LGB DUO coupler system - how it works (on DCC Loco).

[Cobalt6700]

I need to interface with one of the LGB PCBs controlling couplers in the LGB DUO shunting Locos. I did a little investigation into the circuit and thought others might find it useful information.
I’m assuming that all of the DUO system couplers (as in the box with the motor in) work in the same way, however I only have a 23900 to hand.

I have seen options on controlling these with relays (nod to The Shed ) however I wanted to have a look into the LGB PCB and how it works.

The answer to that is - fairly simply with the use of a dual P-Chan MOSFET and a NPN Transistor. I have done a quick and dirty reverse engineer of the circuit and have marked up a photo of the PCB and created a schematic, which are shown below. I have only chosen to show one couplers operation in the following examples, the other coupler is a carbon copy.

It is worth noting that either the ‘UP’ or ‘DWN’ MOSFETs are ‘always’ energised, the power is ‘switched off’ at the end of travel by the switch inside the coupler.

Basis of operation:

• At rest - the ‘UP’ side of the PMos is energised
  • Decoder function is deactivated - floating / ‘high-z’ state, neither pulled high or low.
  • The Base of the NPN is pulled high by its 10K pullup (NPN ON state).
  • The gate of the ‘UP’ PMos pulled low (PMos ON state) by the NPN transistor
  • The ‘direction’ switch inside of the coupler is on the ‘ready to lower’ side, with the path from the ‘UP’ side of the PMos broken and the ‘DWN’ side of the PMos made.

• DWN
  • Decoder function is activated - this switches the output to GND (low).
  • The Base of the NPN is pulled low by the Decoder (NPN OFF state).
  • The gate of the DWN PMos pulled low (PMos ON state) by the Decoder.
  • The gate of the ‘UP’ PMos pulled high (PMos OFF state) by its 10K pullup.
  • The ‘direction’ switch is in the ‘ready to lower’ state, which allows it to drive down When it reaches the end of its travel, the switch is on the ‘ready to raise’ side, with the path from the DWN side of the PMos broken and the ‘UP’ side of the PMos made.

• Back UP
  • Decoder function is deactivated
  • The Base of the NPN is pulled high by its 10K pullup (NPN ON state).
  • The gate of the UP PMos pulled low (PMos ON state) by the NPN transistor
  • The ‘direction’ switch is on the ‘ready to raise’ side, which allows it to drive up. When it reaches the end of its travel, the switch is on the ‘ready to raise’ side, with the path from the ‘UP’ side of the PMos broken and the ‘DWN’ side of the PMos made.

Think that covers it