Finder 77.21.8.230.8251 – 77 Series Modular solid state relays

Why Choose the Finder 77.21.8.230.8251 Solid State Relay?

The Finder 77.21.8.230.8251 is a 25 A modular solid state relay (SSR) designed for reliable, contactless switching of AC electrical loads in industrial automation, heating control and electrical control panels. Part of the Finder Series 77, this single-phase relay combines semiconductor switching technology with a compact DIN rail mounting design.

Featuring a 40–305 V AC control input and a 19–305 V AC load switching range, the Finder 77.21.8.230.8251 provides flexible integration into compatible industrial control circuits. Its single normally open (1 NO / SPST-NO) output supports switching of appropriately rated AC loads without the mechanical contact wear associated with conventional electromechanical relays.

The relay uses random turn-on switching, enabling semiconductor conduction in response to the applied control signal rather than waiting for an AC voltage zero crossing. With an integrated heatsink, 22.5 mm housing width and IP20 protection rating, it provides a practical switching solution for professionally designed industrial control assemblies.

Key Advantages

• 25 A maximum load current for appropriately rated AC switching applications.
• 40–305 V AC control voltage range for compatible industrial control circuits.
• 19–305 V AC load voltage range for single-phase AC switching.
• Random turn-on switching for applications requiring control without zero-cross synchronisation.
• Contactless semiconductor operation eliminates mechanical contact bounce.
• Silent switching for machinery and electrical control equipment.
• High switching endurance without mechanical contact wear.
• Integrated heatsink to support thermal management.
• 22.5 mm modular housing for space-efficient control panel layouts.
• 35 mm DIN rail mounting for convenient installation.
• IP20 protection rating for installation within suitable electrical enclosures.
• Input and output terminals on opposite sides for organised panel wiring.

Key Features

• Finder Series 77 modular solid state relay.
• Type 77.21 single-phase switching device.
• Maximum load current: 25 A.
• Control input voltage: 40–305 V AC.
• Switched load voltage: 19–305 V AC.
• Output configuration: 1 NO (SPST-NO).
• Switching method: Random turn-on.
• Output technology: TRIAC semiconductor switching.
• Integrated heatsink and protective housing.
• 6 kV input-to-output impulse insulation at a 1.2/50 µs waveform.
• Contactless, bounce-free switching.
• Silent operation.
• 22.5 mm housing width.
• 35 mm DIN rail mounting in accordance with EN 60715.
• Box-clamp terminal connections.
• IP20 degree of protection.
• Operating ambient temperature: −20 °C to +80 °C.

Reliable Contactless Switching for Industrial Automation and Heating Control

The Finder 77.21.8.230.8251 provides electronic switching of AC loads using a semiconductor output rather than mechanically operated contacts. When a suitable control voltage is applied to the input, the relay enables current flow through its output circuit. This allows compatible control equipment to operate heaters, solenoids, contactor coils and other appropriately rated electrical loads.

Its random turn-on switching is particularly useful where switching must respond to the control signal without waiting for a voltage zero crossing. Unlike mechanical relays, the solid state design avoids contact bounce and produces no mechanical switching noise. This makes it suitable for applications involving repeated switching operations, subject to the manufacturer's electrical and thermal limits.

The integrated heatsink supports heat dissipation during operation. However, the 25 A rating must be assessed against ambient temperature, load characteristics and installation conditions. Appropriate ventilation, conductor sizing, upstream protection and thermal derating are essential. As with other TRIAC-based solid state relays, a small leakage current can remain when the output is off, so the device must not be used as a substitute for a safety isolator or protective disconnection device.

Typical Applications

• Industrial electric heating control systems.
• Temperature-controlled heating equipment.
• Industrial ovens and process heating installations.
• Electrical resistance heater switching.
• Solenoid and contactor coil driving circuits.
• Industrial automation control panels.
• Packaging and manufacturing machinery.
• Production line electrical switching systems.
• Lighting and compatible lamp control applications.
• Repeated switching of appropriately rated AC loads.
• OEM machinery and electrical panel manufacturing.
• Replacement of compatible electromechanical switching devices where solid state operation is appropriate.