Different types of solid state relays are used in different industries and applications. When planning to use these devices in your project, you need to identify the most appropriate type that will serve the purpose intended. This in-depth guide to solid state relay types will help you do exactly that.
Solid State Relay Basics
A solid state relay refers to a non-contact control device that uses a semiconductor to complete the ON/OFF action. These days, SSRs are found in various applications that include computers, consumer electronics, office machines, and industrial controls.
The solid state relay working is pretty simple. Once the control voltage is applied to the terminals, small current flows through the LED. The light generated by the LED activates a photovoltaic (PV) cell that produces enough voltage to trigger the SCR.
Solid state relays come with many benefits. They have a long lifespan, work at high speeds and don’t produce any sparks, which makes them ideal for use in explosive atmospheres.
Now that you know the basics, let’s take a more detailed look at the different types of solid state relays that are available on the market today.
Types of Solid State Relays
To cater to the different needs of various industries, engineers have developed a broad range of solid state relay types. These include relays that can be used with AC or DC load voltages, those that come with special features, and so on. The different types of SSRs can be broadly classified into the following categories:
- Types of Solid State Relays by Input/Output
- Types of Solid State Relays by Output Power
- Types of Solid State Relays by Isolation Method
- Types of Solid State Relays by Switching Method
- Types of Solid State Relays by Mounting Method
1. Types of Solid State Relays by Input/Output
The types of solid state relays can be grouped by their input/output power type, i.e., whether they are designed to be used with AC or DC load voltages. The types of SSRs in this category include DC-AC, DC-DC, AC-DC, and AC/DC to AC types:
DC to AC SSR
This type uses a DC input voltage to control an AC load voltage. These SSR types will only work with alternating current output supplies. To ensure there’s no reverse of polarity, the DC to AC solid state relay usually come equipped with a protection diode in its circuit.
DC to DC SSR
This type of relay uses a DC input voltage to control a DC load voltage and are mostly composed of IGBTs and MOSFETs. Again, the relays can only allow current to flow in one direction. Just like in the above type, a diode is used to ensure that.
AC to DC SSR
The AC-DC solid state relay uses an AC input voltage to control a DC load voltage. Because AC current can change directions, this type of relay uses a rectifier to convert the AC input into DC. This is the type of control signal that the photocoupler can work with.
AC/DC to AC SSR
The AC/DC to AC types of solid state relays are built for use with both AC and DC voltages. To do that, it comes with separate terminals for DC and AC inputs and the rectifying circuit mentioned earlier.
2. Types of Solid State Relays by Output Power
Output power is another factor that can be used to classify different types of solid state relays. The output power is the type of current supplied by the load circuit. These relays include:
As the name suggests, this type of relay is built to only switch load power supplies that operate on direct current (DC). Because of this, the DC SSR is not suitable (and cannot be used) for loads that require an alternating current (AC).
A DC solid state relay can further be a constant current type or a resistive current type. Their differences are explained below.
- Constant current DC SSR: This type of relay is designed to meet the needs of loads that require a specific current level.
- Resistive current DC SSR: On the other hand, this type is designed for use with resistive loads such as heaters, where current will vary with voltage.
The AC SSR is the exact opposite of the DC SSR and can only be used with loads that require an AC power supply.
These are the most common types of solid state relays and are available in a very broad range of subtypes. The different types of SSRs that are designed for AC power are:
- Zero Crossing SSR: the zero crossing solid state relay is designed to operate only when the AC load voltage is at or near zero. These devices are used in applications where low EMI (electromagnetic interference) and noise are a concern.
- Random Turn-On SSR: The random turn-on solid state relay, on the other hand, can be turned ON at any point during the AC load voltage cycle. These devices are typically used in applications where there is no need for special noise or EMI suppression.
- Peak Sensing SSR: The peak sensing AC SSR is specially designed to turn ON only when the AC load voltage reaches its peak value. These devices are used in applications where it is necessary to minimize the inrush current.
