Rectifier diode and Zener rectifier diode. Rectifier diodes have two contact-terminals, an anode and a cathode. Conduction is forward-biased and will not occur if a positive voltage is applied to the cathode. It consists of either a series of four input rectifier diodes or a four-pin bridge rectifier chip. Zener rectifier diodes , the exception, permits current to flow in the opposite direction.
Conduction is reverse-biased.
The transition to this state is sharp. While in this state, the diode keeps the voltage constant. Many Zener diodes are connected to grounds or will drop excess voltage across an adjacent current-limiting resistor. After the positive voltage is sent trough the gate terminal, an SCR continues to conduct current as long as it is flowing through it, even if there is no longer voltage at the gate terminal. The SCR is switched on during a specific phase of the AC cycle.