Solid State Relay Meaning, Operation and Price

In the early 70’s, a fast-switching, compact relay called solid state relay was made. Today, the relay can be found in a broad variety of applications and industries. This makes it a very important switching device. Read through this post to acquaint yourself with solid state relay modules, their operation, types, and current market price.

What is a Solid State Relay?

A solid state relay, SSR, is a kind of relay that, unlike the traditional mechanical type, contains no physical contacts or moving parts. Instead, it makes use of semiconductor components to provide the switching action. Here is a deeper look at the solid state relay meaning. Also, it’s purpose and key benefits:

Solid State Relay Definition

In the world of electronics, solid state means using semiconductors. So a solid state relay module is a switching device that uses electronic parts instead of the traditional coil and electromechanical contacts.

The semiconductors include silicon controlled rectifiers (SCRs), metal-oxide semiconductor field-effect transistors (MOSFETs), and more. The typical solid state relay construction is also composed of these different sections:

• Input: Terminals where the control signal that switches the SSR ON or OFF is applied.
• Isolation: An important part of the relay that keeps the input and output circuits electrically isolated.
• Driver: The circuit that provides the necessary gate current to turn ON the semiconductor switch.
• Output: The output is the load current circuit.

Solid State Relay Symbol

In electrical schematic diagrams, the solid state relay symbol must be clearly marked, usually as a square block. Note also, that the electrodes are indicated by their symbols, next to the relay symbols. When drawing circuits with more than one SSR, the relay symbols must be marked as SSR1, SSR2, and so on.

For solid state relay schematic diagram, it’s a common practice to indicate the symbols of the semiconductor devices used. This helps to present, in a simple way, the relay’s internal structure and circuitry.

Solid State Relay Function

The main solid state relay purpose is to switch an electrical circuit. Essentially, it lets a small control signal turn a much larger load current on or off. This is why SSRs are often used in devices that need to switch electrical loads. With that said, let’s see where these relays are needed.

Solid State Relay Uses

The most common solid state relay uses, and where the device offers advantages over a mechanical relay, include the following:

• Automotive: The automotive industry uses SSRs to control various functions, such as headlights, wipers, and doors.
• Computers and peripherals: In computers, SSRs are used to control the power supply and cooling fans. They are also used in printers and scanners.
• Consumer electronics: Solid state relay controller circuits are widely used in consumer electronics, such as TVs and game consoles.
• Home appliances: Many home appliances, such as washing machines, dishwashers, and clothes dryers, use SSRs.
• Industrial: The industrial applications of solid state relays include process control, machine tooling, and packaging machinery.
• Medical: The medical industry uses SSRs to control the circuits of various power systems and a range of medical equipment.
• Telecommunications: The use of solid state relay devices in telecommunication equipment systems is one of the most common.
• Hobbyist projects: On the other sides, hobbyists often use a solid state relay with Arduino for circuit power control.

Solid State Relay Benefits

An SSR offers several benefits when compared to the electromechanical relay (EMR), which explains its widespread use. The main solid state relay benefits include:

• They have a long lifespan since they contain no moving parts or arcing contacts.
• They are more resistant to shock and vibration and can withstand harsh environments.
• SSRs can switch large loads with a small control signal
• They have fast switching speeds
• High sensitivity and low EMI
• SSR operate silently
• Solid state relays are generally smaller that electromechanical types.

These relays will also have downsides. One disadvantage of SSRs is that they usually have a higher cost than electromechanical relays. They are also more prone to overheating and output leakage current.

Solid State Relay Types

Solid state relays are largely classified by the type of output power supply they are built for. Therefore, we have the following types of SSRs: DC SSR and AC SSR.

DC Solid State Relay

These types of solid state relays are used for switching DC loads. The key advantage of using a DC SSR is that it can handle high inrush currents that are common with inductive loads. A DC solid state relay mostly uses MOSFETS, BJT, and IGBTs as their switching components.

AC Solid State Relay

An AC solid state relay is used for switching AC loads. These types of relays are available in a variety of configurations, including zero-crossing and random turn-on SSR. The main difference between the two is that zero-cross SSRs will only turn on when the AC voltage is at or near zero, while random turn-on SSRs can be turned on at any time.

