Help me understand LED's?


KB02

KB02

Well-Known Member
With a current project I am working on for the layout, and other future projects that I have in mind, I would like to add some lighting. LED's certainly seem the way to go, but when I start looking for some, I just get confused. Could someone (or a group of someones... like, this group of someones?) help me expand my knowledge so that I at least know what to look for when shopping for LED's?

Here is what I know (and understand): An LED (or Light Emitting Diode) is a solid state relay that emits light when current is passed through it. An LED has an Anode (Positive) and a Cathode (Negative) and is polarity sensitive (won't work if plugged in backwards). AND, LED's are also sensitive to the power running through them and can easily be burnt out if too much power is applied - hence the need, in many cases, for a resistor (which converts a set amount of electrical energy passing through it into heat energy, thus reducing it's output as compared to its input).

I get the basics.

Now, you might notice that I mentioned "Power" in the above statement instead of voltage, wattage or amperage. This is where my understanding of electrical theory fails me. I understand that the bigger the number, the more power. Voltage I get, in and of itself. Wattage I get, in and of itself. But how they relate to each other? I just don't get it. What understanding I do have about electricity as it relates to lighting comes from Automotive and Theatrical lighting (self trained in the former and formally in the latter) where certain principles (voltage) are set and the desired results are clear (Brighter is better... usually). This knowledge does not really translate well to artistic lighting design on the layout, though. I found a great little 12v LED strip in the automotive section of Walmart that I could easily put inside my roundhouse. But the end result, I'm sure, would look like they somehow managed to capture the sun inside the building.

SO, other than physical size, what should I look for when LED shopping? How do you determine an LED's voltage based off the label (I find them confusing compared to the 120v/60w lamp in the fixture beside me)?

I want to install some lighting in the roundhouse I am building (and other future buildings) as well as some exterior lighting. While I appreciate the "ready-made" options that are out there, I would rather know WHY that resistor goes with that lamp - with this knowledge, I can customize the work I do to the situation I am working in.

Thanks for any help you are willing to offer!
 
First a correction, a LED is not a solid state relay, it is a diode, hence the reason it will only work in one direction. On to the other questions.
It takes one volt, to push one amp, through 1Ω of resistance. Power, (wattage) = volts X amps. Look for an Ohm's law wheel on the internet, and this will show all of the relationships. For individule LED's, I just figure 2 volts and 20mA's per LED. The formula to determine the needed resistor is:
(Voltage source - LED voltage)/LED amps, or (15-2)/.02=650 ohms. Or, just to keep it simple, I just use a 1KΩ resistor. With an LED, it is tough to tell the difference in light intensity.
 
Thank you for the correction.
As for your equation, it appears to assume that all LED's are 2 volt, 20mA's. How does one tell the voltage and amperage of an LED?

Or are ALL LED's 2v, 20mA? (Admittedly, that would make things much easier).
 
most LED's that are 5mm and 3mm are 20ma maximum brightness, red / blue/ yellow are 2.0v - 2.2v or so, bright white are 3.0v normally ..
 
The LED specs, (voltage, amperage) should be on the package that the LED's came in. If you bought them off of eBay, then generally in the description, they will give the specs. For the LED's that we are generally going to use in model railroading, if you don't know the specs, then using 2V, 20mA, will give satisfactory results. Personally what I do, is just use a 1KΩ resistor, the difference in light intensity between a 680Ω, and 1KΩ is not enough for my eyes to see. Plus it just makes life easier. An additional plus, I have my track voltage set to 15 volts, if I take my locomotives to another layout that has a higher voltage, the LED's are still protected. Now for a string of LED's, that are wired in series, I would use an IC LED driver, you can get these from Digi-Key, they are made by MicroChip. These are current limiting devices. You would wire these in series with your LED/LED's, and up to 90VDC can be applied to the current limiter, and they will only output .020mAs, or .025mAs, depending on which one you get, ( I would recommend the .020mA ones, CL20). I have tested these devices, at 90VDC, and they do work. With these, as long as the voltage is above the minimum voltage, (I can't remember exactly what that is, 5 or 10 volts) they will light up the LED regardless of the incoming voltage. I have been buying the SMD version of these, and installing them in my locomotives, and this works very well.
 
