LED technical info

[don9GLC]

I have noticed a lot of comments on LEDs and some surprising responses.

So now I have gone and actually tested an LED.

Im not sure this is the best place to discuss the results, though it may be.

Let me know if you think this is 'behind closed doors' or where you think it might be noticed.

 

[Madman]

Looks like you're in the proper place. Then again, some of us post all over the map here. As for where it will be noticed. I am of the solid belief that we're all hungry for any new posts about things we may not understand. So it will be found.

 

[tramcar trev]

Yes I agree it will be found for those who quest the info....

But testing one LED is really inconclusive as there are so many LED variants out there so I'd like to see testing of a whole raft of leds....

Personally I'm into imperical testing as in "ah yes this led is the right size and colour for the job, what are its requirements?" But Don your into a more scientific approach I suspect. The vast majority of us have limited abilty to comprehend the data on the data sheets that come with leds. I would find it handy if your findings were expressed in well " laymans terms".....

 

[mike]

as far as the mods/site, yes, please post away.. leds, a intrest to meny..

 

[Madman]

Yes I agree it will be found for those who quest the info....

But testing one LED is really inconclusive as there are so many LED variants out there so I'd like to see testing of a whole raft of leds....

Personally I'm into imperical testing as in "ah yes this led is the right size and colour for the job, what are its requirements?" But Don your into a more scientific approach I suspect. The vast majority of us have limited abilty to comprehend the data on the data sheets that come with leds. I would find it handy if your findings were expressed in well " laymans terms".....

How about , LEDs for Dummies.
That would fit me, and maybe some others if they care to admit it, better.

 

[trammayo]

Yes an ideal title for the likes of me too!

 

[Rob s]

Yes I agree it will be found for those who quest the info....
But testing one LED is really inconclusive as there are so many LED variants out there so I'd like to see testing of a whole raft of leds....
Personally I'm into imperical testing as in "ah yes this led is the right size and colour for the job, what are its requirements?" But Don your into a more scientific approach I suspect. The vast majority of us have limited abilty to comprehend the data on the data sheets that come with leds. I would find it handy if your findings were expressed in well " laymans terms".....

How about , LEDs for Dummies.
That would fit me, and maybe some others if they care to admit it, better.

I'm up for this too

 

[don9GLC]

All you will ever need to know..and then some.
http://donklipstein.com/ledx.html

Yes, that's an excellent source of technical info, but covers the very wide range of applications for LEDs. Its not exactly 'for dummies' either. In many ways its easier to write for a technically knowledgable readership. I do not know if I will succeed, but my ambition is to produce a guide for those with little understanding of the physics, so that they can readily find the information they need for the reliable and safe installation of LEDs on their G Scale railways.

I suspect that a careful selection of what to leave out will be as important as what to include. 'Its not rocket science' but it can be made to feel like that.

Many thanks for the link. I will use it to cross check my results. I decided to do my own testing after hearing so many conflicting opinions on LEDs and suitable circuits. As I read last week "It has been reported that publishers are turning Wikipedia into a 5,000 page book. However, I don?t think it?s true, because I read about it on Wikipedia".

 

[don9GLC]

I'm about to post some test results and conclusions.

Its not that difficult to carry out a technical test, but translating the results into helpful language for the layman is a bit more of a challenge. Also on a forum posts have to be relatively brief and its difficult to know what to omit.

Please let me know if there are things that could be explained better.


I will carry out some more tests on a range of typical LEDs used in G Scale but I would be surprised if the general conclusions change. The physics of the electrical circuit are the same. The photometric qualities are another matter.

I'm planning more testing: apart from extending the range of LEDs tested, I think there may be some thermal effects that I have not evaluated. And the behavior of LEDs on DCC also requires separate study. What does DCC voltage mean (peak to peak, root mean square, average) and how is it affected by commands? And how does lighting affect RailCom, or does it not matter in G Scale?


What is beyond me is producing a guide on how to illuminate your railway. For a start, such lighting is largely decorative and more an art form than an engineering exercise. Also the photometric data that is used for calculating commercial and industrial lighting designs is not available. Its 'rule of thumb' and your thumbs may be different to mine!

One aspect of photometric characteristics may be helpful: the ultra bright LEDs tend to achieve their brightness by a very narrow angle of view (20-30 degrees), rather like a spotlight. That may be a good choice for loco headlights, for example. The wider viewing angle LEDs are better for general illumination such as platform lighting and interior lighting. But it all depends on YOUR thumb. Remember Rule 8.

Please also note that the human eye does not respond to light in a linear way. 200 lux will appear about twice as bright as 100 lux, but you need to increase to 400 lux to achieve another apparent doubling in light. And then 800, 1600 etc. That's a fairly huge simplification. But you can easily limit a high output LED by varying the resistor. I'm not going to go into photometric calculations: they make Ohm's Law seem simple

 

[don9GLC]

First quick tests done with a bright white 3mm LED from eBay.
Specification: 3.5V 20mA 17000-20000mCd, peak current 40mA




Conclusions:
There were considerable deviations from the specified values for voltage and light output.

The good news is that you should not have to worry about voltage. The resistor required in series with an LED has an essential function in limiting the current, and hence the light output. I suggest limiting the current to about three quarters of the specified current rating and selecting a series resistor to suit. That is to compensate for any tolerance in the supply voltage or actual LED operating voltage.

