Relay is a commonly used component in control circuit. It uses the principle of electromagnetic induction to control the connection or disconnection of a circuit. It can use small current to control large current, so as to reduce the current load of the control switch contact and protect the switch contact from ablation.

Relays are widely used in electrical control circuits, such as intermediate relays, time relays, closing relays, tripping relays, etc.

1、 Structure of relay

Electromagnetic relay is generally composed of iron core, coil, armature, return spring, contact, etc.

2、 Classification of relays

According to the on and off mode of the relay, the relay can be divided into normally open relay, normally closed relay and normally open, normally closed hybrid relay.

3、 Working principle of relay

When the switch is closed, a certain voltage is applied at both ends of the coil, and a certain current will flow through the coil, resulting in electromagnetic effect. The armature is attracted by the electromagnetic force to overcome the tension of the return spring and attract to the iron core, thus driving the moving contact of the armature to attract to the static contact. At this time, the red bulb will light up. When the coil is powered off, the electromagnetic suction disappears, and the armature will return to its original position under the reaction force of the spring, so that the moving contact and the static contact (normally closed contact) attract each other. At this time, the green bulb lights up. In this way, the purpose of turning on and off in the circuit is achieved by the pull in and release.

The normally open contact and normally closed contact of the relay can be distinguished as follows: the static contact in the disconnected state when the relay coil is not energized is called the normally open contact; The static contact in the on state is called "normally closed contact". Generally, a relay has two circuits, one is the control circuit, and the other is the working circuit.

4、 Detection of relay

1. Measure resistance

Use a multimeter to detect the resistance value of the relay coil with electrical blocking, so as to judge whether there is an open circuit in the coil.

2. Power on detection

If the resistance meets the requirements, apply working voltage to the relay coil, and then check the continuity of the contact with a multimeter. If the contact is normally open, the contact should be closed after applying working voltage, and the measured resistance is 0; If it is a normally closed contact, the contact should be disconnected after the working voltage is applied, and the measured resistance is infinite.

Use multimeter resistance R × Check the connection terminals 86 and 85 in gear 100, which should be connected (with a certain resistance value), and the resistance between connection terminals 30 and 87 should be infinite; Apply 12V voltage between terminals 86 and 85, and measure terminals 30 and 87 with a multimeter. They should be conductive. If the test result is inconsistent with the above, the relay is damaged.