Edit: The perils of procrastinating with the post button! NCR gives a better answer above.

KB02 said:
Thank you for the correction.
As for your equation, it appears to assume that all LED's are 2 volt, 20mA's. How does one tell the voltage and amperage of an LED?

Or are ALL LED's 2v, 20mA? (Admittedly, that would make things much easier).
Click to expand...


Actually LED's are mostly 3V 20mA but there is a fair amount of variation. It's probably not going to help you at this point seeing as you are asking the question but the best way is to find specifications is on the packaging they come in. When I first got into LED wiring I made the mistake of mixing them up so I had a lot of trouble at first but using 3V/20ma is a good baseline to start. I tend to use 12V power supplies for LED lighting as they are the easiest to get and I use a 620 ohm resistor in series with each LED. I found doubling them up in parallel on a single resistor doesn't work very well.
 
So, these are some that I am looking at on ebay:
s-l500.jpg

eBay auction link

Where on the label should I look? Most of the LED's I have seen have labels similar to this.
 
That looks more like the manufacturer's labeling for inventory. What the previous posts are talking about are like the picture I attached. That is a screenshot of several different LEDs and their specs. Give you an idea of how they can vary.Capture.JPG
 
Well, I spent about $16 on ebay last night and bought 100 each of LED's and resistors. That should last me a while anyway.
 
I have bought a few to experiment with. Some I got were rated at between 12-18V, and came with a resistor (least that's what shape it looks under the shrink) which while the light itself is quite small (2.5mm?) has a slightly larger rectangular base and the resistor being mounted directly behind the LED, a bit big for close mounting to a lights position, depending on the room available. I applied power from a DC 12V power pack to one and it seemed to handle the voltage, but the resistor got too hot for finger holding comfort. At about 6V, the light was still plenty bright and it was only just warm.

Another lot of similar design, 1/2 the size, also produced bright light at slightly lower volts.

The SMD's I'm messing with at the moment, came wired and with a specified 2.8-3.2V and tested very bright with a 3V battery. I am planning on using 2 of them for the #boards on UP PA's. I will try them with 1K Ohm resistors to see what their light put out is then. They would be too bright as they are.
 
KB02,

I'm definitely not an expert when it comes to the techo stuff with LED's or resistors; however, what I can tell you with absolute confidence is quite a few places (such as Allelectonics) provide the appropriate sized resistors with the LED's you buy which takes the guess work out of "which resistor for which LED" :)

One of the things that always confused me was the "brightness" of an LED. Since building the engine shop, I have learnt one important thing regarding that, and that was a thing called "MCD". The higher the MCD rating, the brighter the LED will be. I have used 530 MCD LED's with my engine shop attached to 2K SMD resistors, mainly to tone them down a little, I could have used a lot lower. In my HO structures, I used 3mm (normal) LED's with 480 ohm (half a K) resistor without any problems at all.

The lowest resistor I will now use with ANY LED (SMD or Normal) is a 1K resistor and the lowest MCD rated SMD I will use is the 530 MCD. Bottom line is, with a 1K resistor you "should be fine" using almost any standard LED.

On a side note, an LED will lower the power/voltage/amps (or which ever it is) getting to the LED. If you use a 1K resistor and the LED you choose is still too bright (for example) add a second or third Resistor. :)

PS: If you want good quality, fast service LED's take a look here:

https://www.electron.com

They are based in Czech Republic BUT are quick and their site gives all of the pertinent information for each LED. They are also very cheap as well ;)
 
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I've been reading this thread over and over for a month, and been just letting it roll, but....

tootnkumin said:
The SMD's I'm messing with at the moment, are wired and with a specified 2.8-3.2V and tested very bright with a 3V battery. … I will try them with 1K Ohm resistors to see what their light put out is then. They would be too bright as they are.
Click to expand...
The way this is written sounds like you were connecting them directly to a 3V battery. That is never a good idea. In a raw LED (one that is, from the factory, not connected to or encased with a built in resistor) its forward voltage rating is a threshold. That is, it is the amount of voltage required to get it to pass current. As soon as that threshold voltage is reached, the diode can be considered to be a short circuit and it will pass as much current as is possible. It is that massive amount of current that will burn out the diode. It is the resistor's job to limit the current to that which the LED can handle. Hooking an LED directly to a low current source will work (at least for a while), but it is not safe.

