DCC common negative

[JGPhoto]

Hi all,

 

Just a quick one, if I have more than one decoder in a train can the current from an accessory go to any of the common negatives or does it have to be the same decoder as the current originated from?

 

James

[Only-Me]

The dcc common is positive matey! Its the blue wire.

Yellow white green purple etc are negative...

In theory you could power the device from any common positive but its the negative wire that is turned on and off by the decoder to create the circuit

[Grovenor]

Decoders don't have a common negative for the accessory (function) outputs, they have a common positive which is the blue wire.

As the blue wires are the positive directly off the bridge rectifier and all source power from the rails you can use any blue wire from any decoder.

The only downside is that it will make it a bit harder to keep track of the load you are putting on each of the rectifiers to make sure you keep below the maximum current rating.

Regards

[JGPhoto]

Sorry guys, half asleep as I'm writing this, I know it's common positive and that's what I was meaning when I wrote the post...

 

The post originates from a DMU I'm working on and have added conductive couplings to. They have four connections, I'm going to use two for track current, one for interior lights and one for orange obstruction lights.

 

As it's only a 2 car so I hope there will be no issue with current draw, but if there is I can change to 4 function decodes and just have track current going through the cars.

[Phil S]

The 6-pin decoder design uses chassis return for lighting, and Lenz instruction booklets for their 8 pin decodeers I think also show this option - it has consequences on brightness, and analogue compatibility, as I recall.   These are downloadable from their website. 

Because itas 'return current' is no longer through the full-wave rectifier in the decoder, it is possible that it may not measure the current taken accurately, and as a result some over-current protection might be lost,

This method has also been used for lamps which are wired directly to the chasssis on one side - as for example some older Fleischmann EMUs  (Older examples will usually be USA or Conitnental, as few British models had any lighting at all!. } 

 

However, the electrical connection has usually been '100% reliable' because it is WITHIN the single loco/coach/hard-coupled unit .... you ALSO NEED TO CONSIDER with wiring passed through to adjacent coaches WHAT HAPPENS WHEN they become disconnnected, or you have to remake the connection (eg after a derailment) and if the track power is on .... what risk is there of mis-connection if the wrong poles on the 4pin plug make contact (as is possible on the Roco coupling, for example).  Some of this can be protected by adding diodes, and choosing which side of the connector to place the current limiting series resisror for LED lighting.

 

With the price of some conductive couplings, it is cheaper to buy a separate decoder for the 2nd coach !!  I have tried a mixture of both methods:  2 pins for the track is often a good idea, so as to ensure the best pickup ... but this may affect any track-circuit detection you use.   (Marklin in '3 rail' actually SWITCH which centre-pickup shoe is used according to the direction of the train, so that it is always the front collector!) .  Without passing track pickups, 4 poles can give Forward, Reverse amd Interior Lighting with a Common Return.

 

I have also tried 2 3 and 3 pin JST plug/sockets (as per Hornby loco-tender connections, and available on Ebay) - but these are additional to the motive-coupling 8-(

 

Adding to Grovenor's comment about the commonality of the blue wire electrically between decoders .... do consider what happens when the train runs into a reversing section or another power district, or braking section etc etc.

[AMJ]

I can understand the logic in power coupling as I had done this with the DLW. I still use 2 decoder but let each take the load for each vehicle. Power coupling ensures both cars keep signal if one loses it.

 

Trailer car has cheap as chips decoder power car has sound.

 

Best to put both on programming track together as they will get same address. I had seen some fail in trailer as no load stops it changing address. Together there is a motor load