Using Miniatronics 14V Bulbsgreenspun.com : LUSENET : Wiring for DCC : One Thread
I am preparing to do my first decoder hardwire instillation on a brass 4-6-2 for a client of my custom paint shop. The client supplied me with Miniatrnics 14V bulbs for the headlight instillation. Since the model has no existing lighting, the lighting installation is basicly from scratch. My question is, do I have to wire in a resistor in this istillation sicne the decoder output is 12V and the bulbs are rated for 14V. If I do have to wire in a resister, what Ohm value do you reccomend, and do I solder it to the Blue lead or the white lead?
Thanks for your assistcance
-- James R. Mitich (firstname.lastname@example.org), March 20, 2003
I believe the 14V Miniatronics lamps draw only about 30-40ma of power. This would mean you do not need a resistor to protect the decoder function. I am most familiar with NCE and Digitrax decoders and I believe they can handle about 80ma of current draw. The 12V supply will be fine for bulbs ranging 12 - 16 volts.
I recommend using the blue (common) lead when hooking up lamps. The decoder manual will have full details. Here is a hint. If the tender is not equipped with a lamp and you wish to have the headlight on even during reverse, then wire the white and yellow wires together to one lamp lead and the blue wire to the other headlamp lead. This will keep the headlamp lit regardless of direction. Again, follow the advice packaged with the decoder.
-- Doug Fraser (email@example.com), March 20, 2003.
There is a second reason to include a resistor if you have space. Incandescent lamps have a nasty habit of drawing 6-10 times normal current when you first turn them on (cold, not glowing). The transistor in your decoder that control the light circuit will also see this turn-on surge and can be damaged by it. Placing a 22 ohms resistor in series with the lamp limits how much current a cold lamp can draw and reduces the likelyhood of transistor damage/failure. For the same reasons, it will also extend lamp life. Consider doing this particularly for flashing fire-box or ditch lights.
It doesn't really matter which bulb lead gets the resistor. But for the sake of consistentcy, put it in the white lead so that you won't later confuse it with other circuits that may also be using the blue (+) supply lead.
Yes, there will be additional voltage drop...about 0.66 volts for a 40 ma load. A 14 V lamp will see less than rated voltage and appear slightly yellowish, just like the prototypes.
Be aware that if there was no lamp there before that a 14 V bulb drawing 30 ma gives off 0.4 watts of heat in a tiny space. It will be hot to the touch and may cause plastic to distort or melt.
-- Don Vollrath (firstname.lastname@example.org), April 17, 2003.