Cut to the chase: Is it ok to use a 12v lamp on a 14v circuit with no resistors?
It’s been insane checking out all the options for wiring lights in an Athearn sd40-2 in DCC. I am using a lenz gold decoder.
First try: 3mm LEDs. Problem: Lenses too big, too many resistors.
Second try: 1.2mm 1.5v bulbs. Problem: bulbs too small, too many resistors, and too much heat for my liking.
My proposed third try: 1.7mm 12v bulbs, no resistors.
It would be real nice if I could program the decoder to send out only 12v to the light function circuitry.
What is the official size of the lamp housing in these GP/SD cabs? I now know it’s somewhere between 1.7mm and 3/32" (2.38mm). I am afraid to get the 2.4mm bulbs thinking they might be too big… but then again, there are 14V 2.4mm bulbs, which would wipe out my biggest concern at the moment. I could always ream out the lamp housing in the cab a little bit (i think!).
But I am defying the commom sense for a moment because I’ve come across advertisements for these 12v 1.7mm bulbs that say “they can be wired directly to the function outputs of model railroad DCC decoders without fear of electronic overload or plastic-melting heat generation.”
Who’s right, who’s wrong, what is assumed by this advertising?
From what I’ve read, trying to operate DCC with only 12v on the tracks is a bad move (if that’s even the correct way to change the voltage coming out of the decoder… I haven’t read that far yet).
The only thing wrong about common sense is that it’s not common.
Anyway, nothing wrong about experimenting with one. I’d suggest trying one out of the loco (just connected to the power supply) and see how hot it gets and how long it last.
If you’re still stuck, another idea is fiber optic wire or cable.
Why not find a source for a single 300 ohm resister (or whatever will do the job, I’m not that knowledgeable), and use that with your bulb. I will bet that you will at least double the span over which you’d have to replace the bulb with all that work, and all that wear and tear on the loco and fine parts.
Reminds me of claims that toilet bowl cleaners, openly stating that they contain sodium hypochlorite (bleach), are “safe for septic systems.” Yeah, sure they are.
I’d love to try just 1 of these as an experiment, but I don’t have a local shop. It comes down to paying shipping again and again, which adds up. I want to buy 20 bulbs in the end and want a few words of advice or whatever before I commit. This is my third try on these *** lights and I want it to be my last!
Thanks gsetter! I know these exist (I mentioned 'em up there in my first post). But before I need to start reaming out the lamp housing, I want to kill all options on the 1.7mm bulb, which I am guessing is the most ideal “fit” physically, but not electronically.
800ohm (a 470 and a 330… couldn’t find the ideal 750ohm resistor at the local Radio Shack). The heat isn’t too bad because all the wires are still out. I just don’t want heat to be an issue once everything is bunched up and stuffed inside the loco body. I really haven’t left the lights on for an extended time to see how hot the resistors will really get… but I am sure it’s not a hazard. There are just too many wires/resistors/etc, and if a 12v bulb really can address these issues, I am all for it.
I don’t know how much money I’ve wasted buying stuff that wasn’t what I expected or got results I hoped for. And I don’t want to know.
Radio Shack is no longer a real electronics store. More like a cell phone / toy store now. Are there ANY real electronic stores near you? If not, try All Electronics. They have a wide selection.
Do you know how many milliamps the 12 volt bulbs draw?
The 1.7mm bulbs may be the best way to go. If you don’t like them, there’s always structures that need lighting. In any case, try a 1/2 watt resistor. Heat won’t be an issue, especially if you’re using 12 volt bulbs. Figure one resistor for the front lamps and one for the rear.
I’m not a DCC user/guru but my electrical engineering background says that if the input side of the decoder is 14V, the output side is likely 12.5V at best - could easily be less. You might try looking up the specs for the decoder light circuits - I suspect they’ll be 12V.
That said, the 14V bulb would be better - last longer, run cooler - than the 12V bulb even with a 12V circuit. The DCC puts out a continuous 12V to the lighting circuit, unlike analog DC which is usually much less than 12V, and is off when engine is not moving.
