A relay is an electronic control device that has a control system (also known as an input circuit) and a controlled system (also known as an output circuit). It is commonly used in automatic control circuits and is essentially an "automatic switch" that uses a smaller current to control a larger current. Therefore, it plays a role in automatic adjustment, safety protection, and circuit switching in circuits.
Relay characteristics
When the input signal x of a relay continuously increases from zero to the operating value xx that triggers the armature to engage, the relay's output signal immediately jumps from y=0 to y=ym, meaning the normally open contact changes from closed to open. Once the contact closes, further increases in the input x will not cause any change in the output signal y. When the input x decreases from a value greater than xx to xf, the relay begins to release, and the normally open contact opens. This characteristic of a relay is called its relay characteristic, or its input-output characteristic.
I. Working principle and characteristics of a relay
1. Working principle and characteristics of electromagnetic relays
Electromagnetic relays generally consist of an iron core, coil, armature, and contact springs. When a certain voltage is applied across the coil, a current flows through it, generating an electromagnetic effect. The armature, attracted by this electromagnetic force, overcomes the tension of the return spring and is drawn towards the iron core, causing the moving contact of the armature to engage with the stationary contact (normally open contact). When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the reaction force of the spring.
Returning to its original position releases the moving contact from the original stationary contact (normally closed contact). This engagement and disengagement achieves the purpose of connecting and disconnecting the circuit. The "normally open" and "normally closed" contacts of a relay can be distinguished as follows: a stationary contact that is in the open state when the relay coil is not energized is called a "normally open contact"; a stationary contact that is in the closed state is called a "normally closed contact".
2. Circuit Principle
A relay is a small-capacity AC/DC control circuit whose contacts (or circuits) connect or disconnect when the input quantity changes to a certain value .
The armature is kept in the released state by a permanent magnet. When the working voltage is applied, electromagnetic induction causes the armature and the permanent magnet to generate attractive and repulsive torques, resulting in downward movement and finally reaching the attracted state.
3. Transistor driver circuit
When transistors are used to drive relays, NPN transistors are recommended . The specific circuit is as follows:
When the input is high, transistor T1 is saturated and turned on, the relay coil is energized, and the contacts are closed.
When the input is low, transistor T1 is turned off, the relay coil is de-energized, and the contacts open.
The functions of each component in the circuit are as follows: Transistor T1 is the control switch; Resistor R1 mainly limits the current and reduces the power consumption of transistor T1; Resistor R2 ensures that transistor T1 is reliably turned off; Diode D1 provides a reverse freewheeling current and provides a discharge path for the relay coil when the transistor switches from conducting to turning off, and clamps its voltage at +12V.
4. Integrated circuit driver circuit
Currently, integrated circuits that integrate multiple driver transistors are used, which simplifies the design process of printed circuit boards that drive multiple relays. Our company currently primarily uses the TD62003AP integrated circuit for driving relays .
When the 2003 input terminal is high, the corresponding output port outputs a low level, the two ends of the relay coil are energized, and the relay contacts are energized.
When the 2003 input is low, the corresponding output is in a high-impedance state, the relay coil is de-energized, and the relay contacts open.
Relay in series with an RC circuit: This type is mainly used in circuits where the relay's rated operating voltage is lower than the power supply voltage. When the circuit is closed, the relay coil generates an electromotive force due to self-induction, which hinders the increase of current in the coil, thus prolonging the pull-in time. Adding an RC circuit in series shortens the pull-in time. The principle is that at the instant the circuit closes, the voltage across capacitor C cannot change abruptly and can be considered a short circuit. This allows a power supply voltage higher than the relay coil's rated operating voltage to be applied to the coil, thereby accelerating the increase of current in the coil and causing the relay to quickly pull in. After the power supply stabilizes, capacitor C becomes inactive, and resistor R acts as a current limiter.
II. Selection of Rated Operating Voltage for Relays
The rated operating voltage of a relay is one of its most important technical parameters. When using a relay, the operating voltage of the circuit in which it is located (i.e., the circuit where the relay coil is located) should be considered first. The rated operating voltage of the relay should be equal to the operating voltage of the circuit in which it is located.
The operating voltage of the circuit is generally 0.86 times the rated operating voltage of the relay . It is crucial that the operating voltage of the circuit never exceed the rated operating voltage of the relay, otherwise the relay coil may burn out. Additionally, some integrated circuits, such as the NE555 circuit, can directly drive the relay, while others, such as CMOS circuits, have low output current and require an additional transistor amplifier stage to drive the relay. In these cases, the transistor output current should be greater than the rated operating current of the relay.
1. Transistor drive circuit
When a transistor is used to drive a relay, the transistor's emitter must be grounded. The specific circuit is as follows:
2. Brief Introduction to Principles
When driven by an NPN transistor: When a high-level input is applied to the base of transistor T1, the transistor saturates and conducts, and the collector becomes low, thus energizing the relay coil and closing contact RL1. When a low-level input is applied to the base of transistor T1, the transistor is cut off, the relay coil is de-energized, and contact RL1 opens.
summary:
This article introduces the working principle of relays and the relay driving circuit. The design of the driving circuit depends on the pull-in voltage and current of the relay coil. The voltage and current must be greater than the pull-in current of the relay to ensure that the relay works reliably.
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