DC Powered

With the connections of Figure 5, the power supply is configured for a DC power input. The diode in the circuit provides reverse-voltage protection. Since the power supply has its own voltage regulator, a wide range of DC input voltages is possible. The manufacturer will indicate the acceptable range in its specifications as well as the current draw. When using a switching regulator, the lower the DC input voltage, the higher the input current. This may appear counter-intuitive, but this is the nature of switching regulators. The hub or switch electronics represents a constant power load so depending upon the efficiency of the switching regulator; the input power will also be constant. That is not the case with a linear regulator. Input current remains constant with increasing input voltage since the linear regulator absorbs the increased power input—resulting in wasted energy.

Redundant DC Powered

It is possible to incorporate one or more redundant DC power sources for backup purposes in case the primary source fails. However, load sharing cannot be assumed so each power source must be capable of providing adequate power in the event that the other source or sources fail. Figure 6 shows such a scheme. Either or both of the AC inputs can be used as DC inputs when the power sources are referenced to 0vdc. However, there is a risk with this scheme since there is no reverse-voltage protection on the AC inputs when referenced to 0vdc. Unlike the DC input connection, a reverse voltage on an AC input connection with respect to 0vdc results in one of the diodes in the bridge conducting to ground, effectively applying a short circuit across the power source. Either the power source will fail or the input diode will fail, so extreme care must be exercised when using AC connections for DC backup schemes. Class 2 power sources usually have internal fuses that cannot be reset if they blow.

AC Powered

The two AC inputs are reserved for the secondary of a Class 2 transformer which then feeds a full-wave bridge as shown in Figure 7. A full-wave bridge is used since center-tapped Class 2 transformers are not common. With full-wave rectification, a wide range of AC voltages is possible. Although 24 VAC is the most common, a 12 VAC transformer could work just as well, depending upon the power supply specification. Notice that the two AC input pins are above 0vdc and that the transformer’s secondary must be floated. A floating output means that the output is not referenced to ground. This will become an important issue when we want to share transformer power with other devices. For now, assume that one transformer is required for each device when using the two AC input connections.

AC Powered with Battery Backup

It is possible to have two potential power sources—one AC and one DC connected at the same time. The DC source could be a battery or simply a DC supply from a redundant power system. Figure 8 shows the connection. This scheme could be tricky to implement since the battery voltage, under charge, must be below the AC voltage level otherwise the battery would be powering the device all the time. The only way to know what power source is actually powering the device is to measure the current to the DC input. If it is zero, then the AC source is doing all the work. When the AC source is removed, the battery voltage must be at a sufficient level to power the device on its own. Notice that there is no provision for charging the battery while in standby mode. This must be handled by the external backup system.