Silicon Controlled Switch (SCS - Four-Terminal Low-Power Thyristor)

Silicon Controlled Switch (SCS) is also called four-terminal low-power thyristor. It is a novel and multifunctional semiconductor device. As long as its wiring method is changed, it can form a common thyristor (SCR), a turn-off thyristor (GTO), a reverse conducting thyristor (RCT), a complementary N-gate thyristor (NGT), a programmable unijunction transistor (PUT), a unijunction transistor (UJT). In addition, it can also form an NPN transistor, a PNP transistor, a Shockley diode (SKD), three kinds of voltage-stabilizing diodes, and an N-type or P-type negative resistance device, respectively realizing the circuit functions of more than ten kinds of semiconductor devices. So far, no device has ever had so many functions as it does. Therefore, it is well-deserved to be known as a novel "universal" device.

The silicon controlled switch is a PNPN four-layer four-terminal device, and its internal structure, equivalent circuit and symbol are shown in Figure 1. The equivalent circuit is composed of an NPN transistor (T1) and a PNP transistor (T2). The four lead-out terminals are anode A, cathode K, anode gate GA, and cathode gate GK. Because its gate trigger current is extremely small (a few microamperes) and the switching time (tON, toFF) is extremely short, it is equivalent to a highly sensitive low-power thyristor. The capacity is generally 60V, 0.5A, mostly metal shell packaging, the tube diameter is 8mm, and the pin arrangement order is shown in Figure 2. Typical foreign products include 3N58, 3N81, MAS32, 3SF11, etc.

The biggest feature of silicon controlled switch is that there is a lead-out terminal in each layer of PNPN, so it is extremely flexible to use. The circuit functions of silicon controlled switch under different wiring modes are detailed in Table 1. Among them, Shockley diode SKD (Shockley Diode) is a four-layer, high-speed, controllable semiconductor rectifier diode, which can be used as a switching diode or trigger in laser pulse generator. In addition to the uses listed in the table, silicon controlled switch can also be used as relay driver, delay circuit, pulse generator, bistable trigger, and high-sensitivity level detector.

When using silicon controlled switches (GTOs), the following points should be noted (see Figure 3):

First: Pole A is connected to the positive pole of the power supply, and pole K is connected to the negative pole of the power supply;

Second: When a negative pulse is applied to the GA pole, the device is turned on. When a positive pulse is applied, the device is turned off.

Third: When a positive pulse is added to the GK pole, the device is turned on, and when a negative pulse is added, the device is turned off.

Here is a method to check the silicon control switch using a multimeter:

1. Check the unidirectional conductivity of the three PN junctions

Set the multimeter to R×1k and measure the forward and reverse resistances between A-GA, GA-GK, and GK -K. The forward resistance should be several thousand ohms to more than ten thousand ohms, and the reverse resistance should be infinite, indicating that the PN junction has unidirectional conductivity.

2. Check the reverse conductivity

Reverse conductivity can be observed by shorting GA and A. But to make the effect more obvious, GK and K are also shorted. At this time, there are only two silicon PN junctions of the same polarity connected in parallel between A and K, and the K pole is connected to the positive pole of the PN junction, and the A pole is connected to the negative pole of the PN junction. Therefore, the forward resistance is measured by connecting the black test lead to the K pole and the red test lead to the A pole, which is about several thousand ohms. This characteristic is called "reverse conduction" (meaning reverse conduction).

If the positions of the test leads are swapped, there will be no reverse conductivity and the resistance will become infinite.

3. Check the trigger capability

First, connect the black test lead to the A pole and the red test lead to the K pole. The resistance is infinite, which proves that the device is turned off. Then follow the steps below:

(1) Touch the GA pole with the tip of the red test lead, and then disconnect it (but keep the red test lead connected to the A pole). This is equivalent to adding a negative pulse to the GA pole. If the resistance decreases rapidly, it means that the device has the ability to trigger.

(2) Touch the GK pole with the black test lead and then disconnect it. This is equivalent to inputting a positive pulse to the GK pole. The resistance value decreases rapidly, indicating that it has trigger capability.

4. Check the shut-off capability

First, use the method described above to turn on the device, and then perform the following operations:

(1) Touch the GA electrode with the tip of the black test lead and then disconnect it. If the resistance becomes infinite, it proves that the device is turned off, see Figure 4(a).

(2) Touch the GK pole with the tip of the red test lead and quickly disconnect it. If the resistance is infinite, it means that the SCS is turned off, see Figure 4(b).