Silicon bilateral switch (SBS) is another breakover device which is capable of triggering triacs, and it is popular in low-voltage trigger control circuits. They have breakover voltages lower than those for diacs, ± 8 V being the most popular rating. V-I characteristic curve of an SBS is similar to that of the diac, but it has more pronounced “negative resistance” region i.e., its decline in voltage is more drastic after it enters the conducting state. The schematic symbol and V-I characteristic of an SBS are shown in figure a and b respectively. When SBS switches into conducting state the voltage across its anode terminals drops almost to zero (to about 1 V). The SBS is said to have a breakback voltage of 7 V when it turns on.
Characteristics of Silicon Bilateral Switch or SBS
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 VB0 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 + VB0 and -VB0 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 VB0 among the different SBSs in a batch is less than 0.1 V. The batch spread among diacs is about 4 VA 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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