Silicon Control Switch (SCS)

Silicon Controlled Switch (SCS), also known as four terminal low-power thyristor. It belongs to novel and multifunctional semiconductor devices. As long as the wiring method is changed, ordinary thyristors (SCR), switchable thyristors (GTO), reverse conducting thyristors (RCT), complementary N-gate thyristors (NGT), programmable single junction transistors (PUT), and single junction transistors (UJT) can be formed. In addition, NPN type transistors, PNP type transistors, Schottky diodes (SKD), three types of voltage regulators, and N-type or P-type negative resistance devices can be formed to achieve the circuit functions of more than ten types of semiconductor devices. So far, there has never been a device with so many functions as it does. Therefore, it is well deserved to be hailed as a novel "universal" device.

Silicon controlled switches belong to PNPN four layer four terminal devices. The equivalent circuit consists of an NPN transistor (T1) and a PNP transistor (T2), with four output terminals: anode A, cathode K, anode gate GA, and cathode gate GK. Due to its extremely small gate trigger current (a few microamperes) and extremely short switching time (tON, toFF), it is equivalent to a high sensitivity low-power thyristor. The capacity is generally 60V, 0.5A, mostly enclosed in metal shells with a diameter of 8mm.

The biggest feature of silicon controlled switches is that each layer of PNPN has a lead out terminal, making them extremely flexible to use. Among them, Shockley diode (SKD) belongs to four layer, high-speed, controllable semiconductor rectifier diode, which can be used as a switching diode or trigger in laser pulse generators. In addition to the purposes listed in the table, silicon controlled switches can also be used as relay drivers, delay circuits, pulse generators, bistable triggers, and high-sensitivity level detectors.

When using silicon control switches, the following points should be noted:

Firstly, the A pole is connected to the positive pole of the power supply, and the K pole is connected to the negative pole of the power supply;

Secondly, when a negative pulse is applied to the GA electrode, the device conducts. When a positive pulse is applied, the device is turned off;

Thirdly, the device conducts when a positive pulse is applied to the GK pole, and turns off when a negative pulse is applied.

The following introduces the method of using a multimeter to check silicon control switches

1. Check the unidirectional conductivity of three PN junctions

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

2. Check for reverse conductivity

Simply short circuit GA to A to observe reverse conductivity. But to make the effect more obvious, GK and K are also short circuited. At this time, there are only two parallel silicon PN junctions of the same polarity between A and K, and the K pole is connected to the positive pole of the PN junction, while the A pole is connected to the negative pole of the PN junction. Therefore, connect the black probe to the K pole and the red probe to the A pole to measure the forward resistance, which is about several thousand ohms. This characteristic is called "reverse conduction" (meaning reverse conduction).

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

3. Check the triggering ability

Firstly, connect the black probe to pole A and the red probe to pole K. If the resistance is infinite, it proves that the device is turned off. Follow the steps below again:

(1) Place the red pen tip on the GA pole and then disconnect it (but the red pen always stays on the A pole), which is equivalent to applying a negative pulse to the GA pole. If the resistance rapidly decreases, it indicates that the device has triggering ability.

(2) Take a black probe and place it on the GK pole, then disconnect it. This is equivalent to inputting a positive pulse to the GK pole, causing the resistance value to rapidly decrease, indicating triggering ability.

4. Check the shutdown capability

Firstly, use the method introduced above to make the device conductive, and then perform the following operations:

(1) Use a black pen tip to touch the GA pole, then disconnect it. If the resistance becomes infinite, it proves that the device is turned off.

(2) Place the red pen tip on the GK pole and quickly disconnect. If the resistance is infinite, it indicates that the SCS is turned off.