- Single phase SSR: The single-phase solid state relay is the most common type of AC SSR. As the name suggests, these devices are designed to switch a single phase of the AC load voltage.
- Three-phase SSR: The three-phase solid state relay is designed for use with loads that require a three-phase power supply. These include large electric motors.
3. Types of Solid State Relays by Isolation Method
Isolation in solid state relays refers to the electrical separation between the input and output sides of the device. There are three main types of solid state relay isolation methods used in SSRs, i.e., optical, transformer, and reed.
Photo Coupled SSR
Optical isolation uses an infrared LED and a phototransistor to separate the input and output sides of the relay. The LED emits infrared light that is used to activate the phototransistor, which in turn, triggers the output side of the relay.
These solid state relay types can achieve very high isolation voltage differences. This allows engineers to design very compact relays without compromising on isolation performance.
Transformer Isolated SSR
The transformer-isolated solid state relay uses a high-frequency transformer to electrically isolate the input and output sides of the device. The transformer is usually coupled with an isolation amplifier to provide the required level of protection.
Transformer-based SSRs are mostly used in industrial applications where high currents and voltages are involved. They are also used in applications where a high degree of protection is required.
Reed Isolated SSR
The reed-isolated solid state relay uses a hermetically sealed reed switch as the isolating element. The reed switch is a type of switching or sensing device that’s activated by a magnetic field generated by an external coil. Read switch in an SSR closes a thyristor to produce the switching action.
The hybrid SSR, as its name implies, is a combination of two (or more) isolation or relay technologies. Essentially, it uses an electromagnetic switch and a solid state relay to provide the switching action.
The SSR part is located on the input and output sides. These are then connected, in parallel, to the electromagnetic part. Hybrid solid state relays combine the advantages of solid state switching and those of the electromechanical relay to provide superior performance.
For example, the relay will spark when switched on or off, and there is no contact wear. Also, the SSR can switch very quickly, while the relay can handle very high currents.
4. Types of Solid State Relays by Switching Method
Different types of solid state relays are built to use different switching methods. Essentially, these include the normally open (NO) type and the normally closed (NC) type.
Normally Open SSR
The NO type of SSR has an open circuit between the input and output when the SSR is in the OFF state. The current will only flow when the input is turned ON, thus closing the circuit and activating the output.
This type of SSR is mostly used in applications where it is necessary to maintain a safe condition when the power is OFF. For example, in emergency stop circuits.
Normally Closed SSR
The normally closed solid state relay, on the other hand, has a closed circuit between the input and output when the device is in the OFF state. The current will only stop flowing when the input is turned ON, which opens the circuit and deactivates the output.
5. Types of Solid State Relays by Mounting Method
Based on their mounting methods, the types of solid state relays can be categorized into the following: DIN rail mounted solid state relay, PCB mount solid state relay, bracket mounted solid state relay, and panel or surfaced mounted types.
DIN Rail Mounted SSR
As the name implies, DIN rail-mount solid state relays are designed to be mounted on a DIN rail. These are mostly used in industrial and commercial applications. The advantage of this type of SSR is that it can be easily installed and removed from the DIN rail.
PCB Mounted SSR
The PCB-mount solid state relay is designed to be installed directly onto a printed circuit board (PCB). These are mostly used in consumer and industrial applications, and be socket-mounted or plug-in types. A PCB solid state relay can be quite compact, which makes it ideal for use in space-constrained applications.
Bracket Mounted SSR
The bracket-mounted solid state relay is mounted on a metal bracket. They are also called flange-mounted relays since they often have a flange for mounting purposes. These types of SSRs are mostly used in where space is not a constraint.
Panel Mounted SSR
Panel mount solid state relay types are designed for installation onto a panel and are also known as surface-mounted relays. Because of their small size, they are mostly used in consumer and industrial applications.
Different types of solid state relays are designed in many different types. This makes it possible to find one that is specifically built for a particular application. In light of this, it’s important to carefully consider the specific requirements of an application before choosing your SSR. It’s also equally important to understand the different types of these relays and their specific characteristics, as covered in this article.