Power supplies are available in both AC and DC in a wide variety of voltage and current values. The most common type favored by model railroaders is the plug-in DC type because it can serve many different purposes and is more versatile than AC. DC power sources are better described as converters since they step down line voltage to a lower value and rectify AC to DC. The output can be either positive or negative depending on how it is configured.
If the negative terminal of the supply is the common return (Fig.1) then the source is positive. If the positive terminal is the return then the source is negative (Fig.2). Fig. 3 shows two units wired to provide both positive and negative 12VDC to a motor driven switch machine.
Batteries and power sources can be wired in parallel to increase the total current capacity or in series to increase the supply voltage as shown below. In the parallel configuration, it is recommended to provide either a fuse or circuit breaker on each supply. In the arrangement shown if either power unit shorted out for some reason, the other unit would be protected. This is not necessary in the series hook-up since if one unit shorted the output voltage would drop to 12VDC, but no damage would occur. In years past batteries were commonly used as model railroad power since they were a source of pure D.C. current. The main disadvantage of this type of power source is the need to periodically charge the battery. The modern type of plug-in electric power source is a well regulated voltage supply and is normally fused to protect against current overload.