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DMU inter car connections


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My Bachmann Cl150 has a 2-pin connection on the drawbar for both head and tail lights on the dummy car.

Do any of the electrical engineers on here have an idea how that is done?

I'm guessing that it is a series of diodes so that the polarity reverses depending on the direction. But can anyone work out the wiring diagram for it in the powered and dummy cars please?

Seconds question, if I was going to do night/day headlights, interior lighting and central locking lighting on one decoder plus bringing the track pickups from the dummy car back, would I be able to save on interconnection pins in any way using these techniques?

Edited by Clay Mills Junction
typo
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Having a look online I can't seem to find an image of the dummy car with the body off. But in its simplest form, the wiring for the lights would look like this. if the lights are controlled buy the decoder my best guess would be that the connection to the locomotive would end up going to the pcb and then to the decoder. If they are dumb then the connection to the locomotive would most likley just go to the pickups. Not having one nor anything similar I couldn't tell you exactly how it works but this would be my best guess.

train.thumb.png.90c98126653f6b87d2a59285e18a0096.png

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Thanks Archie.

The unpowered car is fairly simple then.

I should have specified wiring diagrams for both powered and unpowered cars. I'll try to open up my own at some point and take a picture of the PCB if that helps.

For DCC you have a common positive (blue) and switched negatives head (white) and tail (yellow) which is reversed at the other end. Somehow that common positive has to go to both sides but the opposite to the switched negative that is on?

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Having a deeper dig this diagram from this RM web thread seems to show how be in a simple form the lights would connect to the decoder, unfortunately I think there is a little more to it because, as you mentioned, there is only two connections between the cars of the DMU. It would be useful to know, do the lights activate as the result of one function on the decoder (ie - does one function activate the lights for both the front and rear and change depending on the direction of the train? If they do, if I were to hazard a guess to how it works I would say that the diagram I drew probably still applies and all the processing that takes these three outputs is done on the main PCB in the drive car and then the output is sent over the two connections to the dummy car. I hope what I have said has kind of made sense here.

post-7193-0-86090600-1520965070.png

https://www.rmweb.co.uk/topic/132094-wiring-white-red-smd-leds/

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Thank you.

3 hours ago, Archie said:

 all the processing that takes these three outputs is done on the main PCB in the drive car and then the output is sent over the two connections to the dummy car.

It is that processing on the main PCB in the powered car that I am interested in. I've taken some photos of the top of the PCB in my spare chassis. The CON3 and CON4 on the right are the two feeds for the unpowered car. The light connections for the powered car on the left are the regular Common positive and two switched negatives.

DSC_0840.thumb.JPG.caff1394f1d9e3ebcf15af9569a20448.JPG

DSC_0841.thumb.JPG.eddbf3421de9f170f3682bbb97f22549.JPG

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If I am honest this is a little out of my depth here. If you were to ask on rm web you'd probably get a wider range of replies, with that said it looks like the power is being passed through some diodes before being passed through a series of transistors that then change depending on the direction of travel and the signals of the decoder. This signal then somehow makes it to the lights. Is it possible that there is other components or traces on the other side of the board? from this view it looks like some of the traces terminate at nowhere. Maybe that would show a clearer picture of what is happening.

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I think Archies first explanation is correct.

The "yellow" and "white" wires will be controlling transistors. These will be in a H bridge set up. A H bridge is usually used to control the direction of current through a DC motor to enable it to be run in either direction.

H-bridge.thumb.png.e93d5998701f86c4339d68d0248ddacb.png

In this digram the "yellow wire" would reconnected to Q1 and Q4 and the "white wire" to Q2 and Q3. VCC would be the blue wire and GND would be GND. There will also be some resisters in there to bias the transistors. The fact that function wires in DCC are a path to GND means that the resisters may be in a less conventional setup. But I can't be bothered to work out how. The connection to the trailer car are the inputs to the motor.

The image has been taken form https://www.build-electronic-circuits.com/h-bridge/

 

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Thank you for the explanation Chris, I can follow from that how it would work now.

Unfortunately, it would definitely be beyond me to recreate the H-bridge myself. I'm guessing from what you say that the resistor set up and values would need to be custom to the outputs they are controlling so there won't be an off-the-shelf solution available.

I appreciate the answering of the question though, I had hoped that it would be simpler than that and could be recreated since the interconnects I have are 4-pin which could have carried track voltage one way and lighting the other. I think now I will go with a 4 and 2-pin solution, keeping the track voltage on its own 2-pin with the directional lighting taking three and a spare pin for interior lighting or something.

Thanks both for taking the time to answer.

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OK. How many lights do you want to control? It sounds like just the directional lighting ie forward and reverse?

If that is the case you only need two wires to control it.

Remember that DCC function wires are switchable paths to ground. Normally the VCC is coming from the blue wire, but it doesn't have to come from the blue wire. If you can find a voltage from another place other than the blue wire, you can use it to supply the Anode of your LEDs and their cathodes can be connected to the white or yellow wires via a suitable resistor and your inter-car connector.

You have track voltage in the trailer car. All you need to do is turn the ~14VAC of the DCC supply into a DC voltage for your LEDs. 4 diodes arranged as a bridge rectifier will do that. You could pop a capacitor in there to smooth the ripple off the voltage, you won't be able to see the effect, I don't bother when I use track power for LEDs. Again remember to put a resistor in series with your LEDs to limit the current. Start with a 10KOhm and if they are too dim go smaller, or you could do the Maths and calculate the resistance for the current you require!

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Actually, for LEDs, you don't need to bother with full wave rectification, half wave rectification is fine.

One diode off the black or the red wire will be fine as a power supply in this scenario.

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Thanks Chris, that works far better for me.

I initially wondered if the Bachmann 150 would be like that but there are definitely no pickups on the underside. I will use the supply from track as you suggest but I'm still thinking that I might be best with a separate two-pin for the track voltage just so it is separate and no danger of decoder frying connections being accidentally made. I could then use a 4-pin connector for the 4-function decoder (white, yellow, green and purple) to get head & tail and leave capacity to put interior and central locking lights on later.us

I'm doing my second Bachmann 158 (old model) and also a really old Lima 156 with a CD Motor conversion. The 158 has pickups in the dummy car for the lights to start with and the pickups in the powered car are good enough so don't need the track voltage to come back through. The Lima 156 I'm thinking would benefit from having more pickups spread out over the length of the two cars.

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