Final answer:
An SCR differs from a diode in that it has three terminals, including the gate, allowing it to be switched on and off, while a diode constantly allows current to flow in only one direction.
Step-by-step explanation:
The difference between an SCR (Silicon Controlled Rectifier) and a diode is that an SCR has three terminals. Unlike a basic diode, which has two terminals (an anode and a cathode), an SCR has an additional third terminal known as the gate. This gate terminal allows the SCR to switch between conducting and non-conducting states, effectively acting as a controllable diode. The SCR remains in the off state until a gate pulse is applied, after which it starts conducting until the anode to cathode current falls below a certain threshold.