The solid state relay types on the market will also usually be a standard or industrial version. The standard SSR is the more basic model made for general use, while the industrial solid state relay is more rugged and heavy-duty, and designed for harsher environments and higher loads.

Solid State Relay Operation

Note that our description of the solid state relay working will be based on optocoupler (or photocoupler) SSRs. These are the most popular types – and what you will likely want to use for your project. Here, is the solid state relay operation explained.

• The photocoupler type SSR has an optically isolated input and output. That is, the input and output are completely isolated from each other, electrically speaking.
• The solid state relay optocoupler is usually an LED on the input side that illuminates a phototransistor on the output side, which then turns on/off the power semiconductor.
• When the input voltage (which is the control signal) is applied, it turns on a light-emitting diode (LED) on the input side. This, in turn, activates a phototransistor on the output side.
• The phototransistor then activates the power semiconductor, which operates a circuit to either turn on or off the load.
• When the input voltage is removed, the LED turns off and the phototransistor deactivates, which then turns off the power semiconductor, depending on the relay’s configuration.

Another type of solid state relay uses a transformer to provide electrical isolation between the input and output circuits and other functions. There are also SSRs that use a combination of optocoupler and transformer components.

Solid State Relay Price

One of the first considerations when planning on using solid state relays is the price. The solid state relay price will often start at around $10 per device. A cheap solid state relay may cost less, say $5, but that will be a small relay for general use or for smaller loads. The more specialized or industrial models can cost tens of dollars.

SSR devices are more expensive than EMRs, generally, owing to their construction that requires a more complex process to make. They also often use more advanced features such as zero-voltage turn on and reverse blocking capability.

However, the increased cost of SSRs is often offset by their increased reliability and longer lifespan. In many applications, the solid state relay cost is justified by the fact that they will last for several years, while also offering the ability to use it in complex circuits.

Solid State Terminology

Before concluding this article, it’s worth quickly going over some of the key solid state relay terminology that you’ll need to know. These are terms you will find printed on the relay’s data sheet, in tutorials or relay manual, and so on. With that said, let’s get started:

Solid State Relay Input Voltage

The input voltage is the voltage that must be applied to the SSR’s control input for it to operate. This is typically a DC voltage, but AC voltages can also be used. The most common range of solid state relay input voltage range is 3-32VDC.

Solid State Relay Input Current

This is used to refer to the minimum current that must flows through the SSR’s input to make it operate. As mentioned earlier, the solid state input current can be DC or AC. The typical range is about 10mA for DC or single phase AC inputs and around 30mA for 3-phase AC.

Solid State Relay Output Voltage

The rated output voltage of a solid state relay is the maximum voltage that can be applied to its output terminals Every SSR will have a specific maximum output voltage in line with its intended application and this voltage will be printed on the relay’s data sheet.

Solid State Relay Output Current

This is the maximum current that can flow through the SSR’s output terminals. Just like voltage, rated the output current of an SSR will vary across the devices based on the intended use and other factors.

Solid State Relay Voltage Drop

The solid state relay voltage drop is the voltage that appears across the SSR’s output terminals when the relay is turned on. SSRs contain internal resistance, and this voltage drop is caused by the current flowing through this resistance.

Solid State Relay Leakage Current

The solid state relay leakage current is the current that flows through the SSR’s output terminals when the relay is turned off. This leakage current is usually very small, in the order of a few mA – and usually negligible.

Solid State Relay Inrush Current

The solid state relay inrush current is the maximum surge current that can flow through its output terminals without causing damage. This is usually much higher than the steady-state current, and can be several times the rated output current of the SSR.

The inrush current lasts for a very short period of time, though, typically less than 1 second. It’s usually caused by the inductance of the load, and can be much higher than the normal operating current.

Solid State Relay Resistance

Solid state relay resistance can be used to mean insulation resistance or dielectric strength. SSR insulation resistance refers to the resistance between the input and output terminals when a specified voltage is applied.

Dielectric strength is the maximum voltage that can be applied across the input or output terminals without causing a dielectric breakdown.

Conclusion

So, that’s a basic overview of the solid state relay. Hopefully, this has given you a better understanding of what these relays are and how they work. Also, about their benefits, function, and current market price – especially if planning to use them in your project. In our next articles, we’ll take a look at some of the most common types of SSR relays, their applications and a lot more.