How do reverse loop modules work?

[Trainman440]

Hi, I was just wondering, in a reverse loop module, the module flips the track's polarity when it is running. 

How does it do this without interuppting the train that is running?

Shouldn't the train stop/short when it is being reversed?

Thanks

Charles

[mlehman]

What you're discussing is for DCC. The DCC loco takes its command on direction from the decoder. It switches the polarity fed up to it from the tracks automatically to maintain the commanded direction.

Doesn't work on DC.

[G Paine]

A reverse module works on the same principle as a DCC circuit breaker, like the one formally called Powershield. One end of a reverse loop is always at cross polarity to the main line; a train crossing this would trip the DCC system on a short circuit fault.

When a reverse module detects a short circuit, is changes the polarity of the reverse loop track. This is no problem for the decoder; it does not care about track polarity, so the change is seamless as far as operations go.

[Trainman440]

So if I quickly change my track's polarity, it won't stop the engine when it is running?

[floridaflyer]

If you are running DCC, no. If you are running DC the loco will change direction.

[gregc]

Trainman440

So if I quickly change my track's polarity, it won't stop the engine when it is running?

by polarity, you mean the voltage on the rails of the reverse section relative to the mainline tracks at either end of the reverse loop.

DCC communicates with the decoder by constantly changing the polarity of the track voltage, the time between the polarity reversals represents a 0 or 1.    The decoder uses a bridge rectifier so that it has a constant voltage for power.   The decoder detects the transmitted signal before the bridge and can ignore corrupted packets of data.  The decoder uses an H-bridge  to control the voltage polarity across the motor.

when an DCC auto-reverser detects a short, it temporarily (~25 msec) disconnects power to the reverse section before reversing the connections between the reverse section and mainline tracks, possibly using a DPDT relay.   Momentum allows the motor to continue to rotate without stopping and capacitance in the decoder provides power to the decoder.

[Trainman440]

Okay, I get it now. 

Thanks!