The SIDAC

The SIDAC, or SIlicon Diode for Alternating Current, is a semiconductor of the thyristor family. Also referred to as a SYDAC (Silicon thYristor for Alternating Current), bi-directional thyristor breakover diode, or more simply a bi-directional thyristor diode, is technically specified as a bilateral voltage triggered switch. Its operation is identical to that of the DIAC; the distinction in naming between the two devices being subject to the particular manufacturer. In general, SIDACs have higher breakover voltages and current handling capacities than DIACs. The working of SIDAC is similar to a DIAC used as pulse generators in Triac controlled devices. It is similar to a Triac without gate and is a five layered device. Since the SIDAC handles much current, it can be used to switch on a device and not like the triggering in DIAC.

Sidac Symbol
Upon application of a voltage exceeding the SIDAC breakover voltage point, the SIDAC switches on through a negative resistance region to a low on-state voltage. Conduction will continue until the current is interrupted or drops below the minimum holding current of the device.

Upon application of a voltage exceeding the SIDAC breakover voltage point, the SIDAC switches on through a negative resistance region to a low on-state voltage. Conduction will continue until the current is interrupted or drops below the minimum holding current of the device. The operation of the SIDAC is quite simple and is functionally identical to that of a spark gap or similar to two inverse parallel Zener diodes. The SIDAC remains nonconducting until the applied voltage meets or exceeds its rated breakover voltage. Once entering this conductive state, the SIDAC continues to conduct, regardless of voltage, until the applied current falls below its rated holding current. At this point, the SIDAC returns to its initial non conductive state to begin the cycle once again. Somewhat uncommon in most electronics, the SIDAC is relegated to the status of a special purpose device. However, where part-counts are to be kept low, simple relaxation oscillators are needed, and the voltages are too low for practical operation of a spark gap, the SIDAC is an indispensable component.]

The polarity is different for a diac and a sidac  A diac is a polarized device that acts like a diod, it has one-way current flow, whereas a sidac can conduct current in both directions.

SIDACs can be used:

  1. 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.
  2. 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.
  3. Applications:
  • High Voltage Lamp Ignitors
  • Natural Gas Ignitors
  • Gas Oil Ignitors
  • High Voltage Power Supplies
  • Xenon Ignitors
  • Overvoltage Protection
  • Pulse Generators
  • Fluorescent Lighting Ignitors

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