for errors.

It does not matter what side the resistor goes. The left pictures have the pos side on the right and the right picture has the pos side on the left. Hope this doesn't confuse you.

Since this is AC it does not matter how the wires are attached. They work both ways.









The last picture is a green 3 mm LED and this one will always be on and can connected to AC or DC the 47O ohm resistor will hold the 12 volts DC. The diode box is a rectifier. You will get a postive charge on one side and neg on the other. Regardless the wires are fed into the other sides( the picture shows top and bottom feeds. This is a demo that shows the connection and will leave it exposed on the track to view. You can add wires and soldered it all to a small board. Notice the diode bands are all to the right. That directs the pos current to the right. And the neg is drawn through the left. This is how a bridge rectifier works.



















The picture above to the left is my old method . A 3mmm red LEDwith a 470 ohm resistor on the pos feed. The diode is in series with the neg side to prevent back voltage to the LED on AC current. The light behind it is rigged the same way and has been operating for a few years.


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## T-Man

*Bright Light*

With the LED, brightness is measured in mcd (milli candellas) ((take that!!! lamellas !!!!!!))

The Radio Shack cheepy package go around 2000 to 4000. I bought some blyes at 6000mcd. The white 5mm shown is 13000 mcd. Some are around 25000 mcd You stare into these lights. I am afraid these lower powered LEDs will become hard to find. So if you are happy with one I suggest you by the Anton way and get a fistfull at a time. Warm whites are coming onto the market they resemble the normal incadescent light. Ebay has them all.

I hope you enjoyed this revue. I have a few more displays now for future reference.


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## T-Man

*Bridge rectifier*










Four diodes forming a box pointing in the same direction create a bridge rectifier. For my limited use this definition works just fine. 

This is useful in using AC current for DC applications. The AC is fed to the center and each end has a pos and neg side.
The diodes have little bars on them. If they are on the right the current will flow left to right and the diodes prevents a right to left flow. With four you have a filter to generate DC. 

SO they can be used to get AC to power DC LEDs.









Flashing LED.
Then I jumped into flashing with a timer chip. 
Things get hairy . The 555 timer chip is sensitive and requires constant voltage between 5 and 12 v. SO since a transformer changes the engine speed I need to place a power supply on the train to get my 5v DC. 









Power supply.
The power supply has a voltage regulator and of course a bridge rectifier and I have capacitors to help smooth and filter for use.










It gets interesting very fast. LED, battery,DC,AC,then Flash, add a Power supply.


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## T-Man

*AC Back to Back*

TO continue, I made another little demo. This is two 3mm LEDs wired together at opposite poles with 1/4 watt 470 ohm resistor. This eliminates the need for a diode or bridge rectifier with AC current. The LEDs protect each other from the AC back current. So what happens is that both of them light up. If you had this with DC only one would light up and then the other would when you reversed the current


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## Big Ed

T-Man......I wish I knew what you were talking about?:laugh:


I would like to tackle some of those, but I would think you need years of working with all the whatyouacallit's,to know what they are and do.
And how you go about buying the right type of led lights, diodes,resistors, etc.
Or a good reference book?

Do you have some background working with these parts? 

What I really want to know is what goes in what?:thumbsup: 
Are you using them or just experimenting with them?

Thanks for taking the time to post.:thumbsup:
Another one for your how to do book $$$$$.:thumbsup:


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## T-Man

Ed, people can buy them ready to run, but I wanted to know more so I studied them. Between articles, ebay and Radio Shack and on line sites I learned. 
Knowledge wise I wanted to use LEDs.
I learned how to use them as lights, using DC, then AC, in engines was the greatest challenge. After that I learned how to make them flash.
Now i look for simple diagrams, buy the parts and try to make them work. The fire flicker and welding light are two examples. I also have a electronic diesel horn that sounds kinda flat. 

Parts lingo was a learning curve too because most of the time I had no idea what I was buying.

THis thread shows the simple ways to light them up. I show diodes, LEDS and resistors. AN LED is a diode also.

I also constructed the reverse circuit. TO support that I made a light tower for direction. I experimented with the wall warts to power it. WHen I started using integrated circuits I had to make power supplies. SOme small enough to mount in a train. 

That's my resume.

You will not find a reference book because I had to look at electronic sites because rr sites had ZIp. It was time consuming. It actually turned into a second hobby, Now I buy LED gadgets at a dollar store and scheme what to do with them.

My waste car is a very simple LED set up based on the last post, because I used two LEDs to light it up.

I can down shift and go simpler and start with definitions and properties. I only use one formula. VOlts = Ohms times amps.


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## T-Man

*A reference book*

I just remembered there is a book or rather a kit. Radio Shack sells a snap together electronic projects kit. This thread is a lot cheeper. Actually I have two sets from my yard sale train searches. If you insist I will venture into Electronics BSM (Baby step method) no question too small but you have to type it..

So far my three sources for LEDs are Radio Shack (a bag full), after Xmas sales on single LED bulbs, and ebay (25 to 50). 

Again I do understand the frustration. I have posting for 2 years on the subject. Some aspects have been clarified. I have blown a whole bag of LEDs from testing. I have always tried projects and posted with pictures.

The nice thing is that the subject is universal to scale. Lets compare HO to O

O scale uses AC 6 to 18 volts. Track voltage is around 8 to 12. AC current alternates back and forth yet it does have a pos and neg direction. Reverse is done on board by mechanically switching the leads to the motor.

HO uses DC at a lower voltage of 10 to 16 for modern better transformers. When the current reverses, the motor reverses. 

