The working principle of the relay and the drive circuit

A relay is an electronic control device, which has a control system (also called an input loop) and a controlled system (also called an output loop). It is usually used in automatic control circuits. In fact, it uses a smaller current to control a larger An "automatic switch" of electric current. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.

Relay characteristics

The input signal x of the relay increases continuously from zero to the action value xx when the armature starts to be closed. The output signal of the relay immediately jumps from y=0 to y=ym, that is, the normally open contact changes from off to on. Once the contact is closed, the input quantity x continues to increase, and the output signal y will no longer change. When the input quantity x drops from a value greater than xx to xf, the relay starts to release and the normally open contact is disconnected. We call this characteristic of the relay the relay characteristic, also called the input-output characteristic of the relay.

1. The working principle and characteristics of relay

Relay working principle and driving circuit analysis

1. The working principle and characteristics of electromagnetic relay

Electromagnetic relays are generally composed of iron cores, coils, armatures, contact reeds, etc. As long as a certain voltage is applied to both ends of the coil, a certain current will flow in the coil, which will produce electromagnetic effects. Under the action of electromagnetic force, the armature will overcome the pull force of the return spring and attract to the core, thereby driving the armature. The moving contact and the static contact (normally open contact) are pulled together. When the coil is de-energized, the electromagnetic attraction will also disappear, and the armature will return to the spring’s reaction force.

Return to the original position to release the moving contact and the original static contact (normally closed contact). This pulls in and releases, so as to achieve the purpose of conducting and cutting off in the circuit. The "normally open and normally closed" contacts of the relay can be distinguished as follows: the static contact that is in the off state when the relay coil is not energized is called "normally open contact"; the static contact that is in the on state is called It is a "normally closed contact".

2, circuit principle

2.1 Brief introduction of relay

  basic concepts

Relay is a kind of contact (or circuit) that connects or breaks AC and DC small-capacity control loop when the input quantity changes to a certain value.

2.2 Working principle

Relay working principle and driving circuit analysis

The magnet is kept in the released state, and after the working voltage is applied, the electromagnetic induction causes the armature and the magnet to generate attractive and repulsive moments, resulting in a downward movement to reach the attracting state.

3, transistor drive circuit

3.1 Circuit schematic diagram

Relay working principle and driving circuit analysis

When transistors are used to drive relays, NPN transistors are recommended. The specific circuit is as follows:

Introduction to working principle

When the input is high, the transistor T1 is saturated and turned on, the relay coil is energized, and the contact is closed.

When the low level is input, the transistor T1 is cut off, the relay coil is de-energized, and the contact is disconnected.

3.2 The role of each component in the circuit

Transistor T1 is a control switch.

Resistor R1 mainly acts as a current limiter and reduces the power consumption of transistor T1.

Resistor R2 makes transistor T1 cut off reliably.

Diode D1 reverse freewheeling, which provides a bleeder path for the relay coil when the transistor turns from on to off, and clamps its voltage to +12V.

4, integrated circuit drive circuit

Relay working principle and driving circuit analysis

At present, an integrated circuit integrated with multiple driving transistors has been used. Using this integrated circuit can simplify the design process of a printed board that drives multiple relays. At present, the main integrated circuits used to drive relays in our company are TD62003AP.

When the 2003 input terminal is at high level, the corresponding output port outputs low level, both ends of the relay coil are energized, and the relay contacts are closed;

When the 2003 input terminal is low, the corresponding output port is in a high-impedance state, both ends of the relay coil are de-energized, and the relay contact is disconnected.

24V relay drive circuit

Relay working principle and driving circuit analysis

Relay series RC circuit: This form is mainly used in circuits where the rated working voltage of the relay is lower than the power supply voltage. When the circuit is closed, the electromotive force of the relay coil will hinder the increase of the current in the coil due to the self-inductance phenomenon, thereby prolonging the pull-in time. After connecting the RC circuit in series, the pull-in time can be shortened. The principle is that at the moment the circuit is closed, the voltage across the capacitor C can’t change suddenly and can be regarded as a short circuit. In this way, a power supply voltage higher than the rated working voltage of the relay coil is applied to the coil, which speeds up the current increase in the coil and makes the relay Snap in quickly. After the power supply is stable, the capacitor C does not work, and the resistor R acts as a current limiter.

2. Selection of the rated working voltage of the relay

The rated working voltage of the relay is one of the main technical parameters of the relay. When using a relay, you should first consider the working voltage of the circuit (ie the circuit where the relay coil is located). The rated working voltage of the relay should be equal to the working voltage of the circuit. Generally, the working voltage of the circuit is 0.86 of the rated working voltage of the relay. Note that the voltage of the workpiece in the circuit must not exceed the rated working voltage of the relay, otherwise the relay coil is easily burned. In addition, some integrated circuits, such as the NE555 circuit, can directly drive the relay to work, while some integrated circuits, such as the COMS circuit, have a small output current and need to add a transistor amplifier circuit to drive the relay. This should consider that the output current of the transistor should be greater than that of the relay. Rated working current.

1, transistor drive circuit

When a transistor is used to drive a relay, the emitter of the transistor must be grounded. The specific circuit is as follows:

Relay working principle and driving circuit analysis

2. Introduction to the principle

NPN transistor drive: When the base of the transistor T1 is input high level, the transistor is saturated and turned on, and the collector turns to low level, so the relay coil is energized and the contact RL1 is closed. When the base of the transistor T1 is input low, the transistor is cut off, the relay coil is cut off, and the contact RL1 is cut off.

Editor's comment: This article introduces the working principle of the relay and the drive circuit of the relay. The design of the drive circuit depends on the pull-in voltage and current of the relay coil used. It must be greater than the pull-in current of the relay to make the relay work reliably.