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Light resistor/wiring question when installing a DCC decoder

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Light resistor/wiring question when installing a DCC decoder
Posted by leejax01 on Tuesday, February 26, 2008 8:05 PM

 

I am finally going to try and wire in a Digitrax DH163AO into an Athearn SD70M. I know that I can use a DDH165AO and avoid the resistors all together, but I am saving that one for another project which includes a Soundbug sound decoder.

I went to Radio Shack and bought different ohm rated 1/4 watt resistors from 220,330 and 470 ohms.

1. Which one do I use for the Athearn Genesis lights? I heard that they do burn out easily.

2.Which way do they go in relation to the board and the bulb? The color rings are gold on one end and the other ends are red(220 ohm), orange(330 ohm) and yellow(470 ohm).

3. How many lights can I wire to 1 resistor? I have 2 sets going to the front directional lights, 2 sets going to the ditch lights and 2 sets going to the rear directional lights.

Any help is greatly appreciated. I did look at the manual and the Digitrax DCC look, but I could not find any details or pictures relating to the resistors.

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Posted by jim22 on Tuesday, February 26, 2008 8:16 PM

I am not familiar with that engine, so I can't tell you which resistors to choose, but I think I can help with some of the questions. 

Resistors are not polarized; you can install them in either direction.

You should use one resistor wired in series with each individual bulb.  It does not matter which lead you install it in.

If these are 1.5v bulbs, the resistors may get quite hot.  You may need 1/2 watt resistors, and then you need to be careful how you mount them so they don't melt the shell.  Some quick math gives me 330 Ohms at about 1/3 Watt.

If the decoder does not have onboard resistors, I always replace the bulbs with 14V 30ma bulbs.  These do not need resistors.

Jim 

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Posted by leejax01 on Tuesday, February 26, 2008 8:30 PM
I thought that the resistor needed to be placed on the blue common side(+)? Not sure as I will need to check out the book. The Digitrax booklet said 1/4 watt and I went from there. I see that Atlas has the resistors in their locos and they are in heat tubing, so can I assume that with the correct resistor that it should be cooler?
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Posted by jim22 on Tuesday, February 26, 2008 9:16 PM

You should download and read that section of the manual. 

The reference in the booklet to a 22 Ohm 1/4 watt resistor is for high voltage bulbs (i.e. 12v) drawing more than 80ma, and I'm thinking that really protects the decoder from the lamp inrush current.  Inrush current is a quick spike of current that flows before the filament reaches operating temperature, and can be many times the steady state current.  Since this current only lasts for a few milliseconds, the power rating of the resistor is not a concern.  This resistor could be a single resistor placed in the common lead to all bulbs.

If you have 1.5v bulbs, it does not matter where you put the resistors or how you orient them, but you should have a seperate resistor for each lamp, not a single resistor in the common lead.  If you use a common resistor and a bulb burns out, all the other bulbs will get a higher voltage, burn hotter and burn out faster.  If you have seperate resistors, a burned out bulb will not affect the voltage on the other bulbs. 

Defineatly consider changing the bulbs to 14v 30ma bulbs.  Then no resistors are necessary unless you have more than 2 bulbs connected to a single function output.  If you do have more than 2 bulbs connected to a single function output you should install the recommended 22 ohm 1/4 watt resistor in the common lead.

Note that LED's do not have this inrush problem. They need separate voltage dropping resistors just like 1.5v lamps but a higher resistance value because they use less current.  They are gentler on the decoder function outputs.

