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Thyristors are a family of semiconductor devices that exhibit bi-stable current-voltage characteristics and can be switched between a high-impedance, low-current off state and a low-impedance, high-current on state.
Diacs are bi-directional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. Except for a small leakage current, diacs do not conduct until the breakover voltage is reached.
Silicon controlled rectifiers (SCR) are four-layer (PNPN) thyristors with three terminals: an input control terminal (gate), an output terminal (anode), and a terminal common to both the input and output (cathode). SCRs are used mainly with high voltages and currents, often to control alternating current (AC) where the change of sign causes the device to switch off automatically.
Triacs are three-terminal silicon devices that are configured in an inverse parallel arrangement to provide load current during both halves of the AC supply voltage. They are often used to control motor speed.