Hello! I would like a little info on LED’s and resistors. I am planning to use LED’s on my N-scale layout as turnout indicators. Although size is a factor, prototypical size is definitley secondary to function. A few questions.
(1)Does have anyone have a good source for LED’s?
(2)Are resistors required?
(3) If so what size?
(4) Any wiring tricks?
Thanks for the help!!
if you are looking for bulk LED’s I would try something like www.allelectronics.com. They have a good supply of LEDs and Resistors. Resistors are required if you are using anything more than 1.5 volts as a powersupply. I would say use about a 1k (1000) ohm resistor (Brown Black Red for resistor code if you have them already). I usually get 1/4 watt resistors. hope this helps.
There are also several stores on Ebay selling all sizes and colors of LEDs are very reasonable prices.
What sort of switch motors are you using? If you are using Tortoises, you don’t even need the resistors - the LEDs go in series witht he motor and the motor provides the required current limiting. If you are using some other type and controlling the indicators wih contacts ont he switch motor - then the resistor depends on the supply voltage you are using - if you power the indicator LEDs with a 5 volt power supply you need a lot smaller resistor than if you run them with a 12 volt supply.
–Randy
Great web-site - thanks for the info. Seems to me the bi-color LED’s are a good way to go…using green for open-red for closed. Maybe not prototypical but will look good and is real easy for me to wire and understand - my 7 year old would probably like to do it like real railroaders. Any trick wiring the bi-color? Thanks again.
Yes, not to much heat they will fry. If you are using a Tortoise how are you controling them? DPDT is the best way short of the Hare. I am using the same trick for mine. Randy R. posted it and i tried it and it worked just perfect. I used www.moreleds.com for mine. No teisitors required.
As was pointed out, the size of the resistor (in ohms) depends on the voltage you’re using. Here’s a really simple formula to determine the resistance needed. Subtract 2 from the voltage you’ll be using. Divide that answer by 0.02. That’s the value of the resistor you’ll need. It won’t come out exactly to any standard resistor value, so use the next biggest value resistor. Example: using 12 volts, 12 - 2 = 10. 10 divided by 0.02 = 500. The next largest standard resistor value is 560 ohms, so that’s what you’d use. 1/4 watt resistors are fine, or you can use 1/2 watt resistors, they’re just larger. Each LED needs its own resistor; you can’t run a bunch of LEDs off just one resistor. You can use a larger value resistor if you can’t find the right one; the LED just may be a little dimmer. Just don’t use a smaller value resistor.
LEDs like DC (direct current), they don’t like AC (alternating current). They are directional, that is the polarity of the DC needs to be right for them to light up. The longer lead connects to the positive voltage, the lead nearest the little flat spot on the edge connects to the negative voltage. You won’t hurt an LED by connecting it backwards, it just won’t light up. If that happens, just turn it around.
As for bi-colour LEDs, they come in two flavours: those with two leads and those with three leads. The ones with two leads need to have the polarity changed to show the other colour. This is the type you’d put in series with one of the wires to a Tortoise switch. And as was pointed out, using an LED with a Tortoise switch you don’t need a resistor. The resistance of the Tortoise is enough. If you’re using them by themselves to indicate a switch position, it gets a bit complicated with regards to the power supply. You’d be better off using the three lead kind here. One lead is common and you apply voltage to one of the oth
Bob, found another way to destroy an LED, Resistance Soldering [:)] Pop a few when the stting was to high.
Like your SIMPLE math, better than trying to explain Ohms Law.
(1) Try MPJA:
http://www.mpja.com/listitems.asp?dept=116&main=122&type=1
100 LED (3mm or 5mm) packs (Red/Yellow/Green) for less than 2 cents a piece.
(2) Yes or no, as already mentioned above.
(3) Mentioned above.
(4) Depends on the installation: Control panel mount? Signal mounted?
If you’re trying to install these in an N scale signal, you can try to turn them down with a file and a drill press…just don’t try to take off too much.
The math is simple: If = (Vs-Vd)/R If = forward current of the LED Vs = source voltage (the more the merrier!) Vd = voltage drop across the LED, Typ: 3V R is the series resistance needed to limit the current to the If value. So, if you have a 3V drop across the diode, and a 16V supply, and need 30mA of current, 0.03 = 13/R So, 0.03R equals 13, or 13/0.03 = R, or 433 ohms. So you would need the next higher value. Which probably is 470R. Which will drop .03*470, or 14V. The current will decrease a little to maintain the Vd of 3V. The parameters of the diode should be listed in the catalog, so you’ll know where to start.
And if that’s on continuously it’s probably too much current. Outside of high brightness LEDs, the max current is often around 30ma. You don’t want to run things at the max, they’ll last longer if you don’t.
Red, green, and yellow LEDs more typically have a forward voltage of 2.1 volts, white varieties are 3-3.5V. With a 12-16V suppply, you’re generally safe with a 1K resistor. Heck just assembled a LocoIO board and Hans uses a 1K resistor for a color LED off a 5V supply - and the power on indicator and the activity indicator both are plenty bright with a 1K. That’s driving the LED with less than 3ma!
–Randy
Geez, fellas, even I’M starting to understand this! Interesting topic and great replies. Thanks to all.
Mike