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Solid State Relays: types, uses, advantages and applications

News | Feb 17,2023

Solid State Relays (SSRs) are electronic devices designed to switch electrical loads on and off. They are widely used in various industries due to their many advantages over their electromechanical counterparts.

SSRs can be used to control AC and DC loads and can be used in a variety of applications. Some of the most common applications for SSRs include process control, automotive systems, and lighting control.

If you are looking for a more efficient and reliable way to control electrical loads, solid-state relays may be the right choice for you.

SNC05-P1 Solid State Slim Relay PCB Socket Physical drawing of product application

Components of an SSR

SSRs use a different kind of semiconductor material instead of mechanical contacts to switch electrical signals.

 The main components of the SSR include:

 Control Circuit: This circuit includes input signal conditioning for conditioning the input signal to the output drive circuit. The input signal can be AC or DC, depending on the type of SSR.

 Output Drive Circuit: This circuit consists of output drive transistors or thyristors, which are used to switch the load. Output driver circuits are usually designed to handle load current and voltage.

 Isolation circuit: The isolation circuit is used to isolate the control circuit and the output drive circuit. This is necessary to prevent any interference between the control and output circuits.

 Heat sink: Since the SSR generates heat during operation, a heat sink is required to dissipate this heat. Heat sinks are usually made of aluminum or copper and are designed to provide effective cooling to the SSR.

 Overvoltage Protection Circuit: This circuit is used to protect the SSR from high voltage spikes that could damage the output drive circuit.

 Status Indicators: Typically contain LEDs or other status indicators to provide a visual indication of SSR operation.

Types of solid-state relays

SSRs are classified by their specific input circuitry, which is directly related to the output they achieve. Electrician-approved SSRs fall into three basic categories: reed relay coupled, transformer coupled, and optocoupled.

Reed Relay Coupled SSR

It applies a control signal directly to the coil of a Reed Relay. When the reed switch is closed, it activates a circuit that will trigger the thyristor switch.

Transformer-coupled SSR

It receives the input signal through a DC-AC converter, or directly if the current is AC. AC current is sent to a very low-power transformer, which triggers the thyristors to switch. The exact input-output configuration in such an SSR will depend on how the transformer is designed.

Opto-coupled SSR

It applies a control signal to a light or infrared light source. The photosensitive semiconductor then detects the radiation and switches the load. The beam is the only “coupling path” between input and output in this type of SSR.

Mechanical Relay

The Difference Between Solid State Relay and Mechanical Relay

Relays are electrical switches used to control the flow of electrical current. Mechanical relays use mechanical contacts and electromagnets to switch currents, while solid-state relays (SSRs) use semiconductor devices to switch currents.

Solid-state relays are more reliable and last longer than mechanical relays. They are also more resistant to vibration and shock, making them ideal for industrial applications. However, solid-state relays are more expensive and require more power to operate than mechanical relays.

What is a solid-state relay?

Solid state relays (SSRs) use semiconductor switches such as thyristors, triacs, or MOSFETs to control current flow without any mechanical contact.

The input signal to the SSR is usually a low-voltage DC signal, which triggers the semiconductor switch and allows current to flow through the SSR. The output voltage of an SSR is usually in the form of an AC or DC power source, which is turned on or off depending on the input signal.

What is mechanical really?

The input signal to a mechanical relay is usually a low-voltage DC signal that energizes the electromagnet and closes the mechanical contacts, allowing current to flow through the relay. The output voltage of a mechanical relay is usually AC or DC, which is switched on or off through mechanical contacts.

Solid state relay VS mechanical relay

Unlike mechanical relays, solid-state relays do not change the position of any components when switching between on/off and on/off states. Instead, solid-state relays work by converting incoming electrical control signals into light signals, usually output through infrared LEDs or similar devices.

The choice between solid state and mechanical relays will depend on the specific requirements of the application. Solid-state relays are typically used in applications requiring fast switching times, high reliability, and low noise, while mechanical relays are typically used in applications requiring low cost and high current capacity.

What are the advantages and disadvantages of solid-state relays?

Solid-state relays (SSRs) have several advantages and disadvantages over traditional electromechanical relays. Here are some key advantages and disadvantages of SSR:

Advantage Of Solid State Realy

Advantage Of Solid State Realy

Faster switching:

SSRs can turn on and off much faster than electromechanical relays, typically in the microsecond range, allowing them to respond to rapid changes in input signals.

No Mechanical Parts:

SSRs do not have any mechanical parts that can wear out or fail over time, making them more reliable and durable than electromechanical relays.

Less Noise:

Compared to electromechanical relays, SSRs produce less electrical noise, which can reduce interference with other sensitive electronic equipment.

No Contact Bounce:

SSRs have no contact bounce, which can cause parasitic signals and delayed response times in electromechanical relays.

No Magnetic Interference:

SSRs do not have electromagnetic coils, which means they do not generate any magnetic interference.

No arcing:

SSRs have no contacts that can arc, which can damage the contacts and surrounding components.

Disadvantages of Solid State Relays

More expensive:

SSRs are generally more expensive than electromechanical relays, which may make them less cost-effective in some applications.

Limited current capacity:

SSRs have limited current capacity, which means they may not be suitable for high-power applications.

Heat dissipation:

SSRs generate heat during operation, which may require additional heat sinks or cooling systems to dissipate heat.

Voltage drop:

here can be a voltage drop across the SSR’s semiconductor switches, which reduces the voltage available to the load.

Sensitivity to overvoltage:

SSRs can be sensitive to overvoltage, which can damage semiconductor switches.

SR20F Plastic Retaining Clip For SRC Socket

Applications of Solid-State Relay

Solid-state relays are used in a variety of applications from home automation to industrial motor control. But it is especially well-suited for process applications where a PLC or other microcontroller-based circuit controls a machine tool. Below are some of the most common applications.

Motor Control

One of the most common applications for solid-state relays is motor control. You can use SSRs to control AC and DC motors, from small motors in home appliances to large industrial motors.

Lighting Control

Solid-state relay applications also include switching loads such as light bulbs and LED arrays. In these applications, these relays have the advantage of fast switching speeds, which is important for certain lighting effects.

Heater Control

Solid-state relays are widely used in heating (and cooling) systems for air conditioners, electric ovens, and industrial heaters or furnaces. Other devices can also be used. However, SSRs have the advantage of being able to handle fairly high voltages while still being compact in size.

Medical Equipment

Due to their critical nature, medical equipment requires specialized control systems to turn the power on and off. Controllers with solid-state relay switches are perfect for this requirement and are very reliable and use very low input signals.

Automotive Solid State Relays

In the automotive field, SSR relays are important switching devices. They are rapidly replacing older mechanical relays in engine management systems, headlamp dimming circuits, and fog lamp control applications.

Water Pumps

Water pumps contain electric motors and other systems that need to be turned on and off as needed. You can do this with an AC solid-state relay.


CNC stands for Computer Numerical Control and refers to the process by which computers are used to automate machine tools. This technology is used in various industries such as woodworking, metalworking, and plastics processing.


Communication systems need to switch high currents and voltages quickly and reliably. This is why solid-state relays are often used in these applications due to their better features and functionality.

Applications of Solid State Relay


Solid State Relays (SSRs) are critical components in modern electrical systems. Various types of SSRs, such as AC and DC SSRs, have unique features and are used in various applications such as industrial controls, medical equipment, and heating systems.

If you’re interested in finding out how SSR can be used in your application, we suggest following us on our social media channels or contacting us via telephone (0086-135 8771 2673) or email (sales@shenler.com). Don’t hesitate to contact us, we are here to help!

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