What is a SIDAC - Tutorial

- the SIDAC, SIlicon Diode Alternating Current bi-directional switch is widely used with TRIACs to improve operation of alternating current power switching systems.

DIAC Tutorial Includes

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The SIDAC is a bi-directional voltage triggered switch very similar to the DIAC, and it belongs to the thyristor family of devices. It gains its name from the words SIlicon Diode Alternating Current

The SIDAC generally has greater power handling capabilities than a standard DIAC, but nevertheless it operates in essentially the same manner.

The SIDAC is less common than the DIAC, but is essentially the same, the name indicating more the manufacturer. The device may also be referred to as a SYDAC - Silicon thYristor Diode for Alternating Current.

SIDAC basics

The SIDAC is very similar to the ordinary DIAC. Its operation can be considered to be very nearly identical. However it is always a five-layer device and it has a low-voltage drop in latched conducting state. This makes it more like a voltage triggered TRIAC but without a gate.

Another difference between the two devices is that typically a SIDAC will have higher break-over voltage than a DIAC. Generally one will also have a higher current handling capacity level. This means that they can be directly used for switching and not just for triggering of another switching device such as a TRIAC.

The operation of the SIDAC or SYDAC is very similar to that of the DIAC. It remains in a non-conducting state until the voltage across the device rises to a level above the break-over voltage. At this point the device starts to conduct and enters a negative resistance area of the I-V characteristic.

The device will continue to conduct until the voltage falls below its rated holding current, at which point the device will return to its non-conducting state. It will then only start to conduct again once the break-over voltage is again exceeded.


There are a number of ways in which SIDACs can be used:

  • With a TRIAC:   One of the most popular ways in which devices like DIACs and SIDACs can be used is with other switching devices such as TRIACs that do not have a good firing characteristic - they are asymmetrical leading to a rather undefined firing characteristic. Here SIDACs enable the overall circuit to function with more defined switching limits.
  • Relaxation oscillators:   In applications where cost is premium and component count must be kept low, SIDACs can be used as the major element in a simple relaxation oscillator. Here the switching characteristics can be used to good effect.

Although not widely seen in the industry, SIDACs can be used to good effect, and indeed they can be obtained from a number of sources.

By Ian Poole

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