DCC Topics


DCC auto-reverse and cutout module

I always wanted to make an easy to operate layout that I had not to think about the track polarities. Now I'm sharing with you this circuit that acts as a circuit braker for DCC operations and also is capable of reversing the loop polarity for smooth trains operations. I wanted to make it operate in DC and DCC but it was imposible to make the Mosfets work on both systems.

This circuit is completly based on the www.merg.org site circuit, but I recoded the software for it to be in C language that is easy to mantain and to make upgrades.

I tested the circuit in benchwork and it is working fine. I just finished the PCB and mounted the components. I now can make some more tests

One enhancemente I'm planning for this circuit is to sense the direction where the train enters the loop and when it's going out, the circuit will throw the switch points so there is no human interaction there.

The principle

This text was taken from the www.merg.org site. I just reproduced and recoded the principle of operation using my own interpretation:

The module is fed with the booster output and connects between the booster and the track section. The rectified DCC signal provides power for the module. Track current is sensed with a low value resistors in series with the track feed. For effective auto reversing the module must sense a short circuit in either feed and be able to isolate both feeds. The voltage developed across the sense resistors during a short is sufficient to turn on the transistors Q1 and / or Q7.

All the necessary timing is provided by a cheap 8 pin PIC (12F683). This obviated the need for a considerable amount of conventional logic and monostables. The PIC continually scans for an overload condition. When this is sensed the DCC output is switched off. Each ‘switch’ is two series MOSFETs which carry current in both directions and have virtually instant action. The PIC then reverses the relay, waits 25 milliseconds for the relay to change , then reconnects the track. Disconnecting the track while the relay changes is essential to prevent the main booster from tripping. If the short persists, the PIC disconnects the track again and retries at 0.5 second intervals – without again reversing the relay.

If the short has cleared for about two seconds, the sequence is reset and the next short will again reverse the relay first. This prevents the relay changing on every retry. As the ‘switch’ is purely electronic, it is much faster than any booster and the retry periods are also very short. So short that decoder-fitted locos on other sections are totally unaffected even though, during the retries the main booster is also effectively shorted. Mike Bolton (Merg group)


The circuit

Download the Schematic file in PDF.


The microcontoller sofware is available upon request. (epareja2 @ gmail dot com)