This is why, while technically incorrect, I sort of like the OPs original analogy of a relay. Once the voltage triggers it, the circuit is closed just like the points of a relay. Most electrical things like light bulbs, heating elements, transformers, and motors, will take only the current that the available voltage will push through them ignoring the rest. This is why one can hook an automobile battery with hundreds of amps available to a single 1A bulb and it won't burn out. It will take it's 1 amp and ignore all the rest of the current. It isn't until the voltage is increased that will burn it out. An LED is not a creature of voltage. It hits its threshold, sees all that current sitting there in the battery, and says "come on through". Doesn't matter if it is only 3 volts behind it. Just like a dam breaking, it then crumbles into the flow.

Sure, because of Watt's Law, more voltage will pass more current which in turn will destroy an LED but it isn't the major player.
 
Iron Horseman said:
I've been reading this thread over and over for a month, and been just letting it roll, but....

The way this is written sounds like you were connecting them directly to a 3V battery. That is never a good idea. In a raw LED (one that is, from the factory, not connected to or encased with a built in resistor) its forward voltage rating is a threshold. That is, it is the amount of voltage required to get it to pass current. As soon as that threshold voltage is reached, the diode can be considered to be a short circuit and it will pass as much current as is possible. It is that massive amount of current that will burn out the diode. It is the resistor's job to limit the current to that which the LED can handle. Hooking an LED directly to a low current source will work (at least for a while), but it is not safe.

This is why, while technically incorrect, I sort of like the OPs original analogy of a relay. Once the voltage triggers it, the circuit is closed just like the points of a relay. Most electrical things like light bulbs, heating elements, transformers, and motors, will take only the current that the available voltage will push through them ignoring the rest. This is why one can hook an automobile battery with hundreds of amps available to a single 1A bulb and it won't burn out. It will take it's 1 amp and ignore all the rest of the current. It isn't until the voltage is increased that will burn it out. An LED is not a creature of voltage. It hits its threshold, sees all that current sitting there in the battery, and says "come on through". Doesn't matter if it is only 3 volts behind it. Just like a dam breaking, it then crumbles into the flow.

Sure, because of Watt's Law, more voltage will pass more current which in turn will destroy an LED but it isn't the major player.
Click to expand...
Thanks for that info I.H., certainly not something I knew.
 
Iron Horseman said:
I've been reading this thread over and over for a month, and been just letting it roll, but....

The way this is written sounds like you were connecting them directly to a 3V battery. That is never a good idea. In a raw LED (one that is, from the factory, not connected to or encased with a built in resistor) its forward voltage rating is a threshold. That is, it is the amount of voltage required to get it to pass current. As soon as that threshold voltage is reached, the diode can be considered to be a short circuit and it will pass as much current as is possible. It is that massive amount of current that will burn out the diode. It is the resistor's job to limit the current to that which the LED can handle. Hooking an LED directly to a low current source will work (at least for a while), but it is not safe.

This is why, while technically incorrect, I sort of like the OPs original analogy of a relay. Once the voltage triggers it, the circuit is closed just like the points of a relay. Most electrical things like light bulbs, heating elements, transformers, and motors, will take only the current that the available voltage will push through them ignoring the rest. This is why one can hook an automobile battery with hundreds of amps available to a single 1A bulb and it won't burn out. It will take it's 1 amp and ignore all the rest of the current. It isn't until the voltage is increased that will burn it out. An LED is not a creature of voltage. It hits its threshold, sees all that current sitting there in the battery, and says "come on through". Doesn't matter if it is only 3 volts behind it. Just like a dam breaking, it then crumbles into the flow.