Jim 

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Posted by leejax01 on Tuesday, February 26, 2008 11:00 PM
From a point of overall price, it is becoming a problem as the DH165AO is close to the price of the Digitrax DH163AO and without the resistors and bulb replacement. I do have the book/instructions and I did read them, but it seems a little vague and I actually got the blue common side resistor placement from Digitrax tech support. What is the rating of the Athearn Genesis stock light bulbs as they are the ones that I will use. Thanks.
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Posted by pkeppers on Wednesday, February 27, 2008 7:04 PM
I have a genesis f unit with 1.5 volt bulbs.  They are 15ma so I would check to see if yours are 15ma.  If they are you most likely need a higher value resistor, something like 680 ohms.  The resistors you listed would be for more like 30ma bulbs and if used with a single 15ma would burn it out quickly if the function output is 14 volts which is a common number.  Use a 1.5 volt battery and a multimeter to check the ma of the bulb.
Modeling the NP over Stampede Pass in the mid 50's
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Posted by jim22 on Wednesday, February 27, 2008 8:20 PM

Assuming a resistor for each lamp, and dropping 12.5 volts at 15ma, the resistor value should be 12.5/.015 = 833 Ohms.  The next higher standard value is 910 Ohms, which should probably work ok.  The good news is that less current means the resistors will dissipate less power and heat up less.  You won't need the resistor in the common lead with resistors for each bulb.

You have 6 bulbs.  If you were to use a single resistor in the common lead to drop the voltage for all 6 bulbs, it would carry 6*.015 = .090 Amps.  You would need to reduce 14 volts to 1.5, or drop 12.5 volts accross the resistor.  12.5/.09= 138 Ohms.  Next standard value is 150 Ohms.  This will work if all bulbs are on, but lets say two of them burn out, or are shut off.  Now the current through the resistor is only .06 Amps, and the voltage drop is only 9 volts.  The lamps will see 5 volts, and will probably all burn out simultaneously.  This is why you need seperate resistors for each bulb.  By the way, that one 150 Ohm resistor would dissipate 12.5*.09 = 1.1 watts.  You would need a large resistor with proper heat sync to the frame to prevent melting the shell.  14v bulbs are looking like a better option, yes?  Smile [:)]

Jim 

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Posted by pkeppers on Thursday, February 28, 2008 10:23 PM
I dont know what size the bulbs are in the original posters engine.  The genesis f units have 1.5 v 1.2mm bulbs.  I havent seen 14 volt bulbs that size.  14 volt bulbs also put out a lot of heat which may not work well if the bulb is in an enclosed area like it is in the f unit.  I have been replacing my 2 bulb genesis headlights with a single LED.  Not perfectly prototype for some of them but they are really good looking, last forever, no heat, etc.
Modeling the NP over Stampede Pass in the mid 50's
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Posted by GdennisEdgar on Friday, February 29, 2008 12:09 AM

Hi It does not matter which way round you connect the resisitors.  The gold line is just the tolerance of the resistor.

What you need to do is reduce the voltage to the bulbs.  A 220 ohm resistor should work.  If you want to add lots of lights you must use a higher wattage resistor.

 

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Posted by leejax01 on Friday, February 29, 2008 8:29 PM

I tried the decoder install with a Digitrax DH165AO. It was advertised as a decoder that required no resistors. Well, I found out that it only applied to 1 bulb in the front and rear. If you had more bulbs, then you will have to use F1,F2 etc. Also, 2 bulbs can't be used on the front/rear pads like they were on the factory board.

I wired the ditchlights to F1 and F2 pad with 1K resistors to them and the other wire to blue. No lights and I tried 470 ohm resistors and still no lights. So I called Digitrax and the rep told me to cut the jumper to allow 30ma to go to the directional lights as thay are wired 2 to the tabs. Well, I should not have done that as they went out after 4 seconds. I guess that I can wire the bulbs in series to not allow that to happen?

Back to the ditchlights. I tried them to track power with the resistors still on and they blew out. At this point, I am out 6 bulbs so far and the decoder putting out 30ma to the directional lights. I really want to use the DH165Ao so that I can add sound later on. I will probably take it to the club and see where I went wrong. I decided against high volatage bulbs as they generate alot of heat. I will find out the correct resistors for the bulb configurations. I will call Digitrax as to find out where to go from here as tey told me to cut te jumper and said that it will not void my warranty.

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Posted by jim22 on Friday, February 29, 2008 9:14 PM

Sorry you're having so much trouble.  Hopefully there are some experienced folks at your club who can help.  Most of your difficulty stems from not knowing how much current and at what voltage the original bulbs took.

