The silicon bilateral switch is used for triggering SCR’s and triacs. Basically, it consist of two silicon unilateral switches, connected in parallel but in opposite directions as shown in figure 1. The schematic symbol of silicon bilateral switch is shown in figure 2.

The operation of a silicon bilateral switch is similar to that of a triac. It is in its OFF position so long as the applied voltage is below the switching voltage (about 6 to 10 volts). It turns ON as soon as the applied voltage is increased above the switching voltage. When the silicon bilateral switch is ON, the voltage drops across it, is about 1.7 volts.

Since silicon bilateral switch conducts in either direction, therefore it does not have any reverse voltage rating like silicon unilateral switch. The silicon bilateral switch may be used in phase control of SCR’s and triacs.

Characteristics of Silicon Bilateral Switch or SBS

SBS Characteristics

The characteristic curve of an SBS shown in fig b is for the gate terminal of the device disconnected. The basic V-I characteristic of an SBS can be altered by using the gate terminal. However, the SBS is quite useful even without its gate terminal, just by virtue of the snap-action breakover from A₂ to A₁.  The reason for using an SBS in place of diac is its superiority over the later. Not only does the SBS show a more vigorous switching characteristic as indicated in figure b, but it also more temperature stable and more symmetrical and has less batch spread than a diac. A modern SBS has a temperature coefficient of about + 0.02 % per °C, that is, its V_B0 increases by only 0.02 % per degree of temperature rise, which comes out to only 0.16 V per 100°C which is very temperature stable indeed.

SBSs are symmetrical to within about 0.3 V i.e., the difference in magnitude between + V_B0 and -V_B0 is less than 0.3 V. This yields virtually identical firing delays for positive ‘ and negative half cycles.The batch spread of SBSs is less than 0.1 V i.e., the difference in V_B0 among the different SBSs in a batch is less than 0.1 V. The batch spread among diacs is about 4 V_A by contrast.The main advantage of using gated SBS for the trigger control of a triac is that the hysteresis or flash-on effect is eliminated.

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1. Silicon Unilateral Switch (SUS)

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