Decoder help

[Deadalus]

Hi everyone 

I have started converting to DCC a few months ago.

I had a few questions regarding the hornby 4 function decoder.

I have a flying scotsman(A3) which I have fitted with a R8249 decoder.

It has three coaches which I would like to add lighting to and also like to have a firebox light along with lights for loco and steam.

I would like to know the following:

1) Will the function support lighting in all three coaches. i am not using readymade kits. I am       planning to use filament bulbs three per coach. Or should I go for LEDs

2)I checked the net and found that the purple wire was for a second function and generally        used for steam. So guess I will use it for that.   

3)As it is a steam loco it does not have directional lighting. So I would like to know if I can put    a diode rectifer to power the loco lights (one for cab and two in front). Or instead should I       combine the directional function and other function of the coaches along with the firebox.

I would also like to confirm if this decoder can be used on DC layout with the CV set to allow DC operations.

Thanking in advance.

[96RAF]

I would arrange your coach lighting to be separate from the loco and if you want to have it/them decoder controlled then install the decoder into the rake. See the articles page of my signature linked website for installing lighting in coaches.

 

Directional lighting on F0 (white and yellow wires with common positive blue wire can be used for anything you like but it will be directional. For a steam loco you would probably only have the forward directional lighting connected unless the loco was running light with a red lamp on the back. If running in reverse at night it would swap the red lamp for a white one, so more difficult to model prototypical lighting on steam.

 

Green wire in conjunction with blue common pisitive wire on F1 can be used for anything you want to switch on/off so cab light or firebox glow, etc.

 

Purple wire in conjuction with blue common positive wire on F2 can also be used for anything you want to switch on/off. You mention using this for coach lighting but the decoder limit would be stretched trying to light a rake of coaches with 3 x lamps each via this function. 

 

The thing to watch with those functions is to keep the current draw within the limits shown on the decoder instruction sheet, else you can damage the decoder and for this reason you should look at leds as opposed to filament bulbs.

 

You are correct in assuming you can combine function outputs but with a simple decoder like the R8249 you cannot map them to operate as one, so you would have to individually select both/all to share a heavy load.

 

The decoder can safely be used on DC but the functions will not work as you have no way of selecting them.

 

I hope this clarifies things somewhat for you.

Rob

 

[Chrissaf]

DO NOT USE FILAMENT BULBS they draw far too much current. Subject to the exact bulb spec used, this could be as much as 60/70mA PER BULB. This is potentially more than 200mA for three bulbs. Most decoders are only rated at 100mA MAX on a function output.

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Use LEDs, a carefully designed LED coach lighting circuit (with say 6 or 8 LEDs) can function quite adequately on less than 20mA. Remember that LEDs require current limiting protection resistors in series with them, else the LED will immediately blow. LED protection resistors are typically 1,000 ohms [ 1KR ], but can be significantly higher and still have adequate LED illumination.

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Regarding your question on DC Operation. Yes the R8249 decoder supports DC Operation if Bit 2 of CV29 is enabled. But as Rob has pointed out, the coach lighting will not work on DC if you use a decoder function output to power them.

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In terms of design, your post seems to be inferring that all the lighting in the three attached coaches will be fed from the decoder in the loco. Rather than just having individual electrical pickups from the coach wheels. That is to say that the coach lighting will be decoder controlled rather than 'always on'. If this understanding is correct, then (purely as a rough guide) this is the kind of circuit you will need. The decoder Green wire could also be used instead of the Purple wire if needs be.

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Postscript: Personally, I would (and have done) fit 'always on' LED coach lighting into each individual coach and use pickups on the coach wheels to feed a bridge rectifier with smoothing capacitor on each coach. The difficulty of extending wires from the loco to a whole rake of coaches should not be underestimated. Review the recent thread on 'Half Insulated' Coach wheels for information regarding suitable coach lighting wheel sets.