Practical | In-depth interpretation of the working principle of relays and analysis of drive circuits

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Relay characteristics of relays

When the input signal x of the relay increases continuously from zero to the action value xx when the armature begins to attract, 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 begins to release and the normally open contact is disconnected. We call this characteristic of the relay the relay characteristic, also known as the input-output characteristic of the relay.

1. Working principle and characteristics of relay

1. Working principle and characteristics of electromagnetic relay

Electromagnetic relays are generally composed of an iron core, a coil, an armature, and a contact spring. As long as a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, thereby generating an electromagnetic effect. The armature will overcome the pulling force of the return spring under the action of electromagnetic attraction and be attracted to the iron core, thereby driving the moving contact of the armature and the static contact (normally open contact) to attract. When the coil is powered off, the electromagnetic attraction will disappear, and the armature will return to its original position under the reaction force of the spring, so that the moving contact and the original static contact (normally closed contact) are released. In this way, the attraction and release achieve the purpose of conduction and disconnection in the circuit. The "normally open and normally closed" contacts of the relay can be distinguished as follows: the static contact in the disconnected state when the relay coil is not powered is called the "normally open contact"; the static contact in the connected state is called the "normally closed contact".

2. Circuit principle

2.1 Brief Introduction of Relay

basic concept

A relay is a device that connects or disconnects a small-capacity AC or DC control circuit when the input changes to a certain value.

2.2 Working Principle

The permanent magnet maintains the released state. When the working voltage is applied, electromagnetic induction causes the armature and the permanent magnet to produce attraction and repulsion torques, resulting in downward movement, and finally reaching the attracted state.

3. Transistor drive circuit

3.1 Circuit Schematic

When the transistor is used to drive the relay, it is recommended to use an NPN transistor. The specific circuit is as follows:

Working principle introduction

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

When the input is low level, transistor T1 is cut off, the relay coil is de-energized, and the contacts are opened.

3.2 The role of each component in the circuit

Transistor T1 is a control switch.

The resistor R1 mainly plays the role of current limiting and reduces the power consumption of the transistor T1.

The resistor R2 makes the transistor T1 reliably cut off.

二极管D1反向续流，为三极管由导通转向关断时为继电器线圈中的提供泄放通路，并将其电压箝位在+12V上。

4. Integrated circuit drive circuit

At present, integrated circuits with multiple driving transistors are used, which can simplify the design process of printed circuit boards that drive multiple relays. The integrated circuits that our company uses to drive relays are mainly TD62003AP.

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

When the input terminal of 2003 is at a low level, the corresponding output port is in a high impedance state, the power is cut off at both ends of the relay coil, and the relay contacts are disconnected.

24V relay drive circuit

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 relay coil will generate an electromotive force due to the self-inductance phenomenon, which hinders the increase of the current in the coil, thereby extending the pull-in time. After the RC circuit is connected 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 cannot 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 added to the coil, thereby accelerating the speed of the current increase in the coil and causing the relay to close quickly. After the power supply is stable, the capacitor C does not work, and the resistor R plays a current limiting role.

2. Selection of rated working voltage of relay

The rated working voltage of the relay is the most important technical parameter of the relay. When using the relay, the working voltage of the circuit (that is, the circuit where the relay coil is located) should be considered first. 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 workpiece voltage of the circuit must not exceed the rated working voltage of the relay, otherwise the relay coil will easily burn out. In addition, some integrated circuits, such as NE555 circuits, can directly drive the relay to work, while some integrated circuits, such as COMS circuits, have a small output current and need to add a transistor amplifier circuit to drive the relay. In this case, it should be considered that the transistor output current should be greater than the rated working current of the relay.

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:

2. Principle Introduction

When driving NPN transistor: When a high level is input to the base of transistor T1, the transistor is saturated and turned on, and the collector becomes a low level, so the relay coil is energized and the contact RL1 is closed. When a low level is input to the base of transistor T1, the transistor is cut off, the relay coil is de-energized, and the contact RL1 is disconnected.

Comment: This article introduces the working principle of the relay and the driving circuit of the relay. The design of the driving circuit should be based on the pull-in voltage and current of the relay coil used. It must be greater than the pull-in current of the relay in order for the relay to work reliably.

Source: Network compilation