1.5V lights

[mtrails]

Hello everyone. I run non-DCC trains at home, and would like to install miniature 1.5V lamps in the light openings in my locomotives, vs. the single 12V bulb behind the molded lenses, and need to find the proper resistor to reduce the voltage. Does anyone know? I had installed a white LED in a loco, which used a 100K ohm resistor to reduce the voltage for that particular diode, but am not satisfied with the color of light emitted. Thanks in advance.

[jimrice4449]

Rather than a resistor, try using 2 pairs of diodes connected in series w/ the motor. Current in forward flows through one pair and in reverse through the other. Then tap off of each end of the diode set w/ the leads from your 1.5 V. bulb. The diodes bleed off .7 V. each for a constant 1.4 V to the bulb. The light loss from the .1 V difference will be imperceptable, the life of the bulb will be extended and, best of all, the brightness of the bulb will remain constant throughout the entire speed range of the engine!

[mtrails]

Any particular diode?

[cwclark]

I use this circuit http://www.mrollins.com/constant4.html and attach the components to the roof of the locomotive with double sided tape...it's a constant light (in other words, non directional) but most prototype locomotives keep their lights on all the time anyway no matter which direction they are traveling. the two power leads go to the decoder or can be affixed to the locomotive power as shown in the diagram...

[jim22]

I JUST built a circuit for an Athearn BB (but so far I chickened out at installing it).

What I found is that at the currents that the motor draws (maybe 1/4 - 1/2 an amp), the voltage drop accross each 1N4001 diode is MORE than 0.7 volts. In fact, I was reading about 1.85 volts accross the pair of diodes. Since I was using 1.5 volt bulbs, I added a 10 ohm resister in series with the bulb, which at the 40 ma the bulb draws drops the voltage .4 volts bringing it back down below 1.5 volts.

I had fun building it, and I may put it in. I'm not sure the 1.5V 40ma micro bulbs are quite bright enough. I'd like to find some like the one I burned out in a Life Like Proto switcher. It's a grain of wheat bulb, and draws more like 80 ma at 1.5 volts.

Jim

ps: E=IR where E is the voltage, I is the current, and R is the resistance. Using the formula, you can calulate the resistance necessary to drop the required voltage.

Keep in mind, also, that P=IV where P is power in watts, I is current, and V is voltage. Resistors are rated in watts, and get hot when the power gets high.

[mtrails]

F=BM where F is the above formulas, B is my brain, and M is a malfunction!

I received a message from a guy on Yahoo groups, and he said the resistors needed for the grain of rice bulbs I am using (15mA), is a total of 768 ohms, or at least 700 ohms, if I don't plan to run the locos at 80 MPH. That being said, which diode, in conjunction with the resistor will give me the same current?

[jim22]

Sorry, I was kind of excited about the circuit I built. Let's slow down...

The suggestion was to build a constant-voltage lighting circuit to run the lamps. Here's a web site describing the circuit: http://www.mrollins.com/constant.html . When I built the circuit, I had more than 1.5 volts, so I put a 10 ohm resister directly in series with the bulbs in the circuit.

You're question about using a resistor to drop the voltage to allow a 1.5 volt bulb in parallel with the motor is easier. Suppose the power supply max voltage is 12 volts (you would need to measure this). You need 1.5 volts accross the lamp and the other 10.5 volts accross the resistor. using E=IR, or R=E/I, you need 10.5/.015 or 700 ohms. I think the 768 number is a good one. Resistors come in standard values, so pick the closest or next highest value available. Using P=IV, the power dissipated in the resistor is .015 * 10.5, or .158 watts. A 1/4 watt resistor should be ok. It will get warm. A 1/2 watt resistor or even larger could be used too.

The problem with this approach will be that the light will be very dim until the track voltage is reasonably high. The constant voltage lighting circuit solves this (mostly).

Sorry if I lost you again. I am an engineer (the electrical kind, not the train kind). Check out the web site.

Jim

[areibel]

Now THIS is a great post! Thanks Jim22!!

[modelmaker51]

I have built dozens of these http://www.mrollins.com/constant.html constant lighting circuits.A resister is not needed. The 1.85 v that you came up with is a "no load" value, with the 1.5v light bulbs hooked up in the circuit the actuall voltage to the lights will be about 1.4v and they will light at near full brightness.


Use the black 1 amp diodes (bulk packs at Radio Shack) and here's how:

click on pic for larger view


the diodes stripes all face the same direction on one side and the other 3 diodes face the oposite direction.

Solder (use some flux) all the twisted leads and trim them as shown.

I solder the motor end directly to the Athearn motor clip (after removing it from the motor) and then add the lights as required.

These constant reversing lighting units work great with Athearn BB drives. Kato and Atlas motors are too efficient - the lights don't come on until the loco's halfway down the track.

[jim22]

Thanks for the photos, ModleMaker51. I struggled with how to assembly my diodes. I opted to put them all next to each other like on the web site. I'm still worried about the voltage - I really measured 1.85 with the locomotive running. I traced it down to the specs on the diodes. At very low currents, they do drop 0.7 volts each, but at the currents that the motors draw, the voltages are much higher - up to 1.05 volts depending on the diodes. I know the life-like switchers I have do not have resistors, but I don't know what they use for bulbs. Maybe those bulbs are really 2V bulbs. I've blown one, and Walthers gave me a part number of 922-584760.

Exactly what bulbs do you use? I found some Micro-Scale Models Inc. 1.5V 40 ma micro-miniature bulbs. They are really tiny, and I'm not sure they will be bright enough.

Jim

[Budliner]

hi you have a lot of parts on that circuit
I posted a small circuit a long time ago
it used
the LM334Z
http://www.pollensoftware.com/railroad/index.html


hope this helps

[jim22]

Neat circuit, Budliner. I just went and looked at the specs for the LM334. I can't say I understand why its ok to run it 50% over the recommended current, but I agree power dissipation should be ok. I guess the 10ma limit is for acurracy. I'd like to try out some of those LED's myself. I guess the idea would be to install one of these circuits for each LED.

As for the incandescent circuit, I bit the bullet and installed the directional lighting and diode network in my Athearn GP-35. It took 2 tries - the first try I used wires that were too long and too stiff and interfered with the flywheels and trucks. The 2nd try worked pretty well. While I was at it, I installed Kadee couplers in #27 gear boxes. I may take it apart one more time and install a third lamp in the cab ceiling, but it looks pretty good.

Jim

[modelmaker51]

I use Miniatronics microbulbs (http://www.miniatronics.com) mostly. I've used both the 15 and the 30 milliamp bulbs. I've been lighting up locos this way for over 20 years and I've never used any resistors in this circuit, although you certainly can add some resistance to dim them down a bit and extend the bulb life, but I haven't had to replace very many burnt out bulbs overs the years, they seem to last quite a long time.

Check your LHS, most of them have a Miniatronics rack.