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resistors and Digitrax???????

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  • Member since
    March 2008
  • From: State College, PA
  • 11 posts
resistors and Digitrax???????
Posted by DRGW GUY on Friday, April 11, 2008 2:31 PM

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?

  • Member since
    October 2006
  • From: Western, MA
  • 8,398 posts
Posted by richg1998 on Friday, April 11, 2008 3:02 PM

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 

If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.

  • Member since
    January 2007
  • From: Eastern Shore Virginia
  • 3,290 posts
Posted by gandydancer19 on Friday, April 11, 2008 3:38 PM

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 (assuming the bulbs are wired in parallel) and use it in place of 0.01 or (I), and recalculate.  You will also have to recalculate the Power or Watts, because you are using more current through the resistor.  You CAN do this!!

I know this is technical, and you might be able to find a chart somewhere that will tell you the same thing, but if you can get you head around this, you will learn more.

And this works with LED's too.

Elmer.

The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.

(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.

  • Member since
    January 2001
  • From: Kansas
  • 808 posts
Posted by jamnest on Friday, April 11, 2008 3:38 PM

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 watt for grain of wheat.

Lower resistance values will increase the lamp brightness, minimum value is 100 ohms.

Note: LEDs are sensitive to polarity when hooked up.

Typical resistor 680 ohm 1/4 watt.

Transponder equipped decoders must be hooked up with lamp common connected for proper operation.

Installing Lighting Effects

Adding lights to your locomotives can bring an added degree of realism but there are a few things to consider.

Headlight and Rear Light Operation

Automatic headlight reversing: All Digitrax decoders are shipped with automatic reversing headlight operation as the default mode of operation.

Manual headlight operation: If you do not want automatic reversing headlight operation most Digitrax decoders can be set up so that the white lead (forward light) operates on F0 (Function 0) and the yellow lead (reverse light) operates on F4 (function 4) by programming CV61 to a value of 01 (or 03 if TranspondingTM is also enabled).

Additional Lamp and Function Wiring Considerations

Lamps with current draw over 50 mA: For regular 12 to 16 Volt lamps that draw more than 50 mA when lit, we recommend that you put a 22 to 33 ohm 1/4 watt resistor in series with the lamp leads. This will ensure that the lamp "start-up currents" (which can be up to 10 times normal current draw) do not overload the outputs.

Jim, Modeling the Kansas City Southern Lines in HO scale.

  • Member since
    March 2008
  • From: State College, PA
  • 11 posts
Posted by DRGW GUY on Saturday, April 12, 2008 9:11 PM

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

  • Member since
    March 2008
  • From: State College, PA
  • 11 posts
Posted by DRGW GUY on Saturday, April 12, 2008 10:00 PM

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!!!!!!!!!

  • Member since
    March 2007
  • From: St Clair Shores, Michigan
  • 98 posts
Posted by steve58 on Saturday, April 12, 2008 10:03 PM
 DRGW GUY wrote:

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

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.

**** 'course that's just my opinion, I could be wrong ***********

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