A solid state relay or an SSR is an electronic switching device which can be controlled by applying an external voltage to the control terminals. If we consider function, they are very similar to the electromechanical relays we are familiar with. But the solid state relays don’t have any mechanical parts, instead, they use solid state devices such as triacs as switching elements. Solid state relays function as a switch without the need for any mechanical intervention.
As we know a conventional relay uses electromagnets, springs and mechanical contacts for switching. Unlike conventional relays, SSR uses the electrical and optical properties of solid-state semiconductors for the isolation and switching functions. Like a conventional electro-mechanical relay, SSR provides complete electrical isolation between its control and output terminals. When turned off, the SSR will have an almost infinite resistance between its output terminals, acting as a switch. When turn on, the resistance between the output terminals will be dropped to a very low value, almost negligible, acting as a short circuit path between terminals.
The above image is a simplified solid state relay circuit. It consists of an input and output circuit. The input side is connected to the output circuit through an optocoupler, thus giving isolation between the input and output sides. When the optocoupler is activated by applying a voltage in between the input terminal, the driver circuit in the output section also gets activated, thus turning on the output.
Solid-state relays (SSRs) can be categorized into different groups based on various criteria such as the nature of the input control signal, the type of load they control, and their switching characteristics. Here are some common ways to group SSRs:
Here is a circuit diagram of a DC-controlled AC SSR.
Solid State Relays are widely used across various industries due to their reliability, long life, and fast switching capabilities. Here are some common Solid State relay Uses:
Some of the most popular manufacturers of Solid State Relays are Omron, Crydom, Siemens, Schneider Electric, Rockwell Automation (Allen-Bradley), Panasonic, Fotek, Carlo Gavazzi, IDEC, Phoenix Contact, Opto 22, and ABB.
Here's a comparative table highlighting the key differences between Solid State Relays (SSRs) and Electromechanical Relays (EMRs):
| Feature | Solid State Relay (SSR) | Electromechanical Relay (EMR) |
| Switching Mechanism | Uses semiconductor devices like thyristors, triacs, or MOSFETs. | Uses physical contacts that open and close. |
| Switching Speed | Very fast (on the order of microseconds to milliseconds). | Slower compared to SSRs (milliseconds to seconds). |
| Lifetime | Longer, due to the absence of moving parts and mechanical wear. | Shorter, as mechanical parts wear out over time. |
| Noise | Silent operation as there are no moving parts. | Audible clicking sound due to the movement of mechanical parts. |
| Electrical Noise | Generates minimal electrical noise. | Can generate electrical noise due to arcing when contacts open/close. |
| Size | Typically, more compact. | Generally larger, especially for higher power ratings. |
| Heat Generation | Generates more heat under load, often requiring heat sinks. | Generates less heat, as contacts do not contribute significantly to heat. |
| Cost | Generally, more expensive than EMRs. | Usually less expensive than SSRs. |
| Load Type | Better for high-speed switching and precise control, especially with AC. | More versatile for various load types, including high-current applications. |
| Isolation | Provides good electrical isolation between control and load circuits. | Also provides electrical isolation but depends on the physical gap. |
| Sensitivity to Environment | Less sensitive to vibration and shock. | Sensitive to vibration and shock, which can affect mechanical contacts. |
| Voltage Drop | Typically has a small voltage drop across the switch. | Minimal voltage drops when contacts are closed. |