First, my measurement of 14.7v on the white wire was done with a regular AC/DC ammeter, not calibrated for DCC. Would this be a problem? This isn’t an AC sine wave where I know the regular meters have problems measuring a DCC sine wave. I haven’t changed any settings on the Lenz LZV100, so I assume the default 16v are on the tracks. 14.7v on the output side of the decoder seems right in this respect, but again, measured with a regular ammeter.
Second, to faraway’s suggestion: ABSOLUTELY. I didn’t even put two and two together. The voltage to the bulb can be controlled by the “brightness” CV for lighting outputs A, B, C and D (although it’s a bit strange… there are only two CVs for this control, one for A and C, and the other for B and D). There is also a separate “dimming” CV.
Thanks for the tip on All Electronics. I recently bought a bunch of doughnut magnets for wooden toy train construction (for the 3 year old), and they sent me their awesome catalog. Love it. But a local Radio Shack run works when it comes to a small selection of resistors, capacitors, junk like that, and is great for returning stuff.
I am in Roswell NM, where the latest train excitment happened to “hit” the front pages of the paper this morning (the famous paper that published the story about the UFOs in 1947 that started the whole alien craze):
That’s funny, ripped the engine clean out.
What’s stupid about aliens? The Coneheads are pretty cool. Although, there’s a lot of stupid aliens where I live. Too many just don’t know what a stop sign or red light means.
I have just ordered 40 14v 50ma 2mm bulbs for $10 + shipping.
I took a drill bit set and found the right bit that fit in the lamp housing the best… 5/64"
5/64" = .078")
Converting to millimeters = 2 (2mm = 0.0787")
Searched for 14v 2mm lamps and found ONE PLACE… had them for $2.50 per 10pack (minimum $10 order, so I ordered 40). Incredible price compared with bulb prices at model railroad retailers. Model railroad retailers must be making money hand over fist charging about $30 to $40 for comparable bulb quantities!
Just FYI, the function outputs on most decoders is rectified so you can measure the voltage from any function lead to the blue (common SOURCE (ie POSITIVE)) with an ordinary DC voltmeter.
Unless you’re going for some odd lighting effects, most of the time the twin sealed beam type headlights are both lit at the same time. It’s not the same thing as the two large round lights on, for example, F units, where one was often a headlight and the other a Mars light or something similar. In other words, there’s not much need to put two individual bulbs in place. The place for those tiny lights is for ditch lights, where you DO need to fit the bulb in a tight space - that or use fiber optics. For twin headlights, one larger bulb or LED will light both effectively Fill the opening with MV lenses and put a single lamp or LED behind them. I prefer LEDS, there’s no heat and the resistors don’t get hot either. A typical LED running with a 1K resistor pulling about 10ma is just over 1/8 watt at 14 volts, far below a 1/4 watt resistor’s rating - they won;t even get warm. I fear a 14 volt bulb will get too hot - it’s not the resistors but the bulb itself, especially if you intend to actually mount the bulb in the shell.
I REALLY like locos with light pipes, like the Atlas/Kato RS-3’s. The light pipe includes the lenses for the twin sealed beam headlights, I just trim the pipes back (they are designed to work with a single center mounted lamp) and use shrink tube to attack an LED to each remaining light pipe section. The LEDs and resistors don;t even get warm, and because of the light pipe, both ‘beams’ in the headlight light up nicely.
here in L.A. we have several hobby shops. many have racks of “train electrics” that contain wires, bulbs, and tiny plugs. i think the 12 volt bulbs and the 16 volt bulbs are the same size. the 1 1/2 volt bulbs are tiny.
because i don’t want to ever change one out, i use the 16 volt ones. they’re not as bright as an overpowered bulb, but they last.
sounds like you have an access problem with your supplyline. no hobbyshops to browse. if this is the case; next time you feel backed into a corner, e-mail me with your needs and i’ll browse the electric rack for you and see if they have what you need. this way, shipping should be less than a buck…