Adding a second decoder to a loco to control lights

[Chrissaf]

If you want to connect them together then fit a series 1N4001 diode in each blue to provide electrical isolation. The diodes will prevent each decoder from seeing the other decoder via the blue wires. See drawing below for detail:

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The light coloured bar that is physically on the diode represents the pointed end of the diode symbol on the drawing. Any diode in the 1N4001 to 1N4007 series range will do. The higher the number the higher the voltage supported by the diode, but since the 1N4001 is rated at 50 volts, then any in the range will do. These diodes are rated at 1 AMP continuous current, more than enough for LED lighting.

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Note that the diodes will drop 0.6 volts across them when passing current. This voltage drop is fairly constant and doesn't go significantly up and down with varying current flows. 0.6 volts dropped from the blue power rail is minimal and won't affect lighting operation.

[LMSFan72]

I used diodes as per Chris on some of mine, although, when I have the decoder running it’s own end I normally keep it separate as all I then need is the power feeds to the additional decoder and then  mount it as near to the end as possible for shorter neater wiring

[Gwdriver16]

Thanks both.

LMSFan72, I did notice on your wiring diagram that you had the 2 decoders on separate circuits, but had seen on another site a diagram which showed a common blue feed for both decoders, this diagram does show a diode on each blue wire, I did think that was the reason for them. Chrissaf, thanks for clarifying that the diodes are the solution. 

Thanks both, Rich