Classification and application of relays

Photocoupler

A relay is an electronic control device that has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in an automatic control circuit. It actually uses a small current to control the larger. An "automatic switch" of current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit. In most cases, the relay is an electromagnet whose armature can close or break one or several contact points. When a current flows through the windings of the electromagnet, the armature is attracted by the electromagnet, thus changing the state of the contacts. Relays can generally be divided into electromagnetic relays, thermal reed relays, solid state relays, and the like. The relays configured on the enhanced PIC experiment board are shown in Figure 1.

Electromagnetic relays are generally composed of a core, a coil, an armature, a contact spring, and the like. As long as a certain voltage is applied to both ends of the coil, a certain current flows in the coil, thereby generating an electromagnetic effect, and the armature will absorb the pulling force of the return spring against the iron core under the action of the electromagnetic force attraction, thereby driving the armature. The moving contact is in contact with the stationary contact (normally open contact). When the coil is de-energized, the electromagnetic suction force also disappears, and the armature returns to the original position in the reaction force of the spring, so that the movable contact and the original static contact (normally closed contact) are attracted. In this way, the suction and release are achieved, thereby achieving the purpose of turning on and off in the circuit. For the "normally open, normally closed" contacts of the relay, it can be distinguished as follows: a static contact that is in an open state when the relay coil is not energized, called a "normally open contact"; a static contact that is in an on state It is a "normally closed contact".

The thermal reed relay is a new type of thermal switch that uses temperature sensitive magnetic materials to detect and control temperature. It consists of a temperature-sensitive magnetic ring, a constant magnetic ring, a reed switch, a thermally conductive mounting piece, a plastic substrate, and other accessories. The thermal reed relay does not use coil excitation, and the magnetic force generated by the constant magnetic ring drives the switching action. Whether the constant magnetic ring can supply the magnetic force to the reed switch is determined by the temperature control characteristics of the temperature sensitive magnetic ring.

The solid state relay is a kind of four-terminal device with two terminals as the input terminals and the other two terminals as the output terminals. The isolation device is used to realize the electrical isolation of the input and output.

Figure 1 Relay physical map

Solid state relays can be classified into AC type and DC type according to the type of load power supply. According to the switch type, it can be divided into normally open type and normally closed type. According to the isolation type, it can be divided into hybrid type, transformer isolation type and photoelectric isolation type, and the photoelectric isolation type is the most. Here, we use electromagnetic relay as an example to introduce its usage.

Relay control circuit

In the microcontroller system, the control of the relay is generally driven by a triode. The typical drive circuit is shown in Figure 2:

Figure 2 (a) Figure 2 (b)

Figure 2 general drive circuit of the relay

In the relay circuit, a diode is generally added at both ends of the coil of the relay to absorb the back EMF generated when the relay coil is de-energized to prevent interference. In the above figure, AB is a normally open contact and AC is a normally closed contact. In the figure (a), when the control signal is high level, the relay normally opens the contact to close (AB is on), when the control signal is low, the relay normally open contact opens the normally closed contact to attract (AC guide through). In Figure (b), the polarity of the control signal is exactly opposite to that of Figure (a). This circuit is used on the supporting test board.