ST35

Thyristors - DIACs, SIDACs

A silicon controlled rectifier (SCR) is a thyristor that has three terminals known as anode, gate, and cathode and is applicable in the control of electric power and in voltage regulation. In contrast, a DIAC (diode AC switch) and a SIDAC (silicon diode AC switch) are both form of a thyristor, which has two terminals and is applicable in both AC and DC circuits.

ST35 application field and working principle

ST35 is one of the most popular thyristors mainly applied in the lightning protection, inductor protection, and switching power supply equipment. It is a standard DIAC with two stable modes of operation, reverse blocking or direct blocking, for controlling current and voltage alternately. ST35 is used in high-intensity, high-frequency and high-current circuits. It can be used as a DC switch and can also be applied as an AC switch in AC operation that helps in avoiding the destructive turn-on effects of the large alternating current.

The main functional principle of ST35 is that, when the semiconductor switching operates at the breakover voltage, either forward or reverse bias condition, it changes it mode depending upon the voltage difference between the anode and cathode. In a forward bias condition, it operates as a diode clamp permitting only limited current; while, in the reverse bias condition, it works as an open switch. In addition, the thyristor has a high current carrying capacity when in its reverse blocking state, and can be used as a safe and reliable overvoltage protector.

The ST35 pair of thyristors have features which include high blocking voltage up to 5kV, high current upto 1.2A, and the working temperature range from -40 to 85 °C. Furthermore, the embedded in-built protective diode helps in avoiding reverse damage due to spike in voltage and current when the thyristor is open. The DIAC thyristor also has short rise and fall times up to 70ns which enables it to switch faster than other thyristors.

In comparison, SIDACs are non-linear silicon controlled devices with a two-pin gate, one anode and one cathode which is triggered by a single pulse of power. It is an asymmetric thyristor which facilitates a single-pulse switching, with high current capability, and can be used as an AC switch. SIDACs possess the unique capability of turning on or off with three or four times the current measured by its off state voltage. It does not require a gate input for triggering and, unlike DIACs, it does not require a high and specific voltage to be applied across it for activation. It also functions without a direct gate control and is available for both AC and DC operations.

SIDACs are highly preferred for their versatile characteristics like high peak current up to 5.5A, low turn-off delay time and low turn-off time below 100 ns, high peak-off voltage up to 4.2V and can also be used together with small switching devices such as transistors.

In conclusion, ST35, DIACs and SIDACs are all form of thyristors but with certain differences. ST35 is a standard DIAC with two stable modes of operation, and is mainly used for lightning protection and inductor protection. On the other hand, SIDACs offer a single-pulse switching function, and can be used as an AC switch. Both thyristors are widely used in various applications, offering different electrical and operational characteristics.