When a transistor is used as a relay driver, the relay energizes when which condition occurs?

When a transistor is used as a relay driver, it operates as a switch that controls the relay. In this role, the collector terminal of the transistor is connected to one side of the coil of the relay, while the other side of the coil is connected to the power supply. For the relay to energize, the transistor must be switched on, allowing current to flow through the relay coil, which in turn creates a magnetic field to close the relay contacts.

In this scenario, when the collector goes high, it means that a sufficient voltage level is present at the collector, enabling current to flow from the power supply through the relay coil and into the collector. This current activation is what causes the relay to energize and function correctly, closing its contacts and allowing current to flow through the load it's controlling.

The other options refer to conditions that do not fulfill the requirement for turning the relay on. The emitter going high or low does not directly control the relay operation, as their role in this configuration primarily influences how the transistor is biased and therefore how it conducts. In particular, the important action for energizing the relay is the condition of the collector. Thus, when the collector goes high, it triggers the relay as intended by the circuit design.