Yet another LED question--this time for DC

[dragenrider]

I have not yet gone to DCC, so I'm still playing with DC/analog equipment. One of the things I wanted to do was attempt to create directional lighting using LED's and resisters which would work on DC. After several mistakes and re-do's, I finally got an engine working with gorgeous white lights..sort of.

My problem is that when the engine moves forward it picks up speed before the LED's will come on. [:(] In reverse the lights come on first and then the engine begins to move. [:)] I have two LED's at each end with a single resister to each pair. What did I do wrong?

Please type your answer slowly so I can understand it. If you go too fast I might lose you! Diagrams would be helpful, too.

[rrinker]

Easy - the white LEDs need about 3.5 volts before they light up. Your loco's motor obviously needs less to start moving.
The simplest solution for DC is to use some diodes to drop the voltage to the motor - regular diodes, not LEDs. The downside of any scheme for this is that it will reduce the top end speed slightly.
Each diode in series with the motor will reduce the votlage by 0.6 volts. You will have to experiment to see how many it will take to allow the LED headlight to light before the motor moves. A good diode to use is radio shack #276-1141.
Hook them up before the motor but after the LED - the LED and resistor should be directly across track power, the regular diodes in series with just the motor. Another way, if two diodes will work, is to use a bridge rectifier like RS #276-1152. On the bridge rectifier you will see two terminals with little squiggles ~ and two marked + and -. Connect the two marked + and -. Remove the motor wire from the motor, connect to one of the ~ terminals. COnnect the other ~ terminal to the motor where the wire came from.
This will put 2 diodes in series with the motor in each direction, a 1.2 volt drop. It might be enough. You can use two bridges, connect the + and - together independently on each rectifier, connect the ~ of one to the motor wire, the ~ of one to the second, and the remaining ~ to the motor. This will put 4 diodes in series with the motor, a 2.4 volt drop.
With regular diodes, you most use twice as many as it takes, otherwise the loco will start at one speed in forward and another in reverse. Regular diodes have a band on one end. For a diode symbol here I will use this: >| The band corresponds to the vertical line.
Wire liek this: --->|-->|-->|-->|--- and wire another set the same way.
Flip over the second set, so it looks like this: ---|<--|<--|<--|<---
Connect the ends of the two sets together. Take the wire off the motor, and connect to one end of the diode sets. COnnect the other end of the diode sets to the motor. As draw, it's 4 diodes in series with the motor, 2.4 volt drop. Maybe 5 would be ideal, that would be a 3.0 volt drop - you have to experiment. But you hae to add diodes in pairs, one facing each direction.
It will also vary from loco to loco, some start at lower voltages than others.

Simple enough?

--Randy

[Don Gibson]

Randy 'nailed' it!

Diodes in series with the motor will slow it down and turn on deadlights.Result is lights come on before the locomotive starts to move.

Most resistors have a +/- 10% tolerance, that means a 560ohm can be anywhere between 504 and 616 ohms. Cheapies are +/- 20%, wheras Precision resistors (1% 2%) aren't that expensive.

2 bridge rectifiers in series with the motor should put you in the ball park for constant voltage wheras 1 will work with 1.4V 15ma bulbs.

[dragenrider]

Thanks for the info. I'm grasping the general concept, but having a little difficulty with the details. Rectifiers...isn't that what the fire department puts out after a very bad derailment? [:I]

I think what you all said explains why the light lights first one way and last the other...the electricity is going through the resistor first before the motor when in reverse and last when moving forward.

Now, if I had a decoder I could skip all the resistors to the LEDs, correct? I'm starting to think that If I can do all this LED wiring and soldering then surely I can follow a simple decoder wiring diagram. I'm getting better with a soldering iron, too! [:P] I didn't burn myself at all today. [B)]

[rrinker]

No, with a decoder you still need a resistor for the LED, the function output of the decoder puts out 12-14 votls depending on your DCC track voltage, so you definitely need a resistor to use an LED.
You probably shouldn't have your LED and resistor in series with the motor. I doubt you do, if you did it wouldn;t even run in one direction, the LED works like any other diode and only conducts current in one direction.
What LL does in the Proto2K locos is has a series of diodes like I described above in series with the motor. If you measure the voltage across 2 of them, you get 1.2 volts - always, no matter what speed you have your power pack set to. So you can safely connect a 1.5 volt bulb and have it light before the motor turns AND never get any brighter no matter how fast you go. But it's not enough to light an LED, so it won;t work the same way for an LED. You'd need a lot more than 2 diodes, and you'd start to end up like a BLI sound loco where you have to turn the power pack up past half way before the loco moves - the lights and sound come on sooner, but nothing moves. But conencting the LED to the track pickups with an appropriate resistor, PLUS diodes in series with the motor, will work fine.

--Randy

[xdford]

Hi there Dragan,

You said ..."I think what you all said explains why the light lights first one way and last the other...the electricity is going through the resistor first before the motor when in reverse and last when moving forward."

Are you sure you have the LED's and their dumping resistor PARALLEL to the motor rather than in SERIES as it sounds as though you have the light circuit in Series with the motor. The Motor may chew a little bit of current first plus the LED won't take the higher current levels. Can you be more specific about what engine you are actually fitting?

Feel free to reply off list or visit my website [e-mail] or www.xdford.digitalzones.com

Regards from Down Under

Trevor