I just got a bulk bag of 3mm white LED’s with resistors from the LED guy on ePay. But no instructions. [:(]
First, I can’t remember which leg of the LED is positive, and which is negative.
Second…for DC directional lighting, do I need a diode, or will the LED take care of the reverse polarity? Or will firing reverse polarity to the LED blow it up?
I’m guessing that on DCC, I just need the resistor. The decoder will take care of the polarity. Yes/no?
Rotor
You got me by the short hairs. That’s why I just use incandescant bulbs, and no resistor.
The LEDs will only work in one direction. So if you wire them right they will automatically do directional lighting. The long leg is positive and giving them the wrong polarity wont blow them, they just wont light. I always use a 470 ohm resitor on each one for both DC and DCC.
The long lead on the LED is positive, and the short lead is the negative.
With DCC, the polarity on the LED will not change when you reverse locomotive direction, but it will on DC unless you are putting in into a locomotive that has a constant lighting board.
An LED is a diode, but the reverse voltage they can handle is usually low, so I would not try to rely on the LED’s diode function to serve as the directional lighting on DC.
When wiring for DCC, the resistor is there to limit the inrush current and protect the DCC decoder more so than the LED.
Something that can be used for a DC controlled locomotive to insure that the LED get’s correct polarity is a diode bridge rectifier. These are available in 1 Amp ratings that are as small as the LED itself. You’ll still need a resistor to protect against the current surge.
Just remember that on decoder, the blue wire is the plus, common for all functions.
There is no need to wrry about polarity with a decoder, it’s always blue +, function lead - no matter which way the loco runs.
I find a 470 ohm resistor is a bit light for DCC with 12v or better to the track. I use a 1K resistor for all my LED lights and they are PLENTY bright. With most white LEDs, that amounts to about 8.5ma at 12v, if your track voltage is higher, around 14.5v, then it’s about 11ma. PLENTY. 470 ohms at 12v will be about 18ma, bit on the high side for my tastes.
–Randy
You can’t trust the length of the leads to determine the polarity. The LED will have a flat side and a round side near the leads. That will determine the polarity. I believe the flat side is the cathode (negative), but I could be wrong.
-Tom
Lead length IS one of the standards for identifying the polarity. Most brand new LEDs have both, the flat side AND a shorter lead. That is, until you cut them off… [:D] Surplus stuff, you takes your chances. The flat should always be there. Flat - short - Cathode - Negative.
–Randy
Let’s say we have something like a P2K that already has directional constant lighting. Can you just change the bulb to LED, or do you still need to use a resistor?
Rotor
Definitely need a resistor. And pretty much remove the the included circuit as it won’t work very well with an LED. Problem is, P2K is inconsistent on how they make their circuits. The problem is the ‘constant’ lighting isn’t going to have enough voltage to run the LED - and without a resistor, the LED will blow even if it gets just barely enough voltage to light (has to exceed the forward voltge). So if the LED has a forward voltage of 2.8 volts, and you feed it 3 volts with no resistor, it will pop as surely as if you connected it to 12 volts. And a proper resistor for 3v feeding a 2.8v LED is about 10-20 ohms. Good luck. They DO make them that low. The bigger problem is, does the P2K circuit actually provide 3v? On some it’s the voltage across a string of diodes, usually 4 - that’s 2.4 volts. But of allt he ones I’ve taken apart and put DCC in, each different model seems to have a different circuit.
For straight DC, no constant lighting, where you won’t be running the full 12v most of the time, the 470 ohm resistor might be more appropriate.
–Randy
Randy, I respect your combination of electronic know how and cost consious approach to the hobby. You have often pointed out good cost savings approaches to your solutions (like cutting the tracks on the NMRA dummy plug and using them for wired decoder plugs). I suspect you also have some suggestions on good value sources for your electronic parts. In particular, where do you like to buy your LED’s from? Do you use white or the golden white LED’s?
I’ll agree with Randy about the 1K resistor for those 3mm LEDs. I tried installing both a 3 mm in the front and a pair of red mini-leds (about 1mm) in the back with just a 470-ohm resistor, and I ended up frying the LED and losing that function output on the decoder. (Cheap decoder, and it was probably worth the lesson-learned cost. It’ll end up on a lightless loco, or maybe a turntable.)
There is no loss from using a 1K resistor instead of a 470-ohm. LEDs are pretty much constant brightness, anyway, so if they turn on with a 1K, you’re in business.
LEDs have 2 other advantages for headlights. First, they run much cooler than incandescents. This is important in the confined spaces of locomotives, particularly if you’ve got the bulb stuck through the plastic nose of the engine. Bright, hot incandescents can actually melt the plastic. Also, you’ll find that LEDs are very directional, and the light shines out the front like a headlight beam, not all over the place like a table lamp.
I use golden-whites, since I model the 50’s and they didn’t have halogen sealed beam lights in the locos back then.
My last batch of 3mm size I got from Empire Northern when I ordered decoders. My last bag of 5mm size (I use the bigger ones where space isn’t a problem - and where the headlight lens is HUGE like on the FA) from the LHS, the Miniatronics “Yelo-Glo” Both came out to about $1.25 per LED in 10 packs.
I do want to get some regular white LEDs, the bluish color looks a lot like fluorescent lighting, and I want to put some int he passenger compartmet of my father in law’s P1K RDC when I put a decoder in it - golden white for the headlight, regular white for the interior lighting, I think it would look good.
–Randy
Randy,
For the flourescents, you might look at the SMD (surface mount) white LEDs from LC-LED.com, (UT-652NW.) I’ve used them in gas stations and they look very realistic. They’re frosted and top hat shaped so they disperse the light well, not nearly as directional as 3MMs. Mounting them in an RDC would be easier, too.
The only drawback is you have to order a minimum of 15 @1.35, but you can use them in any industrial/commercial building that has flourescents.
Mike Tennent
Randy, what do you recommend for panels to indicate turnouts? Are you using standard lamps or leds? I see that RS has some LED’s that are mounted in metals sockets with small nuts to panel mount them. The dia hole is 1/4" to drill in panel face for mounting. Thoughts?
I think those LEDs in the fancy silver holders are too expensive. But they do look nice. I use regular LEDs - buy in bulk from Mouser, or there are any number of places on eBay that sell discount parts. At least with Tortoises, LEDs are the way to go - you just wire them in series with the Tortoise motor, no resistor is required. The stall current of the Tortoise motor limits the current through the LED. I’m making my panels out of two pieces of thing plexiglas, with the diagram drawn on heavy card stock and sandwiched in between. The total thickness doesn’t exceed the capacity of small switches, ad for the LEDs I use the little black plastic holders from Radio Shack.
–Randy
Thanks Randy, very good idea !!!I like it.
I’ll second the suggestion to buy the little black plastic LED holders from Radio Shack. They look good and work great.
OK…so simply put, for DCC use the resistor, but no diode.
DC, use the diode and the resistor.
Should the diode go in the positive or negative side of the LED? And I’m guessing the band on the diode goes away from the LED?
Trying to keep this as simple as I can…
Rotor
Where you place the diode determine which way it faces. Think of it as a one-way valve, with electricity flowing out the side with the stripe. (This is not actually what is happening, though, but we won’t go there.) [;)]
If you attach the diode to the positive lead of the LED, you want the strip facing towards the LED. If you put it on the negative lead, you want the stripe facing away from the LED.
Mike Tennent
For DCC AS LONG AS IT IS CONNECTED TO A DECODER you don’t need the extra diode. If it’s going on DCC and directly across the track, you need that extra diode.
–Randy