Silicon one-way switch (SUS)

Silicon Unidirectional Switch SUS (Silicon Unidirectional Switch), also known as unidirectional trigger transistor, is a new trigger device developed after the bidirectional trigger diode (DIAC). The three terminals are anode A, cathode K, and gate G. Compared with DIAC, there are more gates. The silicon one-way switch is essentially an integrated circuit composed of an N-gate thyristor controlled by a voltage regulator tube. Its structure, equivalent circuit, symbols and appearance of typical products are shown in Figure 1. Typical products include US08A, etc., which have the same appearance as plastic-packaged transistors. The voltammetric characteristics are shown in Figure 2. The silicon one-way switch has the following characteristics: First: the voltage stabilizing diode is connected in parallel between the gate and the cathode, and the voltage stabilization value is 6~10V. The conduction process first reversely breaks down the voltage regulator tube, and then the thyristor is turned on, and the forward voltage drop between AK quickly drops to V (ON). Second: the forward and reverse transition voltages are asymmetric, related to the relationship V (BR) > V (BO) > V (ON). The conduction 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 switching speed can be increased by using the integrated resistor R. Silicon unidirectional switches are mainly used in thyristor phase shifting circuits. In addition, it can replace the single-junction transistor to form an oscillator, and can also form a temperature over-limit alarm and a DC motor speed regulator tube. Here's how to use a multimeter and megger to check a silicon one-way switch. 1. Determine the gate G. As can be seen from Figure 1(a), there is a PN junction between AG and KG. The gate is easy to identify based on symmetry. The specific method is to select the R×100 level of the multimeter, connect a red meter to a certain pin, and touch the other two pins in sequence with a black test lead. If the resistance value measured twice is several hundred ohms, and after exchanging the test leads, the resistance The values ​​all become infinity, which means that the red test lead is connected to the gate G. 2. Identify the anode A and the cathode K. First, assume that the remaining leg is the A pole and the other leg is the K pole. Then measure the forward voltage V (ON) according to the circuit in Figure 3(a), and then follow the circuit in Figure 3(b). Measure the reverse transition voltage V (BR). Finally compare the two measurement results. If V (ON) < V (BR), it proves that the assumption is true, otherwise it must be re-assumed and the measurement continues. Example: The silicon one-way switch under test is US08A. For the convenience of description, the three pins are numbered ①, ②, and ③ from left to right. Set the 500 multimeter to R×100, connect the red test lead to ②, and the black test lead to ①, and the measured resistance value is 820Ω. At this time, the countdown deflection number of the meter needle is n1′=22.5 divisions, which corresponds to U1= K′n1′= 0.03V/div × 22.5 divisions = 0.675V. The red test lead does not move, and then the black test lead is connected to ③. The resistance is 840Ω. At the same time, it reads n2′=22.8 divisions, which corresponds to U2= K′n2′=0.684V. Since both 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 below 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. The conditions of V (ON) < V (BR) are satisfied, proving the original Assumed correct. In order to further verify the A and K poles, the reverse breakdown voltage of the PN junction between GK and GA was measured to be 8.3V using a megohmmeter and a multimeter at 10VDC; while the reverse breakdown voltage of the PN junction between GA was 16.5V, which is greater than 10V. . It proves that the voltage regulator tube is located between GK, that is, ① is the cathode, and ③ is naturally the anode. Note: When you first shake the megohmmeter to measure the forward voltage between AK and AK, the voltmeter pointer will suddenly drop from about 10V (corresponding to V(BO)) to below 1V (corresponding to V(ON)) before reaching stability. When measuring the reverse transition voltage between AK, the pointer is always stable.