Digital lighting problem

[Tim Brien]

I am in the process of working out the wiring for my tender mounted headlight. Obviously, I only wish it to illuminate in 'REV' direction. The problem is that I only have five wires in total to connect the tender to the loco mounted decoder.

Two wires are for track power. Two wires for the speaker leads. Only one wire is leftover to operate the tender headlight. I was thinking of picking up the track power (right rail) from the tender pickup (inside tender) and then, using the remaining wire on the connector, feed the negative return to the decoder function terminal (left rail?). My issue is if polarity changes on the rail during loco direction change, will this affect the decoder, or will status quo remain in place as I would think the decoder 'polarity' would be the same as the track polarity?

 

[muns]

Tim the polarity on the rail in DCC is allways changing due to its nature (AC like).

IMHO you will need another wire.

 

[Cliff George]

I'd use another decoder in the tender, connected only to the rear light and having the same address as the decoder in the loco.

Get a cheap one, possibly function only and also possibly a decoder intended for the smaller scales (but make sure 18v (or whatever yours is) on the rail is in spec for it). I've got a Lenz HO decoder I got second hand with the motor drive part of it not working correctly in mine

 

[ntpntpntp]

You cannot assume that the decoder common is directly linked to a rail. I expect the DCC common is normally derived from the positive side of a bridge rectifier: DCC function common is positive voltage and each function acts as a sink to negative/ground.

If you study the instructions for most decoders they're pretty clear that the decoder and lighting needs to be totally isolated from the pickups.

Personally I'd run another wire to the tender, but as Cliff says a 2nd cheap decoder just to run the rear light is another option.

 

[Philbahn]

May be a cheap HO/OO decoder after all its only a light

 

[Tim Brien]

Thankyou to all. For the present, I think that I will just wire the light directly to the track pickups in the tender. This will give me a permanent rear light. It will be simple enough to get another decoder for the light operation in the future.