Yes, the DH165A0 has current controlled outputs for the headlight and tailight.  With the jumper intact, each is supplied 15ma.  With the jumper cut, that is doubled to 30ma.  If your bulbs only took 15ma, the decoder supplied them too much current (by raising the voltage) and blew them.

Yes, only those two function outputs are current controlled.  F1, F2, F3, and F4 are simply off (0v) or on (14v).  For these, you either need 14v bulbs or you need to know how much current your 1.5 volt bulbs take at 1.5 volts and install a resistor in series with each bulb to drop the reamaining 12.5 volts.

Once you get suitable bulbs and resistors figured out, you can wire a pair of bulbs, each with it's own resistor, in parallel between FOF and common.  Similarly, you can wire a 2nd pair, each with it's own resistor, in parallel between FOR and common.  If you use 14v 30ma bulbs, you do not need resistors.  Just put the 2 bulbs in parallel at each end of the engine.

For the ditch lights, you can wire one bulb each, either 14v or 1.5v with a suitable resistor, to the F1 and F2 outputs.  It is possible through CV settings to get the ditch light effect from these function outputs.  It is not necessary to use the throttle function keys to control the ditch lights.  This is significant because you will probably want F1 and F2 on the throttle to be used for bell and horn sounds in the future.

Jim 

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Posted by leejax01 on Saturday, March 1, 2008 6:20 PM
Good point on the F1 and F2 functions as I do plan on adding the Soundbug to the DH165AO in the future. I went to the club and we could not find a meter, but I am getting some direction You ever heard of a resistor decade box? It sets various ohm variables by turning a knob. Does anyone have one or know where I can find one?
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Posted by jim22 on Saturday, March 1, 2008 6:50 PM

A resistor decade box is a box with a bunch of resistors in it and several 9 or 10 position rotary switches.  The idea being that you can dial in whatever resistance you need for a test circuit.  You can do something similar with a variable resistor, called a potentiometer, and an Ohmeter.  I do not think you need a decade box.  What you do need is a meter that can measure the basics: DC voltage (in volts), DC current (in milliamps) and resistance (in ohms).  Meters like this are very inexpensive and can be purchased at places like radio shack.

Before any of this does you any good, you need a basic understanding of electricity, including the use of Ohms Law, which relates voltage, current, and resistance of a simple DC circuit.  I think you have 3 choices, one of which you MUST make.  (1) Get someone who is knowledgable on such things to install your decoders for you,  (2) Follow the advice given which keeps you safely away from the tricky stuff, or (3)  do some studying, some experimenting, and teach yourself how the tricky stuff works.  Maybe there's a 4th choice: purchase ready to run equipment.

You said there are 2 headlight bulbs and 2 tailight bulbs in this engine.  Are they both for the light, or is the second one for light boards?  How do you want them to behave?   Typical headlight/tailight are on in the traveling direction, or maybe bright in the traveling direction and dim in the opposite direction.  I set my lightboards to be on whenever the directional lighting is selected using F0 on the throttle.  In this case, a single headlight was wired to FOF, a single tailight to FOR, front lightboard to F1, and rear lightboard to F2.  That would leave you F3 and F4 for the ditchlights.  I did all mine with 14v 30ma bulbs and no resistors.  Lower voltage lights could be used with resistors, but you need to know the lamps current draw when powered by a 1.5 volt supply to calculate the appropriate resistor value for a 14v supply.

I have not had any shell overheating problems, but that might be a good reason to use 1.5v bulbs or even LED's.

Jim 

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Posted by leejax01 on Saturday, March 1, 2008 10:33 PM
I bought a meter, but I can't find it for my life. I got a run down today and I am getting the hang of it. I will get new bulbs and buy another meter and take it to the ckub and go from there. I will post back when I get some more direction. Thanks for the reply.
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Posted by Arjay1969 on Monday, March 3, 2008 10:21 AM

You might also think about using the TCS A6X decoder.  It's a bare board drop-in replacement for the circuit boards on Genesis, Kato, Atlas, etc.

 

It also has the advantage of having a 1.5V voltage regulator built in for the lights, so that it will put out 1.5v REGARDLESS of the input voltage.  I have a few of these in my locomotives, and they are fantastic.

