bulb size

Discussion in 'FAQs' started by Relic, Apr 24, 2006.

  1. Relic

    Relic Member

    At the Moncton train show last Saturday I got some GP9's for what I thought was a good price BUT I want to run them long hood ahead. I took one apart (as a matter of course) and found that a previous owner had modified the light by glueing a piece of copper tubing to the inside of the hood an sticking a grain of something bulb in it. Lookes a lot easier than moving the stock light
    My question is ( you thought I forgot didn't you?) what voltage ofbulb should I ask for when I go to what used to be Radio Shack (I don't get to town much) I'm assumeing that I want a grain of rice bulb, or wheat ?
  2. pgandw

    pgandw Active Member

    A 12 volt bulb should work fine, unless there is a constant intensity lighting unit installed (doesn't sound like it). I think the bulb you are thinking of is grain of rice. But it doesn't really matter except that the bulb fits the socket and/or space and is rated for the correct voltage. The constant intensity units are usually built for 1.5 volt bulbs.

    Hope this helps
  3. Ray Marinaccio

    Ray Marinaccio Active Member

  4. kf4jqd

    kf4jqd Active Member

    I replaced my bulbs with golden glow LED's. You will need to add a resistor.

  5. Relic

    Relic Member

    Whata can of worms you've opened.Since I'm not going to be handy a lhs for a while and will be getting said led's at Radio Shack (or whatever they'er called now) could you tell me (and meby some other poor ledly challenged souls)what I would need for led's, resistors , and where does the resister go ? I'm thinkin' it goes on the neg side, right?
    Is it a fact that led's take less power than bulbs ?Would it be a noticable amount?
    Tell you why I ask. I Bashed myself a F B unit out of an A and saw no reason to hook up the light ,HA the B was then pushing the A till I hooked the light up again. Also I need all the power I can get 'cause of a couple of hills which aren't all that steep but have turns at the top that make a biiiig diference.
    thanks a heap
  6. pgandw

    pgandw Active Member


    There are 2 ways to hook up light bulbs correctly, depending on the bulbs (or LEDs), and circuits (I am assuming normal DC operation in this discussion, DCC is another story).

    A normal 12 volt bulb circuit puts the light bulb in PARALLEL with the motor. Both the motor and the bulb "see" whatever the track voltage is and turn or burn accordingly. At lesser voltages, the motor turns slower and the bulb burns dimmer. Either motor or bulb could be removed without affecting the other - this assumes power pack can handle the load. This arrangement is generally used in lower cost locos.

    If you are going to substitute an LED in the parallel wiring situation (LED parallel to the motor), you need a resistor in series with the LED (but not the motor) to limit the LED current to less than 20 milliamps. In most cases, the calculated resistor value is 860 ohms, but is difficult to find. Using a commonly available 1K 1/4 watt resistor provides a suitable safety margin, and generates less than 1/4 watt of heat. Resistors are not polarized, so can be added on either side of the LED.

    To get around dim headlights at normal operating voltages, and to provide for directional lighting, the constant intensity lighting circuits were developed. These use 2 parallel strings of diodes (or a full wave rectifier, which is the same thing in one package) in SERIES with the motor. The diode strings siphon off 1.4 volts (could be more in some cases) from the voltage available to the motor to provide to a 1.5 volt light bulb(s). This limits the top speed of the locomotive slightly, which is usually not a problem.

    Substituting an LED for a 1.5V light bulb in a constant intensity circuit requires understanding of the specific LED and a schematic of your constant intensity circuit. Just a bit complicated to explain in a post.

    Your problem with an A unit motor not running with the bulb removed bothers me. This should not happen if wired correctly. If a 12 volt bulb, make sure both the motor and the light bulb have indendent wiring to the track pickup system. Was the motor not running, or was the A unit simply slower at a given voltage? If the A unit was simply slower, that could be accounted for by the voltage drop of a constant intensity lighting unit, but still should not be affected by the absence or presence of the bulb.

    You also hint at a problem with insufficient "power" to go up your grade at the speed you desire. Your locomotives will slow down going up a grade or under increased load. It takes some pretty fancy electronics to prevent this from happening. But if the train is stalling on the grade/curve without the wheels slipping, then you do have a problem. Either the motor is not powerful enough to pull the load, or your power pack cannot supply enough current (an indication of the latter would be the power pack getting hot/very warm). In either case, prolonged power-on stalls will likely overheat a motor, and should be avoided. If the wheels are slipping, then the loco doesn't have enough traction to pull the load up the grade, but no harm is being done to the electrical system.

    Hope this helps. The more specifics you provide, the more we can help.
  7. Relic

    Relic Member

    Thanks for the info , now I have a choice of led or cream of wheat bulb (whatever's cheaper)
    the thing about the hills is more a flange thing than an electrical thing ,because of the curve at the top the hill .
    The F a/b thing, I thought it a tad strange that the b (wich used to be an a) would gain so much speed by unhooking the light (factory bulb cheap Bachmann engine)but it sure did.
    thanks again
  8. Relic

    Relic Member

    Ok I finally got to town and got some bulbs
    Will these do or what?
  9. Thoroughbreed

    Thoroughbreed Member

    Those bulbs should do quite well.
    edit: ignore from here on, as I didnt realize where your from till after post:oops: :thumb:
    If you have a Hobby Lobby nearby, stop in their train aisle and pickup a set of lights made for houses. These are generally 14v bulbs and come in packs of 20 for ~$15.:thumb:
  10. Russ Bellinis

    Russ Bellinis Active Member

    Fred, I think what he was referrng to is that in a lighted locomotive coupled to an unlighted locomotive, the unlighted unit will always start at a slightly lower voltage and run at a slightly higher speed in all voltage ranges than the lighted unit because of the power syphoned off the operate the light. If you don't have dcc with speed table adjustment capability, the only ways to compensate are to add a light, add a resistor to simulate a light, or put enough weight behind the locomotives to make them work to pull the load so that neither one can run away from the other.
  11. Relic

    Relic Member

    There y'go Russ, you hit the nail squarly on the head.I put a 12v grain of something bulb in the "B" and cured the problem.
    thanks to all

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