The following link may be a help to those members that find LED’s difficut to use.
http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng
Charlie,
Thanks for the link on the LED info. Question for you though. What advantage/disadvantage does running an LED in series have over running one in parallel? One I can see right off the bat is: If you have some LEDs in series and the first LED blows, the other LEDs in front of it stop working because of the break in the current. Yes?
Anyway, thanks for your help. [:)]
Tom
The primary reason to have two LED’s in series is when you wi***o have two lights. An example would be to have a light in two different directions or two locations that would both be on or off at the same time.
And yes in a series circut all components are affected if one ‘blows’. But the life span of LED’s, if not abused by over voltage or current, is very long. Many times that of a light bulb.
I hope this explanation is clear.
Charlie,
Yes, I realize the advantages of LEDs over incondescent - e.g. much longer bulb life. I also understand the LED in series issue. So - where, when, and why would LEDs in parallel be used? Thanks!
Tom
Generally LED’s are used in series because for a given current, each LED will have a slightly different voltage across it. The voltage is determined by the device, it varies with the current. If one LED in the series string fails, then they will all go out. The series string arrangement means that each LED in the string has the same current passing through it and thus the light output is kore likely to be the same.
In parallel, with a single resistor setting the current, if one LED fails, then the current through the others will increase. The resistor was chosen to provide a level of current through the parallel group, and that current will now be shared through fewer LEDs. This will result in increased brightness, but will also likely reduce the lifetime of the rmeaining LEDs.
Because the voltage drop across each LED will be slightly different, in a parallel block each LED will likely have a slightly different light output, thus they can look un-even.
In normal usage LEDs would ideally be run in series strings. So to answer the question you raised, LEDs would never normally be used in parallel, if insufficient voltage is avialble for a eries striong, then use a separate resistor for each LED rather than actually putting the LEDs in parallel.
Some other advice:
- LEDs are cheap, so always buy a big batch of them. For high output white 5mm LED’s I usually pay about $31/100 including shipping from Hong Kong on EBAY.
- Always try to use LEDs from the same batch in each string. This way the brightness will be more consistent (and color too!).
- A really good source of LEDs, a bit more expensive than EBAY though,
Try to think of using LED’s much like you would use light bulbs. This means series, or parallel circuits. It is all the same except LED’s require less current/voltage. If you placed a 3 volt bulb in a 12 volt circuit you would experience the same results as if you used an LED in the same circuit… a bright flash!
So using/switching to LED’s is no real mystery. Just abide by omhs law and provide the proper resistors just as you would using bulbs that have a voltage rating less than the supplied voltage.
I guess the reason most people have a problem converting to LED’s is you don’t have as much selection in voltage ratings as you do in bulbs. This is the reason you must use a dropping resistor with almost every LED application.
If you swapout the common 12 v transformers for lower voltage transformers the resistor problem goes away.
John & Charlie,
Thanks for the edumacation on LEDs. [:)] I like learning about this stuff and I have installed two already in a couple of locomotives. One was easy because the headlight board already had a resistor in it. The other one I had to solder a resistor in line. (Pretty straight forward.) I’m also very familiar with Digi-Key. At work, we order stuff from them quite frequently. Their catalog must easily be 3" thick.
Anyhow, thanks again, fellas! [tup]
Tom
I previously posted this link but I will show it here again. Great suff for LED user’s that lends itself to model railroading.
I have used their dwarf lights and used pieces of soda straws to add sun shades.
I have no connection with this firm except as a customer.
A little elaboration is needed here. LEDs do not actually “share” current very well. In fact, in the above situation if you have LEDs in parallel that are sufficiently disparate in draw, one may light but the rest won’t. The “larger” one draws off all the current. Mixing colors of LEDs, even from the same manufacturer, most often results in the above. Reds seem to be major hogs. [:)]
Mike Tennent
That would only be true if using one resistor for both LED’s. You would have to select the proper resistor for each LED based on their current/voltage specifications. If you did not use a resistor at all and each parallel LED was rated at the same voltage as the supply voltage they would each light properly. This assumes that the supply current was adaquate for the sum of all the parallel LED’s.
Here’s an LED that I use as panel lights:
This one has the correct resistor already installed, and is available in orange or green as well as red.
LED’s are current controlled devices. Their brightness is proprotional to current. More current, more bright.
They also have a voltage drop as noted above. All LED’s have different voltage drops. The same type LED will have slight differences. The internal resistance per LED is low. The lowest voltage drop will lower the voltage drop accrossed the other to a point of dim light if any. Usually red is the lowest at around 1.7.
Parellel works well if you use a resistor per LED. One LED is usually all a quarter watt resistor can take. A higher wattage resistor could be used, but it usually counter productive in terms of differences in lighting. You can get away with one resistor if you place the LED’s in anti parellel. If the current flows one directon, one LED lights. Switch the current and the other LED lights.
Series you can get away with one resistor for a string of LEDs. The current is the same throughout. You do have to add the voltage drops for each LED. The source voltage must exceed the voltage drop or your current will be el goose egg. The resistor will simply limit the current to the desired value. The norm is usually 15-20 mA.
I’d rather have a slightly brighter structure than one that doesn’t light at all. You can adjust the resistor if one of the LEDs fails, but I’d rather not rip out the entire structure to fix a simple lighting problem. I don’t buy the argument.
-Tom
Most people think of LED’s in a similar fashion to light bulbs but they dont really work the same way. For one thing, if used correctly and LED will last so long that for the purposes of model railroad you might as well assume its lifetime is infinite. If you wire it up incorrectly however you WILL destroy it. You can get away with considering really only two things:
- The forward voltage drop of the LED which varies due to a number of factors (which we can do nothing about and therefore dont concern ourselves with).
- The current draw at which maximum brightness occurs along with the maximum current draw.
Both of the above can be found on datasheets or often from the catalog from your preferred electronics supplier. Like a few people have said above, LED’s are CURRENT controlled devices. That means that it is the current that controls how bright the LED is and also it is the current that usually kills an LED (ie. too much current). Its evident to anyone who has ever experimented with LED’s that varying the current varies the brightness. I find usually that an LED at full brightness on a layout is actually too bright (depending what you’re using it for) and might light up everything around it. In this case limiting the current will reduce the brightness.
Generally when using LED’s in parallel, one resistor should be used in series with each of the LED’s. This ensures that each LED gets the correct amount of current since they all might require different amounts. Usually different colour LED’s require different currents to shine with a similar brightness. Also in this case it acts as protection mechanism to ensure that if one part of the circuit fails the remaining LED’s wont be overloaded and destroyed by extra current.
Series is the preferred way of connecting LED’s however, especially if u are not concerned about minute differences in brightness between them (differences you probably wouldnt notice). When you do this you only need one resistor to limit t