Head and Tail Lights on Older Bachmann Multiple Units

[Gray69]

I get the feeling that I am missing something obvious, but I can't think of the answer.

On the Bachmann DEMU, 150 and others, the coupling bar between coaches has 2 through power connections.

I have been fitting head and tail light kits to a Bachmann class 166. To save having to add decoders to each end I wanted to run the wires required for the lighting functions through the coupling using a Viessmann 5048 coupling, which has 2 wires. It only dawned on me once I had the coupling that there are 3 wires required, the white, yellow and blue.

So how does Bachmann operate the head and tail lighting on the trailer with only 2 connectors, and is this something that can be replicated.

[H2O]

57 minutes ago, Gray69 said:

I get the feeling that I am missing something obvious, but I can't think of the answer.

On the Bachmann DEMU, 150 and others, the coupling bar between coaches has 2 through power connections.

I have been fitting head and tail light kits to a Bachmann class 166. To save having to add decoders to each end I wanted to run the wires required for the lighting functions through the coupling using a Viessmann 5048 coupling, which has 2 wires. It only dawned on me once I had the coupling that there are 3 wires required, the white, yellow and blue.

So how does Bachmann operate the head and tail lighting on the trailer with only 2 connectors, and is this something that can be replicated.

I don't know how Bachmann do it but it could be done by having the 2 LEDs in inverse parallel.  Current going one way lights up one LED  current going the other way lights up the other LED.  Two protection resistors may be needed, one in each 'arm', value of each based on the different forward voltage, current drawn and brightness wanted from each LED.

Edited April 5 by H2O

[Nigelcliffe]

"Blue" is positive volts.   

 

You can also get the positive from the track, though (unless you fit a rectifying circuit) this will be half-wave power.   At its simplest, just connect the pickup on one side of the trailer car to the positive side of the LEDs .  

 

( This is how all 6-pin decoders work, and its documented in many of the more thorough decoder maker's manuals  ).  

[Gray69]

Thanks Nigel, I have never had anything that uses a 6 pin decoder so was unaware of the method of operation.

 

 

[RHF9019]

On 05/04/2024 at 09:00, Nigelcliffe said:

"Blue" is positive volts.   

 

You can also get the positive from the track, though (unless you fit a rectifying circuit) this will be half-wave power.   At its simplest, just connect the pickup on one side of the trailer car to the positive side of the LEDs .  

 

( This is how all 6-pin decoders work, and its documented in many of the more thorough decoder maker's manuals  ).  

 

Taking this a step further, if you were to put a diode on the red wire and a diode on the black wire, effectively half a bridge rectifier, and connect the blue to the output, would this give full wave power? This can then connect to the positive side of the led.

 

Then is it possible to use the white and yellow on the negative side of the led to control them via the decoder?

Edited April 8 by RHF9019
Clarity

[Nigelcliffe]

55 minutes ago, RHF9019 said:

 

Taking this a step further, if you were to put a diode on the red wire and a diode on the black wire, effectively half a bridge rectifier, and connect the blue to the output, would this give full wave power? This can then connect to the positive side of the led.

 

Then is it possible to use the white and yellow on the negative side of the led to control them via the decoder?

 

Works but usually unnecessary.   

 

Simply connect the LED+resistor negative to the decoder output (white or yellow, or whatever) and the LED=resistor positive to one of the track pickups.