DCC "keep alive" systems

Overview

From the inception of DCC, interruption of power to the DCC decoder has caused issues. In the beginning often the decoder would reset, and this was especially problematic with sound systems.

Even the briefest of interruptions would cause this, and has led to the popular "old wives tale" that DCC only works on perfectly clean track, and cannot be run outdoors. Certainly this was often the case 20 years ago.

Over the years, many decoders have a small capacitor storing charge to run the on board microprocessor in case of brief interruptions. Usually this just prevents reset of the decoder, but lights, sounds and motors stopped.

Later, larger capacitors allow continuous operations over brief interruptions.

But as people desired more protection (or had poorer track pickups or other issues), the "need" for a "keep alive" power source appeared. This power source can allow much longer interruptions of power.

Types of Keep Alive systems

The basic idea is a "bank" of capacitors to charge up, and supply power if track power to the decoder is interrupted. To keep things compact, "super caps" are typically used, and they do not have a high enough voltage rating so that one capacitor can do the job alone. To raise the working voltage level, you put capacitors in series, the working voltage is then the sum of the voltage ratings of the individual capacitors. But, as you put capacitors in series, the capacity is reduced proportionally.

Therefore, capacitor banks that can handle 24 volts DCC can be expensive. Be very aware of the voltage rating when purchasing a pre-built KA system.

A great and detailed page by Mark Gurries is here: https://sites.google.com/site/markgurries/home/decoders/keep-alive-compatibility

Application of the KA system

Normally, the KA system is connected at the output of the full wave bridge rectifier in virtually all decoders. The track pickups go the "input" of a full wave bridge rectifier (usually 4 discrete diodes) and now you have DC to power the motors, lights, and the microprocessor on the "output" of the full wave bridge.

The KA system is normally connected to the "output" of the full wave bridge.

Now on to the types of systems:

First type is just simple capacitors

Next is addition of some basic features, resistor and diode to limit inrush current and diode to allow full output current. Also variation with an inductor instead of the resistor for inrush.

Finally is a "smart" unit that has basically a DC to DC inverter to raise input voltage to charge caps higher. Nice for analog, unneeded for DCC.

Soundtraxx sells the correct harness, 810158,