Headlights not working

[tmiller]

Hey guys, have an interesting questions for you today. I am running a DCC system with 4 DCC locomotives equipped with sound. Two of the engines with sound are using MRC decoders which I installed myself. Everything has been working fine up until a few weeks ago, when I noticed that the two diesels using the MRC decoders, the headlights are not working. And on one of them, both the front and back lights are not working. Other than that, all functions and sound are working.

Is this a coincidence, a problem with the decoder, bad bulbs, etc. From time to time when a engine/car derails, it will momentarily show a short, but then quickly resolves the issue. Could this be a reason to blow out bulbs. Or should I be seeking help from MRC?

And if it is the bulbs, how is the best way to test, instead of just replacing the bulb, which isn't that big a deal?

Thanks for any help.

Ted

[NIMT]

Ted,
Are they actual bulbs or L.E.D.'s?
If they are bulbs...well not that uncommon to toast a bulb during a derail, Also if you don't have a resistor in line with the bulb that makes them burn out faster, all it takes is like a 47 to 100 ohm res to extend their life out along way!
If you have LED's that have burned out...:eek:...Did you have large enough resistors on them to begin with 470 to 780 Ohm is normal.
Testing the output's with another bulb is the simplest way to check it out quick.:cool:

[tmiller]

Sean,

They are bulbs, and the same ones that were in the loco when I bought it. According to Walthers, the bulb is a 3v approx. There are no resistors in place.

I just purchased a 3.6v LED to see if that will work. Otherwise, I will get replacement bulbs from Walthers. Should I think about adding a resistor if the LED works?

Thanks for the help.

Ted

[gunrunnerjohn]

I'm guessing the decoder puts out more than three volts, probably why the bulbs are going up in smoke. :)

[NIMT]

If you put the LED on there with out a resistor it will also go POOF real quick!
Common problem with retro fitting a loco is the way things work in DC will not work in DCC.
Every lamp Or LED needs it's own resistor! Even if there are 4 headlights each needs to have it's own resistor! 470 ohm min. Run all of one side of the bulbs to blue and the others to it's own resistor then to white (for head light)
Same for any other lamps.:cool:

[Massey]

to be safe find out what the voltage is at the decoder for the lights. I have found that some decoders are 3V and others are track voltage which on DCC can get up to 18V AC. And also dont just assume that each manufacturer has the same voltage across the line of all of their decoders. I have some Digitrax decoders that have 12V at the light and a similar shaped but different model that has only 5V at the light. A quality volt meter can solve this issue fast quick and in a hurry, and prevent you from blowing up an LED... Trust me they can pop hard enough to break the tip off.

Massey

[tmiller]

Hey guys, working headlights! Went to Radio Shack and bought some 5mm 3.6v LED's. Placed a 470 ohm resistor in series and they look great. Nice and bright!

I would have liked to used 3mm LED's because of space requirements, but just had to do some retrofitting to make them fit.

As always thanks for all your good advice. Never a dull moment and always something new to learn.

Ted

[gunrunnerjohn]

You can also increase the size of the resistor if you feel the lights are too bright. I typically run 3mm LED's for headlights on my O-scale stuff, and I limit the current to around 10ma, the full 20ma rating makes them too bright IMO.

[tmiller]

Good idea on the resistors. I think I may just try and order some 3mm LED's and save me some cutting.

Thanks again.

Ted

[NIMT]

I use these in a lot of my installs the will usually go in the holes for grain of rice bulbs. They come in all the different colors!:D
12169

12170

[gunrunnerjohn]

Sean, I got a bag of those 2mm LED's on eBay for a few bucks, I have at least 50 of each color. I end up using more 3mm LED's than anything else on O-scale stuff, sometimes I use 5mm ones. I have used the 2mm ones for marker lights, notably on the MTH Genesis locomotives I did, they fit perfect.

[tmiller]

I like the size of those 2mm LED's. Will have to get some.

Now one of the locos that I replaced the headlight with an LED, blew out. I replaced the LED thinking it just blew out, but it still does not work. It does have a 480 resistor ins series. How is the best way to test the circuitry to see where the problem may lie? My guess is maybe the decoder may have a problem.

Any advice from anyone?

Ted

[gunrunnerjohn]

I'd guess that if you have an LED going out repeatedly, I'd guess that you are subjecting it to reverse voltage, something that they don't tolerate very well. Try a diode in series with it in addition to the 480 ohm resistor, I predict long life for it.

There are 2mm LED's with no big base available, but they're considerably more expensive, but if you have a place with no room... :)

[gc53dfgc]

Also make sure you have the positive piece of the LED going to the positive wire and then the neg to the neg. I have run into several cases where this was the only problem.

[NIMT]

When you say you have a 480 Ohm res you have me worried, the normal resistor value is 470 Ohm's????:confused:
It is possible that the resistor bought the farm with the LED or it went and not the LED! If the wattage of the resistor is too low it will not hold up too.:D
And like GC said, some miss installing the LED in the proper direction, Blue is Anode (+) and all others output wires are Cathode's (-)! ;)

[tmiller]

Hey guys, problem fixed. I simply reversed the leads and it now works. I di not know there was a polarity issue. Learned something new. Adding a diode makes sense. I will see how all three of them do and if I have to replace them, I will certainly place a diode in series. When I initially replaced the three lights, I just wired them in, and they worked. Now I am feeling rather lucky that all of them worked in regards to polarity.

How does the reverse polarity work? And, how can it happen, just a derailment, which is what caused it I think, unless something else happened that I am unaware of?

Thanks guys for all your help.

Ted

[gunrunnerjohn]

If you truly have just DC, the extra diode is not necessary. I just figured that you had a working LED, but it was popping, my experience puts that at reverse voltage many times. I'm not an HO man, in O-scale we have AC running all over the place. :)