Silicon Bilateral Switches – SBS – How they work (ART099)

Silicon Bilateral Switches (SBS) are devices of the thyristor family. It can be considered the same as two 888 wired in anti-parallel mode as shown in Figure 1.


   Figure 1 – Equivalent circuit for a SBS
Figure 1 – Equivalent circuit for a SBS


They are used as triggering elements for circuits using triacs. The operation is the same as the SUS; the internal zener diodes determine the trigger point of each SUS and the gate can be used to alter this value.

External zener diodes can be wired between the gate and the two other terminals (A1 and A2) to lower the trigger point of the device.



Figure 2 shows the symbol adopted to represent the SBS.


Figure 2 – The SBS
Figure 2 – The SBS




A part number is adopted by the manufacturer to indicate the device is an SBS. However, only with a data sheet or other manufacturer’s document is it possible to access the electrical characteristics of the device, which are the maximum current and voltage between A1 and A2 and the voltage of the internal zener diodes.


Where they are found

The SBS isn’t a common device in modern equipment. They can be found in some equipment using SCRs as motor controls, timers, alarms, sensor interfaces, etc.



The multimeter can only reveal if an SBS is shorted. The operation test must be done using an appropriated circuit.