The TISP83121D is a dual-gate reverse-blocking unidirectional thyristor designed for the protection of dual-voltage ringing SLICs (Subscriber Line Interface Circuits) against overvoltages on the telephone line caused by lightning, a.c. power contact and induction. The device chip is a four-layer NPNP silicon thyristor structure which has an electrode connection to every layer. For negative overvoltage protection the TISP83121D is used in a common anode configuration with the voltage to be limited applied to the cathode (K) terminal and the negative reference potential applied to the gate 1 (G1) terminal. For positive overvoltage protection the TISP83121D is used in a common cathode configuration with the voltage to be limited applied to the anode (A) terminal and the positive reference potential applied to the gate 2 (G2) terminal. The TISP83121D is a unidirectional protector and to prevent reverse bias, requires the use of a series diode between the protected line conductor and the protector. Further, the gate reference