Hello everyone, I am Rose. Welcome to the new post today. Today I will introduce relay to you. Including its definition, parameters, working principle and so on.

Ⅰ. The basic meaning of the relay

The switch-type electronic control devices known as relays are frequently employed. They can be categorized into electromagnetic relays, solid state relays, time relays, temperature relays, and photorelays based on various operating principles, structures, and functions. The electromagnetic relay is the major topic of this article.

The coil, armature, and contact make up the bulk of an electromagnetic relay. The coil is the relay's input component, and the contact is its output component. According to the electromagnetic effect, a magnetic field will be created when the coil has a proper current flowing, attracting the armature and causing the contact click action. To manage high currents with small currents, electromagnetic relays are frequently utilized in control circuits. The internal structure of the electromagnetic relay is shown in Figure 1.

Figure. 1 Structural composition of the relay

Ⅱ. Basic parameters of the relay

Understanding a relay's basic characteristics is important when choosing one. These characteristics include things like coil voltage, contact capacity, contact resistance, release voltage, etc.

When the relay is operating normally, the coil voltage is the voltage applied to the coil's two ends. When purchasing a relay, the question "How many volts of relay do you want?" is frequently posed.

The term "contact capacity" describes a relay's ability to handle a load, such as 2A/30VDC or 20A/220VAC;

The voltage at which the contacts are reset as the voltage across the coil decreases is referred to as the release voltage.

Figure. 2 Relay physical map

Normally open, normally closed, and normally open/normally closed combination types of relay connections are all common.

Ⅲ. How to operate the relay

Relays are essentially standard on the microcontroller development board, as my friends who have studied microcontrollers know. You can learn how to set up the microcontroller's IO port as an output by controlling the relays. Transistors may be used to drive low-power relays. The circuit diagram for driving relays with NPN and PNP transistors is shown below.

NPN transistor drive relay

It is necessary to connect the relay to the transistor's collector and a reverse diode in parallel with the coil when using an NPN transistor to operate a relay. Figure 3 displays the standard schematic diagram.

Figure. 3 NPN transistor drive relay

In the diagram above, the pull-down resistor between the base and emitter and the current-limiting resistor on the base work to prevent the base current from damaging the triode. The level may be zero when the microcontroller's IO port is initially initialized. The pull-down resistor disables the transistor in an unknown condition by lowering the base to a low level, keeping the relay from failing.

The triode is turned on, the relay coil is energized, and the relay contacts are activated when the base signal is at a high level;

The triode is turned off, the relay coil is de-energized, and the relay contacts are reset when the base signal is low;

PNP transistor drive relay

When using a PNP transistor to drive a relay, it is necessary to connect the relay to the collector of the transistor, and connect a reverse diode in parallel to the coil. The typical schematic diagram is shown in Figure 4.

Figure. 4 PNP transistor drive relay

In the diagram above, the pull-down resistor between the base and emitter and the current-limiting resistor on the base work to prevent the base current from damaging the triode. The level may be zero when the microcontroller's IO port is initially initialized. The pull-down resistor disables the transistor in an unknown condition by lowering the base to a low level, keeping the relay from failing.

The triode is turned on, the relay coil is energized, and the relay contacts are activated when the base signal is at a high level;

The triode is turned off, the relay coil is de-energized, and the relay contacts are reset when the base signal is low;

The role of freewheeling diodes

A freewheeling diode, which is connected in parallel above the coil in the first two driving techniques described above, is placed there. Due to the relay's coil's inductive properties, the reverse electromotive force will be induced and the current flowing through the coil won't change abruptly when the power is turned off. The triode could be harmed if the reverse voltage exceeds its withstand voltage rating. should stay away from this scenario. In order to protect the triode in the event of a power outage, a reverse freewheeling diode is added to provide a discharge channel for the reverse electromotive force.

Ⅳ. How does the relay save energy

Users today are concerned about power usage and want it to be as low as possible without sacrificing performance. For relays, this is pretty evident. Some relays require a significant amount of current to start and run due to their extremely low coil resistance. The relay needs a big current to activate, but just a little maintenance current to maintain the action state after the activation. Therefore, to achieve the energy savings of the relay drive, the current going through the coil

Figure. 5 Relay energy saving

There are two techniques to conserve energy:

Conserving energy by descending

The rated voltage is applied to the coil's two ends when the relay is turned on. The voltage at both ends of the coil is lowered to above the release voltage once the relay action has been stable for between 100 and 500 ms. This results in a small voltage applied to both ends of the coil and a modest current flowing through the coil. Figure 5's left side illustrates this.

PWM technique for energy conservation

The most popular energy-saving technique reduces the average voltage at both ends of the coil by using PWM to drive the control terminal of the triode or MOS tube, which results in coil energy savings.

Fig. 6 depicts the rough waveforms of the aforementioned two modes.

Figure. 6 Relay energy saving

Ⅴ. Basic knowledge of solid state relays

The electromagnetic relay's contacts are mechanical. A large current can be broken with little difficulty, producing an arc. After working in the arc environment for a prolonged period of time, the contacts will become corroded and invalid. As a result, the electromagnetic relay's contacts have a useful life. The contacts of the solid-state relay, which is an entirely electronic relay, are implemented using MOS tubes or thyristors. Electronic connections don't experience the arc problem, hence solid-state relays' contacts have an infinite lifespan and have a quick reaction time. There is less noise and no jitter issue.

The fundamental elements of solid state relays

AC solid state relays and DC solid state relays are the two categories of solid state relays. Optocouplers and thyristors are the major components of AC solid state relays, whereas optocouplers and MOSFETs are the main components of DC solid state relays.

Detection of Zero Crossing in Solid State Relays

There are two types of solid state relays: those that detect zero crossings and those that do not. If there is a trigger signal at the input end for an AC load without zero-crossing detection, the contact will act immediately. If there is a trigger signal at the input end for a load end with zero-crossing detection, the contact will wait until the load end has a zero-crossing Only move.

Figure. 7 Solid State Relay

Solid-state relays have electronic contacts, and when the current becomes excessive, a large-area heat sink is needed. Prices are higher when compared to electromagnetic relays.

Both solid state and electromagnetic relays have benefits and drawbacks.