A Silicon unilateral switch (SUS) is used for triggering SCR,s and triacs. Basically, a silicon unilateral switch is a four-layer diode that resembles the SCR in which the contacts are made only to anode and cathode.

Silicon Unilateral Switch

Figure shows the schematic symbol of silicon unilateral switch. The operation of a silicon unilateral switch is similar to that of SCR. A typical silicon unilateral switch is in it OFF position until the applied voltage is below the switching voltage(about 6 to 10 v). The voltage drop across, a silicon unilateral switch when it is ON, is about 1.5 v. The maximum forward current capability is about 200mA. The holding current ranges from about 0.5mA to 1.5 mA.

A silicon unilateral switch can be turned OFF by reversing the anode voltage or disconnecting the supply. The maximum reverse voltage rating of silicon unilateral switch is about 30V.

V-I characteristic of SUS

The schematic symbol and V-I characteristic of SUS are shown is figure a and b respectively. The SUS, like the SBS, has gate terminal which can change the basic breakover characteristic shown in figure. By connecting a Zener diode between the gate and the cathode of an SUS, the breakover voltage can be reduced to (V_Z + 0.6 V).This is accomplished by connecting Zener diode cathode to the SUS gate and Zener diode anode to the SUS cathode. SUS can be fired at a very low anode to cathode’ voltage (approxi­mately 1 V) if gate current flows from anode to gate. SUSs are low-voltage, low-current devices. Most SUSs have a breakover voltage of 8 V and a current limit of less than 1 A.

Four-layer diode

Four-layer diode is another unilateral breakover device. Its schematic symbol is shown in figure. The V-I charac­teristic for four-layer diode-.and SUS are the same and are illustrated in figure. The V-I characteristic of four-layer diodes and SUSs is similar   to that of an SBS except that only forward breakover voltage is possible. Reverse break-down can happen, but only at a much greater voltage level than + V_B0. Reverse breakdown is destructive for the device. The breakover behaviour of a four layer diode cannot be changed. Four layer diodes are available with breakover voltages ranging from 10 to 400 V. They can carry large pulsed currents if the pulses are of short duration. Some four-layer diodes can carry 100 A current pulses.

 

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