My 2 cents [2c]Smile [:)]

Robert Beaty

The Laughing Hippie

-----------------------------------------------------------------

The CF-7...a waste of a perfectly good F-unit!

-----------------------------------------------------------------

Then it comes to be that the soothing light at the

end of your tunnel, Was just a freight train coming

your way.          -Metallica, No Leaf Clover

-----------------------------------------------------------------

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Posted by jim22 on Monday, March 3, 2008 9:51 PM

That looks like just the ticket!  I like TCS decoders, maybe because they have sent me a few free ones in exchange for submitting installation photos.  Unfortunately, I accidentally allowed the headlight output to short on one recently and smoked that function.  I could send it back for a replacement, but I don't see asking them to take the hit for what was clearly my fault.

Jim 

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Posted by leejax01 on Tuesday, March 4, 2008 10:39 PM

I got it to work. I ended up using a DH163AO instead. I was given a  meter to double check everything. I bought 6 new bulbs as I burnt out the original set. I used 1/4 watt 1K resistors from Radio Shack and some 1.5V 12ma bulbs from Model Power as that was all the lhs had. I wired the ditchlights to F1 and F2 so that they can alternate when activated and everything works great.

I saw the TCS decoders and are their resistors like the Digitrax ones in the DH165AO? How many bulbs can be used on 1 function like the stock Athearn set-up? Thanks for the replies.

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Posted by Arjay1969 on Tuesday, March 4, 2008 11:05 PM

The TCS decoder doesn't use resistors to drop the voltage.  It uses a voltage regulator to keep the output voltage at 1.5 (actually, I think it's closer to 1.3) volts, regardless of what the input voltage is.  The downside to using resistors is the fact that if you take your locomotive to another layout that has the track voltage set higher or lower than what you run at home, the lights will be brighter or dimmer. And of course, brighter means they'll fail more quickly.

I've had no problems with putting two bulbs on one function on these decoders.  If I'm remembering correctly, each function is rated for 120mA, so you could conceivably put 3-4 30mA bulbs on each one.  You'd be better off limiting it to two, though, and putting other bulbs on separate outputs and remapping those to the ones you want controlling them. 

Hope it helps! Smile [:)] 

Robert Beaty

The Laughing Hippie

-----------------------------------------------------------------

The CF-7...a waste of a perfectly good F-unit!

-----------------------------------------------------------------

Then it comes to be that the soothing light at the

end of your tunnel, Was just a freight train coming

your way.          -Metallica, No Leaf Clover

-----------------------------------------------------------------

  • Member since
    January 2008
  • 116 posts
Posted by leejax01 on Wednesday, March 5, 2008 1:10 PM
I was looking at the TCS decoders before and they seemed interesting. Which ones work for the Genesis line of locos? I assume that they work for the non-DCC ready RTR locos also after the motor is isolated and a wire is ran to the top clip?
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Posted by Arjay1969 on Wednesday, March 5, 2008 2:28 PM

The A4X and A6X decoders are both drop-in replacements for the Genesis boards.  The difference between the two is 4 vs 6 functions (A4X=4, A6X=6), plus the A4X does NOT have the voltage regulator, so you'd have to use resistors for 1.5v bulbs.

As far as using them in RTR or BB Athearns, absolutely!  I have them in my BB FP45's, and they work quite well.  You have to make sure they are insulated from making contact with the motor or frame,  but other than that, they install the same.  Oh, and just like their Digitrax cousins, all of the function outputs have "special effect" settings you can use.

 

Can you tell I REALLY like these decoders? Smile [:)]

 

(And no, I'm not affiliated with TCS in any way, shape, or form.  I just like their products!) 

Robert Beaty

The Laughing Hippie

-----------------------------------------------------------------

The CF-7...a waste of a perfectly good F-unit!

-----------------------------------------------------------------

Then it comes to be that the soothing light at the

end of your tunnel, Was just a freight train coming

your way.          -Metallica, No Leaf Clover

-----------------------------------------------------------------

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