Sure, because of Watt's Law, more voltage will pass more current which in turn will destroy an LED but it isn't the major player.
Click to expand...
Why did you let it go for a month? You should get a time out or one of those goofy yellow cards at least.

Holy maceral Andy... Cheese and crackers!
:)
 
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Iron Horseman said:
I've been reading this thread over and over for a month, and been just letting it roll, but....

The way this is written sounds like you were connecting them directly to a 3V battery. That is never a good idea. In a raw LED (one that is, from the factory, not connected to or encased with a built in resistor) its forward voltage rating is a threshold. That is, it is the amount of voltage required to get it to pass current. As soon as that threshold voltage is reached, the diode can be considered to be a short circuit and it will pass as much current as is possible. It is that massive amount of current that will burn out the diode. It is the resistor's job to limit the current to that which the LED can handle. Hooking an LED directly to a low current source will work (at least for a while), but it is not safe.

This is why, while technically incorrect, I sort of like the OPs original analogy of a relay. Once the voltage triggers it, the circuit is closed just like the points of a relay. Most electrical things like light bulbs, heating elements, transformers, and motors, will take only the current that the available voltage will push through them ignoring the rest. This is why one can hook an automobile battery with hundreds of amps available to a single 1A bulb and it won't burn out. It will take it's 1 amp and ignore all the rest of the current. It isn't until the voltage is increased that will burn it out. An LED is not a creature of voltage. It hits its threshold, sees all that current sitting there in the battery, and says "come on through". Doesn't matter if it is only 3 volts behind it. Just like a dam breaking, it then crumbles into the flow.

Sure, because of Watt's Law, more voltage will pass more current which in turn will destroy an LED but it isn't the major player.
Click to expand...

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What you have stated, would be correct in an ideal diode, but there is no such thing as a perfect diode, there is still an internal resistance within the diode, know as forward resistance. As long as the rated voltage of the LED is not exceeded, the current flow will not exceed the current rating of the LED. I just put a LED on my test bench, at 2.9 VDC, there was ~0.003mA current draw, at 3.5 VDC, the current draw was ~0.025mA.
 
Bruette said:
Why did you let it go for a month? You should get a time out or one of those goofy yellow cards at least.
Click to expand...
I'll take a yellow card for that. There is always time, and I was out on vacation last week, but one always has to consider if providing more information will help the conversation or just confuse it. Analogies always fall apart at some point of detail so to use or not use?!? On the forum on the other side of the tracks there was a fellow who was always technically perfect. But sometimes a person's question would be answered until he interjected a more technically correct description or solution. It just confused the OP and it could take another two weeks of posts to get it straightened out into a usable set of info again. So I always consider it a lot before I jump in, especially if there is a working "solution" on the table which in this case there was.

It does bother me though when voltage is talked about for LEDs as if they are light bulbs.
 
Iron Horseman said:
I'll take a yellow card for that. There is always time, and I was out on vacation last week, but one always has to consider if providing more information will help the conversation or just confuse it. Analogies always fall apart at some point of detail so to use or not use?!? On the forum on the other side of the tracks there was a fellow who was always technically perfect. But sometimes a person's question would be answered until he interjected a more technically correct description or solution. It just confused the OP and it could take another two weeks of posts to get it straightened out into a usable set of info again. So I always consider it a lot before I jump in, especially if there is a working "solution" on the table which in this case there was.

It does bother me though when voltage is talked about for LEDs as if they are light bulbs.
Click to expand...

I think the reason that people talk about LED's and voltage is, that is the way most people control them, use a resistor, to drop the voltage to the LED. Myself, I prefer to use the current limiters from Microchip.
 
Iron Horseman said:
It does bother me though when voltage is talked about for LEDs as if they are light bulbs.
Click to expand...
I guess that would be because that's what most of us train modellers use them for, so the expectations for them is to work in that manner is understandable. But information that corrects or adds to the understanding how something works, should be shared to aid in that. Especially if the info might avoid the possibility of a damaging mistake. It's a bit like the motor manuals produced for car repairs these days that go "when reassembling, reverse the instructions given to dismantle". Anyone who works with cars, or model trains knows that reassembly has more pitfalls than the pulling apart.
 
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