Relays, originally developed in the 1800s for telegraph systems to carry electrical current over long distances, are electrically controlled switching devices used to open or close current flow in a circuit when triggered by specific conditions.
Although electromechanical relays are widely used and practical, they can be insufficient under certain conditions. Invented in 1971, the Solid State Relay (SSR) — known in Turkish as katı hal rölesi — differs significantly from electromechanical relays, primarily because it contains no moving mechanical parts. Below are the essential technical details.
What Does an SSR Do?
Solid state relays are externally controlled switching components that operate using a low-voltage input signal to regulate high-power circuits, similar to electromechanical relays. Despite performing the same core function, they provide several advantages due to their fully non-mechanical architecture.
SSR devices consist of two electrically isolated circuits (input/control side and load/output side). The input voltage powers an internal LED, and the generated photons are detected by a photodiode or optical sensor, completing the load circuit. This design ensures full galvanic isolation, enabling the control and load circuits to operate at different voltage levels (AC or DC) without physical electrical contact.
How Does an SSR Work?
The key difference between SSR and electromechanical relays lies in the switching mechanism:
No mechanical switch exists inside an SSR.
Switching is performed using an opto-isolator (opto-coupler).
The control signal activates an internal LED.
The emitted light is sensed by a photodiode/photodetector, triggering conduction.
When LED power is cut, the SSR turns OFF.
Depending on design type, the photodetector output may activate:
a TRIAC (for AC switching),
an SCR (for DC or phase-controlled AC),
or a series of MOSFETs (for fast DC switching).
Because switching is performed via light and semiconductor conduction, input and output circuits remain fully isolated.
What Are the Advantages of SSRs?
The advantages include:
No moving particles or mechanical parts, enabling very long service life with minimal failure probability.
Compact, lightweight design compared to electromechanical relays.
Orientation-independent installation (vertical or horizontal placement does not affect performance).
Silent operation, with no mechanical noise or electrical switching chatter.
Very high switching speed, equivalent to the LED activation response time, making SSRs ideal for rapid ON/OFF automation.
Low power consumption.
No spark or arcing, allowing use in flammable or chemically sensitive environments.
High resilience to humidity, dust, vibration, temperature, and contamination, making SSRs suitable for harsh industrial conditions.
Safe operation for embedded automation, motor drivers, HVAC, security panels, medical measurement equipment, and similar domains.
What Are the Disadvantages of SSRs?
The disadvantages include:
Higher cost compared to electromechanical relays, though this is often acceptable due to reliability and service life.
Heat generation during operation, which may require heatsinks or cooling modules in high-load scenarios.
Limited current capacity, and exceeding limits may cause permanent semiconductor damage.
Voltage drop across semiconductor switches, reducing the available load voltage.
Vulnerability to over-voltage or surge conditions, which may damage internal semiconductors.
Where Are SSRs Used?
Typical applications include:
Industrial automation equipment (production machines, panels, switching sequences)
Sensitive medical measurement devices (silent, arc-free operation)
Chemical, flammable, or explosive environments (no arcing risk)
Portable energy systems
Embedded motor drivers and power-control sequencing
Security interlock and digital locking panels
Rapid switching IoT, microcontroller automation, HVAC, conveyor timing, robotics load control, etc.
Users should select the correct SSR type depending on current, voltage, switching speed, thermal limits, and environmental constraints.
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