proper wiring for LED's

[astapleford]

This is a subject that comes up a lot, I'm sure. But, here goes again.......

I have looked at many articles, and posts here, and am still a little confused about the correct method of wiring LED's using limiting resistors. Are the resistors wired onto the anode (positive) or cathode (negative) leads of the LED. And, when wiring onto decoder solder pads, does the anode get wired to the "F0" pad? It is often noted that the "blue" wire goes to the common. When you are adding LED's, resistors, and your own wires, there is no "blue" wire. Sure would like to see a good wiring diagram explaining all this, not using generic and/or specific terms relative to a particular loco or decoder manufacturer, but rather terms that are pretty much universal, if possible.

Thanks for any clarity that can be given.  Sorry for yet another lighting question!

Al

 

[cacole]

It doesn't matter which LED lead the resistor goes into.

The blue decoder wire is the POSITIVE voltage output and goes to the positive (longer) lead of the LEDs.  Blue has to go to every LED.  The other (short) LED lead goes to the function wire of the decoder (white, yellow, green, or purple).

The white wire is normally for the headlight; yellow for the backup light; and green and purple for ditch lights, strobes, etc.

The wiring diagram that comes with a decoder shows how to wire LEDs as headlights, backup lights, etc. pretty clearly.

[narrow gauge nuclear]

The advice above is all good, but if you use surface mount LEDs, there are no leads.  You have to add them.  A small test gig is in order.  I tend to not worry at all about the in-line resistor with these until I have wires on them.  Once wired, I throw a 560 ohm resistor in series with the micro LED using clip leads, (radio shack), and then feed 9 volts from a small transistor radio battery to the other lead of the resistor and the remaining free lead of the LED.  If it doesn't light, reverse the battery leads and it will or should now light. (note all micro surface mounts are marked, but with a mark so tiny that I ignore it.)  You can't blow out an LED if connected backwards provided a resistor is in the line and it is only there electrically when the LED is correctly hooked up and is on.

Now, take a black magic marker and mark only the negative or minus lit LED wire as black on the lit LED.  Now you know the correct polarity for later hookup.  This test serves two purposes.  It helps mark a highly visible polarity for hookup and tests the lamp to make sure your soldering heat on the tiny wires did not ruin the tiny device (happens often) and that your connections are solid.

I tend to use 1/8 watt  surface mount resistors as well since I am often cramped for space (steam engines) or there is a convenient solder pad on the controller for one end of the resistor.  Standard in-line resistors with normal leads are fine if you have the room.

As noted by others, the installed resistor value is based on the decoder's line voltage found only in the literature related to the decoder.  With a 12 volt decoder voltage, (most common), a 680 to 2200 ohm resistor is usually fine with 1000 ohms being quite common. These values typically allow a current of from ~5 - 15ma through the LED. 

Note*  Even if you are using larger LEDs with nice, well marked leads, it is always wise to make sure they light once you have soldered to them with the battery test above before going to the hassle of installing them.

Richard  

[astapleford]

Thanks guys. I was wondering if there WAS any correct way for resistors to be attached to LED's. Bulbs usually have no polarity, as LED's have. I've never seen any words in all the articles in MR, the Kalmbach books about DCC and posts here, that say that it doesn't matter which lead of an LED the resistor should be attached to........now I know!  Thanks again.

-Al

[Truck]

I am with Dave on this one. All my resisters are on the function wire (NEGATIVE) side of the LED's.

Though it may not be decoder law to do it that way, I am anal about how I wire things, and to me it seems to be the senceable way to do it.

                                                                        Truck.

                                                                                      

[CharlieM90]

astapleford

I've never seen any words in all the articles in MR, the Kalmbach books about DCC and posts here, that say that it doesn't matter which lead of an LED the resistor should be attached to........now I know!  Thanks again.

Electrically it doesn't matter. But if you're planning on installing/using a lot of LED's then do yourself a favor and standardize your practice as to which side you add the resistor. Can help to save you a lot of time in the future should you need to alter or troubleshoot the circuit it's in.

Just like wire color-coding or standardizing switch machines wiring, it can save you lots of aggravation/frustration.

I'm a firm believer in adopting a standard practice and sticking to it (when it comes to electronics).

 

[rrinker]

 Good idea to keep notes as well, because you will NEVER remember what you did last year. Now, if I could only find my notebook....

 Standardizing is definitely worthwhile as well. So if you DON'T note what you do in one install, you can still figure it out because you used your standard, just like others that you DID write down.

                --Randy