Just how much light you require is a matter of personal choice and circumstances. If the LED is too bright then increase the value of the resistor until you are satisfied.



Would a table of suggested resistor values for different supply voltages and LED operating currents be useful? There are several examples of how to calculate values elsewhere, but a table may be a better tool for those how are uncertain of electrical circuits.




Test results:
The operating voltage of the tested LED was just under 3.2V (specification 3.5V)
Although the voltage in the operating range is not constant, the variation can be ignored when calculating the circuit values.
There is no light output until the voltage is approximately 80% of nominal.
The light intensity varies proportionally with the current.


Test results are shown in the two graphs:The horizontal axis is voltage.
the first with actual values,

the second with values as a percentage of maximum.

 

[tramcar trev]

Trust me..I have worked with them for over 25 years

I remember when they were 3/16" dia and only came in red...... 1978..... I bought a sanyo radiogramme and it had one on the dial and they made a big deal about it "With LED"....

How about , LEDs for Dummies.
That would fit me, and maybe some others if they care to admit it, better. Followed by Transistors for imbeciles... Oh I like it....

 

[don9GLC]

Trust me..I have worked with them for over 25 years

I remember when they were 3/16" dia and only came in red...... 1978..... I bought a sanyo radiogramme and it had one on the dial and they made a big deal about it "With LED"....

How about , LEDs for Dummies.
That would fit me, and maybe some others if they care to admit it, better. Followed by Transistors for imbeciles... Oh I like it....

I had a Sinclair "Black Watch' around 1978. One of the first digital (LED) watches. It had a timer! I used it at work for battery discharge testing. Needed a new battery for the watch after a week Got an LCD display as soon as they became available! Right now, just look at my Avatar

 

[Madman]

I know some here are much smarter than I at this LED thing, as can be seen in the graphs and nomenclature that has been posted. But for me, it's take one LED, attach a resistor to it, hook it up to a power source. If it doesn't light, switch the power leads. It's been working for me since I started using LEDs about three years ago. In my first LED projects, I simply took some LEDs, glued them to the underside of some passenger car roofs, tied them into a circuit powered by two AA batteries, no resistors, added a single pole switch, and presto, lighted passenger cars that are still operating on the same AA cells. Can't get much dumber than that. But it works for me.
None of the above is meant to ctitisize anything that you more technical gentlemen have posted.

 

[tramcar trev]

Dan, mate you're not alone. For me understanding the nomenclature is the tricky bit.... I have however learnt heaps on this forum and even now use LM317 circuits with confidence to drive LEDS, far better than resistors as the LEDS can be on and no one need be home so to speak.....
Don I too mean no disrespect but Cobber "plain English" is best for us non initiates....

 

[trammayo]

For me, it has to be simple - I have too much on my hard drive as it is (walk in to a room for a specific purpose, get there, and then wonder why I went there!).

What we really need to know (well I do) is why you need resistors - presume Led's are only made to operate at certain voltages - what resistor you need for different voltages (can any be used for a range of voltages), colour coding of the said resistors, best place to get these things, etc etc.

As Dan says, hook it up and go!

Not all of us are on DCC. I can remember when lighting was measured in candlepower - not that it has any relevance to the uninitiated (like me). I thought the grain of wheat bulbs were fantastic (over the pea bulb) but the only things that make me think (and I am only thinking) about changing to Led's is low power consumption (ideal for battery lighting) and longevity!

Yours sincerely,

Numb Nutz

 

[tramcar trev]

Wellllllllllllllll Numb Nutz have I got a deal for you..... Alas she works out of rooms in Canberra a bit far for you to travel for a very sensitive massage to restore some feeling......

 

[nicebutdim]

Just to make it a little easier with wiring an led, the leg adjacent to the flat bit on the side of the case is the negative. Try not to reverse wire an led, although 'diode' is in the description they will die very quickly with a backwards current.

 

[MRail]

Would a table of suggested resistor values for different supply voltages and LED operating currents be useful? There are several examples of how to calculate values elsewhere, but a table may be a better tool for those how are uncertain of electrical circuits.

Yes, please, Don.
I think this would help most guys.

 

[don9GLC]

Thanks for the feedback guys.


Generally LEDs are used with a fixed power supply much greater than the rated LED voltage and need to have a separate series resistor to prevent the LED from burning out. But this is not always the case in G Scale.

It seems from your comments that it is difficult to determine what to do when using a voltage source close to the LED voltage, for example with a separate battery. It is difficult, because it depends on the battery as well as the LED.

Another problem situation is when the voltage is variable, such as analog track voltage, and the brightness of the LED varies too much with voltage. Despite all the advice, a voltage regulator may be the best solution in this case.

Also the amount of data provided for LEDs varies and it can be confusing to identify those characteristics that are important from those that don't really matter. Incidentally Candlepower is still around, its just called Candela now (abbreviation Cd - or mCd when divided by 1000), but you don't need to know that, and this explanation is a bit of a simplification: all the more reason why you don't need to know.


Please let me know if that covers the main concerns, and I will try to develop simple guide. Are you sure you all don't want to study for a degree in electrical engineering? It would far far easier to explain this technically

 

[CoggesRailway]

I think I am getting this now.... one last dumby question - If you get an LED working at an intensity you like on a stable voltage can you assume it will continue in service like that. Do they either pop or work as opposed to a bulb which may work but only for a short time if the voltage is too high...??