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Home » Technology List » The introduction of the SCR

The introduction of the SCR

Published:7/19/2018.

SCR(Silicon Controlled Rectifier) is also called thyristor, it is short for silicon controlled rectifier element, since the advent of the 1950s has grown into a big family, its main members are unidirectional thyristors, bidirectional thyristors, Optical thyristors, reverse thyristors, turn-off thyristors, fast thyristors, and so on. 

SCR is a kind of high power semiconductor device with four layers of three PN junction. It has the characteristics of small size, relatively simple structure and strong function, and it is one of the more commonly used semiconductor devices. 

The device is widely used in various electronic equipment and electronic products, and is used for controllable rectification, inverter, frequency conversion, voltage regulation, and non-contact switching. 

The thyristor device is also commonly used in household appliances such as dimming lights, speed control fans, air conditioners, televisions, refrigerators, washing machines, cameras, stereos, sound and light circuits, timing controllers, toy devices, radio remote controls, cameras and so on.

The structure of the SCR component

Regardless of the shape of the SCR, its core is a four-layer P1N1P2N2 that is composed of P-type silicon and N-type silicon. It has three P-N junction (J1, J2, J3), from the P1 of J1 structure layer anode A, cathode K from the N2 layer, from the P2 layer control G, in summary it is a kind of four-layer three terminal semiconductor devices.

The structure of the SCR component


The working principle of SCR?

The SCR is a P1N1P2N2 four-layer three-terminal structural component with three PN junctions. When analyzing the principle, it can be regarded as consisting of a PNP tube and an NPN tube. The equivalent diagram is shown in the figure below.

The equivalent diagram


When anode A is added with a forward voltage, both BG1 and BG2 tubes are in an amplified state. At this time, if a forward trigger signal is input from the control electrode G, the BG2 has a base stream ib2 flowing through it, and the collector current ic2=β2ib2 is amplified by the BG2. Since the collector of BG2 is directly connected to the base of BG1, ib1 = ic2. At this point, the current ic2 is amplified by BG1, so that the collector current ic1 of BG1 = β1ib1 = β1β2ib2. 

This current flows back to the base of BG2, which is positive feedback, which increases the ib2. As a result of the forward feed loop, the current of the two tubes increases sharply, and the SCR turns the saturation on.

Due to the positive feedback effect of BG1 and BG2, Once the SCR is turned on, even if the current of the gate G disappears, the SCR can maintain the conduction state, because the trigger signal only acts as a trigger, and does not turn off, so this SCR is not switchable.

Since the SCR has only two working states, on and off, it has switching characteristics, which require certain conditions to be converted. The conditions are shown in the following table.

SCR’s characteristics

How to use multimeter to measure the electrode of SCR?

Jotrin Electronics can easily test the position and performance of the SCR. We can use a special instrument to accurately measure the thyristor's turn-on voltage curve and reverse voltage.

1. The Detection of one-way SCR

The multimeter selects the resistor R×1 file, and the red and black meter pens respectively measure the forward and reverse resistance between any two pins until a pair of pins with a reading of several tens of ohms is found. At this time, the black pen is connected to the control pole G. The red pen is connected to the cathode K and the other is the anode A. At this time, the black meter pen is connected to the anode A that has been judged, and the red meter pen is still connected to the cathode K. At this point the multimeter pointer should not move. Short-circuit the anode A and the control pole G with a short wiring. 

At this time, the pointer of the multimeter should be deflected to the right, and the resistance reading is about 10 ohms. If the anode A is connected to the black test lead and the cathode K is connected to the red test lead, the pointer of the multimeter is deflected, indicating that the one-way SCR has broken through.

2.The Detection of two-way SCR

Use the multimeter resistance R×1 file to measure any two-pin forward and reverse resistance with red and black test leads. The two sets of readings are infinite. If a group is tens of ohms, the two pins connected to the set of red and black test leads are the first anode A1 and the control electrode G, and the other empty foot is the second anode A2.

After determining the A and G poles, carefully measure the forward and reverse resistance between the A1 and G poles. The pin with the relatively small reading is connected to the first anode A1, and the pin connected to the red test lead is controlled. Extremely G. 

The black meter pen is connected to the determined second anode A2, and the red meter pen is connected to the first anode A1. 

At this time, the pointer of the multimeter should not be deflected, and the resistance value is infinite. Then use a short wire to short-circuit the A2 and G poles, and add a positive trigger voltage to the G pole. 

The resistance between A2 and A1 is about 10 ohms. Then disconnect the A2, G very short wiring, the multimeter reading should be kept about 10 ohms. The red and black pens are interchanged, the red pen is connected to the second anode A2, and the black pen is connected to the first anode A1. Similarly, the pointer of the multimeter should not deflect and the resistance is infinite. Short-circuit the A2 and G poles with a short wire, and add a negative trigger voltage to the G pole. 

The resistance between A1 and A2 is also about 10 ohms. Then disconnect the short wiring between the A2 and G poles, and the multimeter reading should be unchanged, and keep about 10 ohms. Jotrin Electronics limited believes that the above rules are met, indicating that the tested two-way SCR is not damaged and the polarity of the three pins is correct.

3. Jotrin Electronics limited uses the advanced XJ4829 semiconductor tube characteristic diagram instrument to accurately measure the open voltage curve and reverse withstand voltage curve of the SCR. 

First find the corresponding product specification to determine the product pin and the range of parameters to be measured, and adjust the plotter to measure. The specific turn-on voltage curve and the reverse withstand voltage curve are as follows.

Turn on the voltage curve:
Turn on the voltage curve


Reverse pressure curve:

Reverse pressure curve:


What issues should we pay attention to when using SCR?

As the use of SCR becomes more and more widespread, Jotrin Electronics limited believes that we must pay attention to the correct way of use in order to ensure the performance and service life of SCR.
1. When selecting the rated voltage of the SCR, refer to the peak voltage under the actual working conditions and leave a certain margin.
2. When selecting the rated current of the SCR, in addition to considering the average current passing through the component, attention should also be paid to factors such as the conduction angle during normal operation, heat dissipation and ventilation conditions. In the work, it should also be noted that the temperature of the shell does not exceed the allowable value of the corresponding current.
3. Before using SCR, a multimeter should be used to check whether the SCR is good. When a short circuit or open circuit is found, it should be replaced immediately.
4. It is strictly prohibited to check the insulation of the components with a megohmmeter (ie, a shaker).
5. The SCR with a current of 5A or more shall be equipped with a radiator and ensure the specified cooling conditions. In order to ensure good contact between the heat sink and the SCR tube, a thin layer of silicone oil or silicone grease should be applied between them to help with good heat dissipation.
6. Using overvoltage and overcurrent protection devices for the SCR in the main circuit as specified.

7. To prevent forward overload and reverse breakdown of SCR control poles.

The introduction of the SCR
 

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