How to make a solid state relay circuit

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/ 2019-04-22 / Tech

Solid state relays are non-contact switches consisting of microelectronic circuits, discrete electronics, and power electronics. The isolation between the control terminal and the load terminal is realized by the isolation device. The input of the solid state relay uses a small control signal to directly drive a large current load.

Solid state relay is a new type of non-contact switching device consisting entirely of solid-state electronic components. It utilizes the switching characteristics of electronic components (such as switching transistors, bi-directional thyristors, etc.) to achieve no The purpose of the circuit to switch on and off the circuit without sparks is therefore also referred to as "contactless switch". The solid state relay is a four-terminal active device in which two terminals are input control terminals and the other ends are output control terminals. It has both amplifying drive function and isolation function. It is suitable for driving high-power switch-type actuators. It has higher reliability than electromagnetic relays, and has no contact, long life and fast speed. It has little interference to the outside world. It is widely used.

SSR can be divided into AC type and DC type according to the use occasion. They are used as load switches on AC or DC power supply, and cannot be mixed.The AC SSR is taken as an example to illustrate its working principle. Figure 1 is a block diagram of its working principle. The components 1 to 4 in Figure 1 form the main body of the AC SSR. As a whole, the SSR has only two inputs ( A and B) and two outputs (C and D) are four-terminal devices.

1.jpg

As long as a certain control signal is added to A and B during operation, you can control between C and D. “通” and “broken”, to achieve the "switch" function,The function of the coupling circuit is to provide a channel between the input/output terminals for the control signals input to the A and B terminals, but electrically disconnect the (electrical) connection between the input terminals and the output terminals in the SSR to prevent The effect of the output on the input, the component used in the coupling circuit is "optical coupler", which is sensitive, high in response speed, high in insulation (withstand voltage) between the input/output terminals; since the load at the input is a light-emitting diode This makes the input of the SSR easy to match the input signal level, and can be directly connected to the computer output interface during use, that is, controlled by the logic level of “1” and “0” The function of the trigger circuit is to generate a suitable trigger signal to drive the switch circuit 4 to work, but since the switch circuit does not add a special control circuit, it will generate radio frequency interference and pollute the power grid with high harmonics or spikes. “ Zero-crossing control circuit & rdquo;. The so-called "zero crossing" means that when a control signal is added and the AC voltage crosses zero, the SSR is in an on state; and when the control signal is turned off,When the SSR waits for the junction of the positive half cycle and the negative half cycle of the alternating current (zero potential), the SSR is in an off state. This design prevents interference from higher harmonics and pollution to the grid. The absorption circuit is designed to prevent the spikes and surges (voltages) transmitted from the power supply from impacting and interfering (or even malfunctioning) the bidirectional thyristor of the switching device. Generally, the absorption is performed in series by R-C” Circuit or non-linear resistor (varistor).

The following video mainly shows how to make a solid state relay circuit diagram.

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