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An experienced engineer seeks to manage the switching and protection of electrical circuits effectively. A fundamental component used for such purposes is the relay. In this article, we will explore various types of relays to help simplify this task.

Introduction

In power systems, relays are considered essential electrical components, particularly in applications requiring the switching and protection of control circuits and other connected electrical loads and components. Relays function by opening or closing their contacts when exposed to voltages or currents. This article will introduce you to several types of relays available on the market, ensuring you are well-equipped when designing your engineering projects for power systems. Let’s dive into the details!

Relay Types

Relays are categorized into various types based on their electrical and construction characteristics. Each type of relay is designed for specific applications, and selecting the appropriate relay is crucial when integrating it into an electrical circuit. Relays can be classified as follows:

• Poles and Throws
• Operating Principles

Poles and Throws Relays

A relay is characterized by poles and throws. Poles refer to the number of switches in a relay, while throws indicate how many electrical circuits are controlled by each pole. Therefore, relays are classified based on the number of poles and throws within their electrical circuit.

Figure 1: Pole and Throw by Simon Mugo

Under this category, you will encounter the following types of relays:

• Single Pole Single Throw (SPST)
• Single Pole Double Throw (SPDT)
• Double Pole Single Throw (DPST)
• Double Pole Double Throw (DPDT)

Single Pole Single Throw (SPST)

A Single Pole indicates that the relay controls only one circuit, and a Single Throw means the relay has only one contact point. The diagram below illustrates this configuration.

Figure 2: Single Pole Single Throw Relay by Simon Mugo

Single Pole Double Throw (SPDT)

A Single Pole means controlling one circuit, while Double Throw refers to the relay having two contact points. The diagram below shows what this configuration looks like.

Figure 3: Single Pole Double Throw by Simon Mugo

Double Pole Single Throw (DPST)

A Double Pole means the relay controls two independent electrical circuits. The Single Throw indicates that each pole has a contact point to complete the circuit. This type of relay is useful when simultaneously switching two circuits, either open or closed. See the diagram below for reference.

Figure 4: Double Pole Single Throw by Simon Mugo

Double Pole Double Throw (DPDT)

This relay features two poles, each controlling a separate circuit, and each pole has two contact points, making it a Double Throw. It can be viewed as two SPDT relays in a single device, with the ability to switch both circuits simultaneously. Check out the image below for more details.

Figure 5: Double Pole Double Throw by Simon Mugo

It’s worth noting that you can find relays with up to 12 poles on the market.

Operating Principles Relays Category

Relays can also be classified according to their operating principles. These include:

• Electromechanical Relays (EMR)
• Solid State Relays (SSR)
• Hybrid Relays
• Reed Relays

Electromechanical Relays (EMR)

Electromechanical Relays (EMR) consist of a movable contact and an electromagnetic coil. When energized, the coil generates a magnetic field that attracts the movable contact. Upon de-energization, the magnetic field disappears, and a spring returns the contact to its original position. These relays can be used in both AC and DC applications. In DC circuits, a freewheeling diode is often used to protect the circuit from back EMF and coil de-energization.

EMRs are generally non-polarized, meaning they can function in both directions. However, when back EMF is present, the polarity must be carefully considered to protect the diode. A downside of EMRs is that they can create arcing when breaking, which results in high resistance and a shorter lifespan.

Figure 6: EMR Courtesy of Simon Mugo

Solid State Relays (SSR)

Solid State Relays (SSR) use semiconductors instead of mechanical parts and operate by isolating low-voltage control circuits from high-voltage power circuits using an optocoupler. The control input activates an LED, which emits infrared light that is received by a photosensitive semiconductor. This converts the light into electrical signals, which then switch the electric circuit.

SSRs offer faster operation and lower power consumption compared to EMRs, with the added advantage of a longer lifespan, as there are no moving parts to wear out. However, a disadvantage is that the semiconductor introduces a voltage drop, leading to some power loss.

Figure 7: Solid-State Relay Courtesy of Simon Mugo

Reed Relays

Reed relays consist of an electromagnetic coil, a flywheel diode to protect against back EMF, and a reed switch. The reed switch is made from ferromagnetic material and is sealed in a glass tube filled with inert gas. When energized, the magnetic field attracts the ferromagnetic blades, causing them to close the circuit.

Figure 8: Reed Relay Courtesy of Simon Mugo

Hybrid Relays

Hybrid relays combine elements of both EMRs and SSRs. They aim to overcome the limitations of each by eliminating the power loss associated with SSRs and the arcing issues related to EMRs. The SSR handles the load current, preventing arcing, while the EMR controls the switching without experiencing arcing, since the SSR is in parallel. Once the EMR completes its switching, the SSR is deactivated.

Figure 9: Hybrid Relay Courtesy of Simon Mugo

Summary

• Relays come in various types, which can be categorized by poles & throws and by operating principles.
• Poles represent the number