Hi. I recently purchased an Automatic Transfer type power supply which can deliver 0 to 30 volts DC. What does this mean? How do Automatic Transfer power supplies work? Just curious. Can't find answer on Google.Thanks.

There are several sites on google explaining how automatic transfer supplies work. I put in 'automatic power supply transfers' and the website eHow came up with:

1. A transfer switch is an electrical device that transfers the source of power being used from one power supply to another. Typically, this transfer is between the local utility and an emergency source of power, and back. The transfer switch not only switches the power source, it keeps the power sources isolated from one another. This prevents any power surges from traveling up or down to either power source from the other source.
2. An electrical transfer switch that operates without the need for manual intervention is called an automatic transfer switch. These transfer switches maintain constant monitoring of voltage levels on the electrical circuit. In the event that there is any type of voltage problem that exceeds the pre-set acceptable levels, the automatic transfer switch acts to prevent any damage by switching the power.
3. Automatic transfer switches monitor the circuit for voltage sags, brownouts, power surges, or power spikes. They also take action in the event of a complete loss of power. As soon as a problem is detected, the automatic transfer switch begins the process of starting up the emergency power supply. The automatic transfer switch continues to monitor the main power supply, as well as the emergency power supply during the start-up process. As soon as the emergency power supply has achieved the proper level of voltage and consistency, the automatic transfer switch transfers the power from the main power supply to the emergency power supply.
4. While the emergency power supply is online, the automatic transfer switch continues to monitor the main power supply, waiting until it has resumed an acceptable level of power and consistency. Once the main power supply has resumed normal operation, and maintained that level for a pre-set amount of time, the automatic transfer switch will switch power back to the main power supply and shut down the emergency power supply.

tick

For DC, its easy, Just a relay, when power is lost, relay deenergizes, making a connection with secondary Power.
Little confusing, as what you have May plug into the wall, When AC is Lost, DC is lost as well. Batteries are a good Back up.
What are you wanting to do?