Advanced DCC Topics

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Excellent resource site:

Mark Burries' web site: https://sites.google.com/site/markgurries/home

Premature or unexpected booster shutdown:

I've lifted some text found on a forum by Bob Grosh.

The basic point is that when using the DCC power or the output of a decoder for input to a regulated power supply, excessive filter capacitor size can create large current spikes as the DCC signal changes polarity. This can be interpreted as a short by the booster, resulting in shutdown.

Any modern decoder that is capable of delivering track voltage without a lot of loss will have Schottky Diodes in the bridge rectifier. Any such decoder can not be used in a locomotive with large filtering capacitors connected directly to the decoder's bridge rectifier output.

These components cause current spikes at every DCC signal transition through zero volts.

Any decoder that uses Schottky diodes have a very low forward resistance. These are used to reduce heat in the decoder and provide better control of the loco. They can deliver much higher current to the loco. To use the decoder output, you must follow a simple rule, limit inrush. Capacitors should never be connected directly to a decoder output. Schottky diodes will deliver enough current to instantly shut down most brands of boosters. Regular diodes, because of their higher internal forward resistance still SPIKE the booster but not enough to shut down the boosters. BUT THEY STILL SPIKE the booster if there are capacitors connected directly to the decoder. The spikes from multiple locos will combine. THIS SEVERELY LIMITS THE NUMBER OF LOCOS THAT CAN BE RUN ON A SINGLE BOOSTER TO WAY FEWER THAN WHAT THE BOOSTER SHOULD HANDLE.

Bob recommends using a twin "T" filter to limit inrush. Going to a Twin "T" filter circuit also reduces the size of the capacitors needed.

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