The importance of electrically separating the controller circuit from the one being controlled is paramount in many electrical systems. For instance, consider a battery-powered circuit where a sensor activates an LED whenever it detects an object. While this setup is useful to some extent, we are likely to require additional actions triggered by the detection of an object.
Suppose we wish to replace the LED with a conveyor belt. One might assume that by simply changing the wires from the LED to a motor, the system will work. However, this would not work because although the circuit could detect the object and send an electric current to the motor, the current would be too small to activate it. Feeding the system with a higher current and voltage, capable of running the motor, introduces other complications. The components in the control circuit are designed to withstand only low voltages, and even if they could endure higher voltages, the system would need to switch from direct current (DC) to alternating current (AC), requiring additional components.
Fortunately, the use of a relay can solve all these problems by isolating the controller circuit from the one being controlled. The signal that previously activated the LED now controls the relay, which either opens or closes the passage of electricity to the motor, without the two circuits coming into direct contact.
Electromagnetic Relays: Basic Operation
Relays come in several types, but one of the most commonly used is the electromagnetic or electromechanical relay. These relays consist of three main components: an electromagnet, a moving armature, and a set of contacts. The electromagnet is connected to the controller circuit. When electric current flows through the electromagnet, it generates a magnetic field similar to that of a permanent magnet. According to Ampere’s law, an electric current in a conductor generates a magnetic field in a circular shape around it. By winding the wire in a spiral or coil shape, the magnetic fields generated by each turn of the wire accumulate, resulting in a stronger magnetic field. This effect is further enhanced by an iron core, which guides the magnetic field.
The moving armature is a lever made of insulating material, with one end containing iron, which is ferromagnetic and attracted to magnetic fields. When the electromagnet is activated, the armature moves to one of two possible states, depending on the direction of the magnetic field. The contacts are connected to the second circuit and remain separated, preventing the passage of current until the armature is activated.
How a Relay Works
When electric current reaches the electromagnet, it generates a magnetic field, which attracts the piece of iron in the moving armature. This causes the armature to change position and push its other end to one of the contacts, completing the circuit and activating the motor. The operation of the relay is simple yet highly effective, and various configurations of the relay allow for versatile applications.
Different Types of Relays
Relays can operate as either normally open or normally closed switches. In a normally open relay, the contacts are open when no current is passing through the electromagnet, and the circuit is closed only when the electromagnet is activated. In contrast, a normally closed relay allows current to pass when the electromagnet is not energized, and when it is activated, the contacts separate, halting the current flow.
The choice between a normally open or normally closed relay can have significant consequences depending on the specific application. For example, using a normally closed relay on a conveyor belt system ensures that the motor will only stop when the electromagnet is activated. If the controller fails or runs out of power, the motor would continue running until the issue is resolved. Additionally, selecting between these two types of relays can have energy-saving implications. For example, if a lamp needs to remain on for 20 hours and off for 4 hours, a normally closed relay would be more efficient, as it only needs to remain energized for the 4 hours it is off.
Single Pole and Double Throw Relays
Relays can also be categorized based on the number of poles and throws. A single pole, single throw (SPST) relay controls a single circuit, with a single set of contacts. By modifying a relay to a double throw (SPDT), it gains an additional connector, providing the ability to switch between two circuits. When the electromagnet changes state, the armature switches the current between the two circuits, offering more flexibility in control.
Switching from a single pole to a double pole relay allows for even more complex configurations. A double pole, double throw (DPDT) relay activates two identical relays simultaneously, allowing for applications such as reversing the polarity of a motor to change its direction. In this case, each time the relay changes state, the polarity of the current passing through the motor is reversed, changing the direction of rotation. However, stopping the motor would require an additional relay, such as a single pole relay.
Applications of Double Pole, Double Throw Relays
A practical example of the DPDT relay is in reversing the direction of a direct current (DC) motor. Since the direction of rotation depends on the polarity of the wires, a DPDT relay can switch the polarity by connecting one motor terminal to the normally closed contact of one pole and the other terminal to the normally open contact of the other pole. This configuration reverses the direction of rotation every time the relay switches states.
While this setup allows for direction control, it does not provide an option to stop the motor. This can be remedied by integrating additional relays, such as the single pole, single throw relay discussed earlier.
Relay Limitations and Alternatives
Despite their versatility, relays have certain limitations. Although they can separate circuits with different voltages and amperages, relays may not handle extreme configurations. For instance, if the voltage is too high, it can ionize the air between the contacts, resulting in an electrical arc that may cause the contacts to overheat, posing a fire risk.
Furthermore, electromagnetic relays rely on moving parts to create electrical contacts, which introduces a delay in the switching process. This delay, known as the rebound effect, occurs when the contacts repeatedly connect and disconnect before settling into position. While this may not be problematic for motors, it can affect the performance of digital circuits operating at higher frequencies.
To address these limitations, solid-state relays can be used, which have no moving parts and do not experience the rebound effect or electrical arcing. However, if electromagnetic relays are preferred, integrating an RC circuit can help mitigate some of these issues by smoothing out the switching process and reducing the likelihood of arcing.
