resistors and Digitrax???????

Model Railroader Electronics and DCC
1

OK- I’m in the midst of rewiring a few old Blue Box Athearn tunnel motors, and am trying to get my cab headlights to run. All that aside here we go…is there a simple formula for what resistors I need to power a single PAIR of 1.5V lamps? Also, what about for a single lamp?

I plan on using the blue as common(obviously), and I believe the White(forward), and Yellow(reverse), and the Green and Violet(one for each lamp of the nose Pyle, as functional options). How many OHMS of resistance do I need for the headlight pairs, and how many OHMS for each individual bulb? All lamps are 1.5v. Also, does 1/4Watt, 1/2Watt resistors mean anything that applies here?

Lastly, does it matter what side/polarity these resistors go on? I would assume so; ei: resistors to F4+ ? Then to lamp?

2

Look in this recent post. There is info on using LEDs. The two with dcc wiki will be of interest for LED resistors.

http://cs.trains.com/forums/1405338/ShowPost.aspx

Rich

3

OK, there is no easy way. If you need to figure these things out, you need to know how to do it.

You start with the voltage that the decoder puts out for the lights. 12vdc(?), then the voltage of the light bulbs that you want to use, 1.5vdc in your case. Now you have to figure out how much voltage to drop across the resistor, so 12vdc minus 1.5vdc = 10.5vdc.

Now you have to know the current that your light bulb will draw when lit by 1.5vdc. This should be provided by the maker of the bulb and noted on the package, in most cases. *** If you can’t get that information, everything from here on out is all guesswork. *** So let’s assume for this lesson that the current is 10ma. (10x10 to the -3) Thats 0.01 amps.

Now we have to use a formula. R=E/I That is the Resistance needed equals the Voltage (E) divided by the Current (I) of the bulb.

So 10.5 (The voltage you want the resistor to drop) divided by 0.01 (The current of the bulb) equals 1,050 Ohms of resistance.

Because 1,050 is not a common value, a 1Kohm resistor is close, BUT, it is too small by 50 ohms and you could burn out the bulb, so you should go higher in value, which could be 1.1Kohm which is 1,100 ohms. Depending on where you get the resistor, you may have to go a little higher.

Now comes the power rating of the resistor, or the Wattage. Again a formula. P=IxE. That’s Power (in watts) = the Current of the bulb (I) times the Voltage (E) the resistor is dropping. So, P= 0.01 x 10.5 which is 0.105watts. So a 1/8 watt resistor would work, but it will get warm because it is close to the 1/10 watt the resistor will dissipate. So to be safe, a 1/4 watt, or even a 1/2 watt would be better.

You will need one resistor per bulb, wired to the white / yellow wires. Since the blue wire is common to each bulb, don’t put the resistors on the blue wire. Resistors have no polarity to worry about.

If you wanted to figure one resistor for two bulbs, you add the current of each bulb together (assumi

4

The following information is from the DIGITRAX DECODER MANUAL which you can downoald for free from the Digitrax website:

What the wires are for Wire Color

Power Pick-up Right (Engineer’s Side) Red

Power Pick-up Left (Fireman’s Side) Black

Motor + Right Brush Orange

Motor - Left Brush Gray

F0(Fwd)-Forward Light White

F0(Rev)-Reverse Light Yellow

Lamp Common Blue

F1-Function 1 Green

F2-Function 2 Violet

F3-Function 3 Brown

F4-Function 4 White w/ Yellow Stripe

F5-Function 5 White w/ Green Stripe

F6-Function 6 White w/ Blue Stripe

Reverse Lamp Yellow

Lamp Common Blue

Forward Lamp White

Reverse Lamp Yellow

Forward Lamp White

Reverse Lamp Yellow

Operation with Lamp

Common Connected

Operation without Lamp

Common Connected

Lamp Common Blue

Forward Lamp White ForwardLamp White

Reverse Lamp Yellow

Lamp brightness won’t be affected by operation of analog locos on the layout.

This is the preferred wiring method but, in some locomotives (particularly in N-Scale and smaller HO units) it may not be convenient to wire the lights with lamp common.

Lamp brightness will change depending on the direction of the analog locomotive being operated on the layout. If you don’t run analog engines on your layout, you won’t notice any difference between these two ways of connecting the lights.

Note: Current setting resistor to suit the lamp used. Typically 560 ohm 1/4 watt for grain of rice and 250 ohm 1/4

5

Thanks for the responses. I guess I was expecting “Plug and Play” decoders to be just that. I don’t understand why these things have to be so difficult, or that I’d need a degree in electronic engineering.

My grain of rice lamps are 1.5v, @ 15mA each. So, Elmer, next question. Is in parallel what you normally find in certain factory applications? ie; one wire from each lamp tied to the others the same, then to their respective poles? And, in respect, then in series would mean + pole to lamp lead A1, from lamp A2 to lamp B1, and B2 to - pole?

Finally, without getting into the COMPLETE “mine is better” melee, does anyone recommend a true plug and play decoder that doesn’t cost an arm and a leg? I’m looking for simple user freindliness…1.5v ready, front and rear constant lighting depending on direction, and at least 2 functions to supplement Pyle lights, or ditch lights depending on the application.

I appreciate any responses, and your patience…

RICK

6

Elmer- tell me if this sounds right…

Each lamp is 1.5v @ 15mA. By itself, that means that 10.5V/.015=700 ohms.

Also, if two are wired in parallel, then 9V/.03=300 ohms, because you add the mA of both lamps and get 30mA=.030A…and 12vdc minus 3.0vdc=9vdc?

If this is the case… THANK YOU THANK YOU THANK YOU THANK YOU!!!

7

TCS makes the A6X decoder that will run 1.5 volt bulbs without resistors. It still requires careful reading of the installation manual, though, bacause the bulbs are soldered to pads, rather than the tabs on the end of the board.