Question on Rule 17 Dimming

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This is the second time I am posting this question. The other post seems to have disappeared?

My question is on Rule 17 dimming. I noticed in the Digitrax decoder manual that you can implement rule 17 dimming of the headlights. I have never been able to get this to work. I assume this is where the rear light would be dim when going forward and vice versa.

What brought this to my attention again was when I read this month's Model Railroader. The author had attached a 22K resistor between the forward light and rear light to achieve thes effect.

Has anyone ever successfully turned on Rule 17 dimming with a Digitrax decoder or used the method of applying a resistor between the forward light and rear light to make the lights dim?



Lake Shore Lines

I haven't done one yet with a Digitrax decoder, however rule 17 is set to allow you to dim the lights when either enter a passenger station or meeting an engine coming in the opposite direction. Usually set so that a separate function key is used for diming. I will have to look at the digitrax manual tonight.

Which decoder are you using? I'm presuming the instruction sheet that came with the decoder says it supports Rule 17 dimming; or there's some function available to allow you to do that.



Lake Shore Lines
The Digitrax manual I see on line for any decoder does not go into any explanation. You need to download the Full Decoder manual from their web site. it is about 78 pages long.

I have read the NCE's decoder manual and basically it says to do the following:

1. Set CV33 = 1
2. Set CV120 = 32 (if you want the F4 function key to dim the light or)
Set CV120 = 36 if you want the F8 key to do so.
3. Set CV121 = 2 if you want the rear light to be on in Reverse and off in forward.

These values should also work with a Digitrax decoder.

Hope this helps.

What I Found

Well I did some experimenting and this is what I found. Digitrax has set up CV49 to program the effects for F0 (fwd light). CV50 and up are the same, just repeated.

I set CV49=68, so that Rule 17 dimming is in effect for the fwd light. What happens is that when use turn on F4, the headlight goes out. It does not go dim, it just goes out. The ditch lights remain lit. I'm not really sure what good this is. If I wanted to turn off the lights, I would just turn off F0. Also, the ditch lights remain on, this is not prototypical to have the ditch lights on and the headlight off.

What I was hoping for was to be able to dim the headlight, not turn it off.



Lake Shore Lines
I finally got a chance to review the Digitrax manual. Not an easy document to read. Unfortunately, what you tried is what they talk about. I would suggest that you call Digitrax tomorrow and ask them why it isn't dimming.
Another Solution

Here's another solution, although it is not very elegant. This would require an electrical mod to the loco wiring. The F4 function could be used to dim the forward light. Although with this idea the function of F4 is reversed. With F4 on, the light is at full intensity, with F4 off the light is at half intensity. I am beginning to see why Digitrax may not implement the Rule 17 dimming properly. They may not want to put an extra resistor on the decoder to disipate the power to the lamp to dim it.

I've had no problems with Rule 17 dimming on Digitrax decoders. My loco with a DH163 as you have has ditch lights and Rule 17 lighting, no problems. Are you using the right number base? The CVs are given in hexadecimal, but the programmers typically work in decimal. You have to convert. I have CV49 set to 104 (base ten) and CV50 set to 120 (base ten). 104 (base ten) is 68 (base sixteen). This gives dimming in reverse direction, or with F4 on.

As far as I know, the ditch lights don't respond to F4, which is rather silly. If you only have ditch lights on the front of the consist, you can simply reverse the direction to turn off ditch lights and dim the headlight.
I are an electrical engineer, so I always work in hexidecimal. I programmed CV49 = x68 (hex), just as you have done. There is one thing that occurs to me, that I might have my voltage dropping resistor too large. I think they are 470 Ohm. The lights on my locos are not that bright. I did this to conserve the bulbs. Most run at around 1.4 VDC. Perhaps the dimming is really working, but the dropping resistor is too large and I cannot see the light being lit. I could try reducing the resistor to something less like 440 Ohm (2 - 220 Ohm resistors in series). I'll put a voltmeter on it when I get around to opening the loco.

I did more investigation and discovered that when I activate F4, the lights really are dimmed. They are just so dim that you cannot see them except in the dark (and then just barely). I'm sure this is due to the fact that I run my lights with a little less than 1.5 VDC. I'm sure if I reduced my dropping resistor size to icrease the bulb voltage, they would be visible when dimmed.

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