On board lighting is more complicated than from a single voltage output transformer. The motor accelerates and decelerates causing voltage spikes. As a minimum I use the bridge rectifier to baffle this with success. In Ho I am not satisfied and engine lighting still causes an LED to burn up. This of course is all non DCC and traditional operation. My mock ups are powered from the track and some have worked for years. To date I have two engines with LED lighting in O scale. TO guarantee success it is possible to mount a voltage regulator on board as part of a small power supply. This way, you can even use it to power integrated circuits or chips. These chips can make the LED blink.


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## T-Man

The Led is a diode that gives of light. The diode only works one way with a directional currrent. The common LED Light Emitting Diode operates at 2 volts , and draws 20 milliamps. New ones with Power operate up to 3.5 volts and 2.5 miliamps. That is why I like to buy in bulk the manufacturer will give me these values.

To explain my circuit I have 3 volts DC form two batteries, next a 100 ohm resistor, Two LEDs and a switch.

Ok first picture lets apply the formula Volts = amps times resistance. THE bateries place out 3 volts ,my Led uses 2, That leaves One left. If this number was larger it would mean My LEDs would be toast so I need to add Resistance to make and save the LEDs.So one / .02 ma = 50 ohms. In the demo I use 100. Often you can go higher to offer more protection Too much and you loose brightness. Here, the values are so close I can use it without the resistor but it is bright and may not last long.

The first shows two LEDs in parallel. Both are lined up the same, and both are on, but, if I reverse one. It stays off because the current is in the wrong direction. If this was AC both would be on. DIodes have a crash value of back voltage but two in opposing directions protect each other.

The green may operate at a slightly higher voltage so the red is brighter.


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## bradimous1

Is this a description of a sentence in your last paragraph of your prior post...

_"The motor accelerates and decelerates causing voltage spikes. As a minimum I use the bridge rectifier to baffle this with success. In Ho I am not satisfied and engine lighting still causes an LED to burn up"_

I am not sure how you get this into the loco so that the light doesn't dim... or am I reading this wrong.


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## T-Man

They don't dim they burn out. PHZZZT! I use the grain of wheat bulbs for HO lighting in an engine. I f you have had success post it on a thread. With anything electrical I try it out and post how to do it. LED on an engine in HO is not one of them. I have posted a blinker set on an engine but not a replacement for a head light.


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## T-Man

This is a comparison for the 1.8 mm to the 3 mm and the 5 mm LEDs.


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## tjcruiser

T-Man,

The pics and descriptions in the initial posts of this thread are enormously clear and helpful to anyone jumping into the LED, rectifier regime. Seeing the actual diodes and resistors wired in such simple operative circuits is somehow much clearer to understant (for me, at least) than just a wiring diagram.

Well done, as always,

TJ


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## T-Man

*Update 2011*

I wanted a clearer picture of the bridge rectifier working the LED.This set up will work on AC or DC layouts. Except engines. The secret to get it to work and not blow up on an engine is the insertion of a 1 uf capacitor. The iprotects the LED from voltage spikes when the motor changes speed.

Evidently the modern engines come with this capacitor. Mine didn't so check out the second picture.






















Also a member, Artieiii, found a source of pre wired for the solder challenged enthusiasts. Here.

The discussion starts here

Also note these are not constant lighting but will dim with lower voltages.


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## dlbraly

Cool info, thanks


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## Bloomfield Station

*LED Reverse lighting*



T-Man said:


> TO continue, I made another little demo. This is two 3mm LEDs wired together at opposite poles with 1/4 watt 470 ohm resistor. This eliminates the need for a diode or bridge rectifier with AC current. The LEDs protect each other from the AC back current. So what happens is that both of them light up. If you had this with DC only one would light up and then the other would when you reversed the current


WOW! Never thought of this wiring diagram. I plan to wire engines simply in a square with polarity reversed. It automatically lights in direction traveling.

how ingenius


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## cid

> The resistance
> is 470 not 750. 750 ohms is used for 18v DC. This is 18 v AC, but because of the RMS voltage it is only 70 per cent of the DC.


I guess I would question why your 18VAC is not an RMS value? Typically AC volts are not called out as Peak Voltage. Just asking...


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## gunrunnerjohn

Remember that an LED is a diode, so it's only lighting on one half of the AC waveform. OTOH, there is another factor, that being the reverse voltage specification of the LED, typically around 5V peak.


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## T-Man

Thanks John electrical theory is not my strongpoint


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## cid

I do like the way that the parallel LEDs clamp each other's reverse voltage. I think that really makes this arrangement successful.:thumbsup:

At the voltage peak of 25.5 v +/- , the peak current is pushing 50mA using the 470 ohm resistor. But it's very short duration. I don't know enough about the physics of the LED to say that's excessive. It's just a lot more than the steady 20mA or so you would normally expect on DC. But, check my numbers.

I guess the longevity of the LED is the true test. If it works, excellent!


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## gunrunnerjohn

T-Man said:


> Thanks John electrical theory is not my strongpoint


You hide it well. 



cid said:


> At the voltage peak of 25.5 v +/- , the peak current is pushing 50mA using the 470 ohm resistor. But it's very short duration. I don't know enough about the physics of the LED to say that's excessive. It's just a lot more than the steady 20mA or so you would normally expect on DC. But, check my numbers.


The important thing with LED's is the average current as long as you don't exceed their peak current specification. Most multi-segment LED displays, and many discrete LED's in multiple unit applications, are driven with PWM signals that supply peak currents that can easily be 100ma or more, but the duty cycle of the waveform limits the average current to below the LED's continuous current rating, typically 20ma for common LED types.


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