Finder 77.A3.9.024.8671 – 77 Series Modular solid state relays

Why Choose the Finder 77.A3.9.024.8671 Three-Phase Solid State Relay?

The Finder 77.A3.9.024.8671 is a high-performance 25 A three-phase solid state relay (SSR) designed for reliable, silent and contactless switching of industrial AC electrical loads. Part of the Finder Series 77 industrial solid state relay range, this device combines a nominal 24 V DC control input, three normally open equivalent outputs and random turn-on switching technology in a robust hockey puck-style housing.

Featuring a 4–32 V DC control input range and a 24–660 V AC output switching range, the Finder 77.A3.9.024.8671 is suitable for appropriately rated three-phase heating systems, industrial machinery and motor control applications. Its 3PST-NO equivalent output configuration enables three AC phases to be controlled simultaneously from a single DC control signal.

Unlike conventional electromechanical contactors, the Finder 77.A3.9.024.8671 uses semiconductor switching technology without moving power contacts. This provides silent operation, rapid switching response and freedom from mechanical contact bounce. Its random turn-on characteristic allows the relay to activate without deliberately waiting for an AC voltage zero crossing, making it useful in applications requiring fast switching response.

Designed for panel or heatsink mounting, the Finder 77.A3.9.024.8671 features screw terminals and a contactor-style terminal arrangement, with input and output connections positioned on adjacent sides. It is an effective switching solution for industrial electrical cabinets, process heating equipment and automated production machinery where suitable thermal management is provided.

Key Advantages

• 25 A three-phase solid state relay for industrial AC load switching.
• Three normally open equivalent outputs allow simultaneous switching of three AC phases.
• 24 V DC nominal control input for compatible industrial automation systems.
• 4–32 V DC input operating range for flexible electronic control.
• 24–660 V AC output voltage range for suitable three-phase electrical installations.
• Random turn-on switching enables rapid output activation without waiting for a voltage zero crossing.
• Contactless semiconductor operation eliminates mechanical contact wear and contact bounce.
• Silent and arc-free switching during normal operation.
• 300 A peak current capability for 10 ms under specified conditions.
• Contactor-style terminal arrangement supports convenient wiring within industrial control panels.
• Panel or heatsink mounting for integration into industrial electrical cabinets.
• Screw terminal connections for secure electrical wiring.
• 47–400 Hz output operating frequency range for compatible AC applications.
• −30 to +80 °C ambient temperature range, subject to thermal derating.
• IP20 protection rating for installation within appropriately protected enclosures.
• Suitable for industrial heating, machinery, automation and compatible three-phase motor control.

Key Features

• Finder Series 77 industrial solid state relay.
• Three-phase AC load switching.
• Output configuration: 3 normally open equivalents (3PST-NO).
• Rated output current: 25 A.
• Output switching voltage: 24–660 V AC.
• Nominal control voltage: 24 V DC.
• Control input operating range: 4–32 V DC.
• Switching characteristic: Random turn-on.
• Output operating frequency: 47–400 Hz.
• Maximum peak current: 300 A for 10 ms.
• Off-state leakage current: Up to 10 mA under specified conditions.
• Input-to-output dielectric strength: 4 kV.
• Repetitive peak off-state voltage: 1600 V peak.
• Hockey puck-style industrial housing.
• Contactor-style input and output terminal arrangement.
• Panel or heatsink mounting.
• Screw terminal electrical connections.
• LED status indication.
• Operating ambient temperature: −30 to +80 °C.
• Protection rating: IP20.
• Silent, contactless semiconductor switching.

Advanced Three-Phase Random Turn-On Switching with 24 V DC Control

The Finder 77.A3.9.024.8671 is designed to control three-phase AC electrical loads using a compatible low-voltage DC signal. When the control input is energised within its specified 4–32 V DC operating range, the relay activates its semiconductor output channels, allowing electrical power to be supplied to the connected three-phase load.

Its random turn-on switching characteristic means the relay does not deliberately wait for the AC voltage waveform to reach zero before switching on. This enables rapid switching response and provides flexibility for suitable motor control, industrial heating and electronic switching applications.

Unlike zero-crossing solid state relays, which are commonly selected for certain resistive heating applications, random turn-on devices can be useful where the load or control method requires activation at different points in the AC waveform. However, the resulting electrical transients depend on the load characteristics and switching conditions. Appropriate surge protection and electromagnetic compatibility measures should therefore be considered.

The three-pole output configuration enables all three phases of a compatible load to be switched together. This makes the Finder 77.A3.9.024.8671 suitable for three-phase industrial heaters, process equipment and selected motor control applications where electronic switching is preferred over a conventional electromechanical contactor.

With a 25 A rated output and a 24–660 V AC switching range, the relay provides flexibility for industrial power systems. Its specified 300 A peak current capability for 10 ms describes short-duration surge withstand and must not be confused with its continuous current rating.

The relay's 24 V DC nominal control input is particularly useful in industrial automation systems employing low-voltage control circuits. It can be operated from a compatible PLC output or other suitable electronic control interface, provided the controller meets the relay's input voltage and current requirements.

Thermal Management and Safe Installation

Solid state relays generate heat during conduction because their semiconductor outputs have an inherent voltage drop. At rated load, the Finder 77.A3.9.024.8671 can dissipate approximately 90 W, making appropriate heatsinking essential for reliable operation at higher load currents.

The relay is designed for mounting on control cabinet sheet metal or a suitable heatsink. The required heatsink capacity depends on the continuous load current, ambient temperature, enclosure ventilation and mounting conditions. The manufacturer's thermal derating information should be consulted to ensure the semiconductor junction temperature remains within its specified limits.

The 25 A rated output current should not be interpreted as an unconditional continuous rating under every installation condition. At elevated ambient temperatures or with insufficient heat dissipation, the permissible current may be significantly reduced.

Appropriate upstream short-circuit protection must be installed. Where the relay controls a motor, suitable motor overload protection must also be provided. Solid state relays do not independently replace circuit breakers, fuses, motor protection devices or safety-rated disconnecting equipment.

The Finder 77.A3.9.024.8671 can allow off-state leakage current through its semiconductor outputs. Therefore, the load may not be completely electrically isolated when the control signal is removed. A suitable mechanical isolator or disconnecting device must be used wherever safe isolation is required.

Typical Applications

• Three-phase industrial heating element control.
• Industrial ovens and process heating systems.
• Temperature-controlled manufacturing equipment.
• Industrial machinery and automated production lines.
• Three-phase resistive load switching.
• Compatible three-phase motor control applications.
• Packaging and processing machinery.
• Industrial process control systems.
• Automated electrical load switching.
• Factory automation and industrial control panels.
• PLC-controlled switching using compatible 24 V DC outputs.
• OEM machinery requiring silent three-phase switching.
• Industrial installations requiring rapid semiconductor switching.