Sidacs are bidirectional silicon switches that provide greater power handling capabilities than standard diacs. These four-layer (PNPN) semiconductor devices are triggered by thyristors and act as open circuits that are capable of withstanding a specific rated voltage until triggered. Triggering a sidac turns it on and causes it to become a low-resistance current path until the current is reduced to a specific level or the device is triggered off. The breakover voltage is not polarity dependent. To ensure the proper distribution of an applied voltage, high-value resistors are often placed in parallel with sidacs. Sidacs are used in a variety of applications such as high intensity discharge (HID) lighting, ion generators, and gas furnaces.
Performance specifications for sidacs include breakover voltage, breakover current, output voltage, repetitive peak on-state current, repetitive peak off-state voltage, holding current, and maximum root mean square (RMS) power. Breakover voltage (VBO), the voltage at which devices begin to conduct, is measured between the input and output terminals when sidacs switch on. Breakover current (IBO) is the instantaneous current flowing at the breakover voltage. Measured during the on state, output voltage (VO) is the voltage across a 20-ohm resistor in series with a sidac during the discharge of a specified capacitor. Repetitive peak on-state current (ITRM) is the maximum limiting peak on-state current, including all repetitive transient currents, for which sidacs are rated. Repetitive peak off-state current (VDRM), the maximum instantaneous value of the off-state voltage occurring across a sidac, includes all of the transient repetitive voltages and excludes all of the non-transient repetitive voltages. Holding current (IH) is the minimum principal current required to maintain a sidac in the on state. Root mean square power is the RMS value of the principal current when the device is in the on state.
IC Package Types
Sidacs are available in a variety of IC package types with different numbers of pins and diodes. Basic IC packages types for sidacs include discrete packaging (DPAK), power packaging (PPAK), and in-line packaging (IPAK). Other package types include diode outline (DO), transistor outline (TO), small outline transistor (SOT), and small outline diode (SOD) packages. Sidacs that use metal electrode leadless face (MELF) packaging have metallized terminals at each end of a cylindrical body. Other available package types for sidacs include thin small outline package (TSOP), thin shrink small outline L-leaded package (TSSOP), and thin small outline J-lead (TSOJ) package. Dual in-line packages can be installed either in sockets or permanently soldered into holes extending into the surface of the printed circuit board. Ceramic dual in-line package (CDIP) and plastic dual-in package (PDIP) variants are often available.