Silicon unidirectional switch (SUS-unidirectional trigger transistor)

Silicon unidirectional switch SUS (Silicon Unidirectional Switch) is also called unidirectional trigger transistor, which is a new type of trigger device developed after the bidirectional trigger diode (DIAC). The three terminals are anode A, cathode K, gate G, and gate is added compared to DIAC. Silicon unidirectional switch is essentially an integrated circuit composed of N-gate thyristor controlled by voltage regulator. Its structure, equivalent circuit, symbol and appearance of typical products are shown in Figure 1. Typical products include US08A, etc., with the same appearance as plastic-encapsulated transistors. The volt-ampere characteristics are shown in Figure 2.

Silicon unidirectional switches have the following characteristics:

First: The voltage regulator diode is connected in parallel between the gate and the cathode, and the voltage regulation value is 6 to 10 V. The conduction process first breaks down the voltage regulator tube in reverse, and then the thyristor is turned on, and the forward voltage drop between AK drops rapidly to V (ON).

Second: The forward and reverse breakover voltages are asymmetric, with the relationship V(BR)＞V(BO)》V(ON). The on-state voltage drop is only a few tenths of a volt, and the on-state resistance is only about 2Ω.

Third: The switching characteristics are good, and the conduction time is only about 0.2μs. The integrated resistor R can increase the switching speed.

Silicon unidirectional switches are mainly used in thyristor phase shift circuits. In addition, they can replace single junction transistors to form oscillators, and can also form temperature over-limit alarms and DC motor speed regulators.

The following describes how to check a silicon one-way switch using a multimeter and a megohmmeter.

1. Determine gate G

As shown in Figure 1(a), there is a PN junction between AG and KG, and the gate can be easily identified based on the symmetry. The specific method is to select the R×100 position of the multimeter, connect the red probe to one pin, and touch the other two pins in turn with the black probe. If the resistance values ​​measured twice are both several hundred ohms, and the resistance values ​​become infinite after exchanging the probes, it means that the red probe is connected to the gate G.

2. Identify the anode A and cathode K

First, assume that the remaining leg is pole A and the other leg is pole K. Then measure the forward conduction voltage V(ON) according to the circuit in Figure 3(a), and then measure the reverse breakover voltage V(BR) according to the circuit in Figure (b). Finally, compare the two measurement results. If V(ON) < V(BR), the assumption is established. Otherwise, the assumption must be re-established and the measurement must be continued.

Example: The silicon unidirectional switch under test is US08A. For the convenience of description, the three pins are numbered ①, ②, and ③ from left to right. When the 500-type multimeter is set to R×100, the red probe is connected to ② and the black probe is connected to ①, the resistance value is 820Ω. At this time, the needle deflects n1′=22.5 grids in reverse, corresponding to U1= K′n1′=0.03V/grid×22.5 grids=0.675V. The red probe does not move, and the black probe is connected to ③. The resistance value is 840Ω, and n2′=22.8 grids are read at the same time, corresponding to U2= K′n2′=0.684V. Since the two resistance values ​​are very small, and U1 and U2 are consistent with the forward voltage drop values ​​of the two silicon PN junctions, it is determined that ② is the G pole.

Assume that ① is the K pole and ③ is the A pole. According to Figure 3, V(ON)=0.8V and V(BR)=8.6V are measured respectively, which meets the condition of V(ON)

To further verify the A and K poles, the reverse breakdown voltage of the PN junction between GK was measured with a megohmmeter and a multimeter at 10VDC, and the reverse breakdown voltage of the PN junction between GA was 16.5V, which was greater than 10V. This proves that the voltage regulator is located between GK, that is, ① is the cathode, and ③ is naturally the anode.

Note: When the megohmmeter is used to measure the forward voltage between AK, the pointer of the voltmeter will suddenly drop from about 10V (corresponding to V(BO)) to below 1V (corresponding to V(ON)), and then reach stability. When measuring the reverse transition voltage between AK, the pointer remains stable.