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Finder 77.21.9.024.8251 – 77 Series Modular solid state relays
Finder 77.21.9.024.8251 is a 25 A modular solid state relay with a 4–32 V DC control input, 19–305 V AC TRIAC output, random turn-on switching, integrated heat sink and 35 mm DIN-rail mounting for industrial automation and AC load control.
£50.13
Why Choose the Finder 77.21.9.024.8251 Solid State Relay?
The Finder 77.21.9.024.8251 is a 25 A modular solid state relay (SSR) designed for reliable, contactless switching in industrial automation, electrical control panels and machinery applications. Part of the Finder 77 Series, this relay combines semiconductor switching technology with an integrated heat sink and compact DIN-rail mounting construction.
Featuring a 4–32 V DC control input and a 19–305 V AC switching range, the Finder 77.21.9.024.8251 provides an effective interface between low-voltage electronic control systems and AC electrical loads. Its single normally open (SPST-NO) output is rated at 25 A under specified operating conditions, making it suitable for appropriately rated heating, motor control and industrial switching applications.
The random turn-on switching function allows the relay to respond to an applied control signal without waiting for the next AC voltage zero crossing. Combined with silent operation, high switching speed, 6 kV input-to-output insulation and a slim 22.5 mm housing, this model offers a practical solution for industrial installations requiring frequent and dependable electronic switching.
Key Advantages
- 25 A rated load current for suitable industrial AC switching applications.
- 4–32 V DC control input for integration with compatible electronic control systems.
- 19–305 V AC switching range for a broad selection of AC loads.
- Random turn-on operation for applications requiring rapid switching response.
- Contactless semiconductor switching eliminates mechanical contact bounce and arcing.
- Silent operation compared with conventional electromechanical relays.
- Integrated heat sink supports thermal management during operation.
- 6 kV input-to-output insulation under specified impulse test conditions.
- 22.5 mm modular housing for space-efficient control-panel installation.
- 35 mm DIN-rail mounting for straightforward installation.
- Box-clamp screw terminals for secure electrical connections.
- IP20 protection rating for installation within suitable electrical enclosures.
Key Features
- Finder 77 Series modular solid state relay.
- Model: 77.21.9.024.8251.
- Switching technology: TRIAC semiconductor output.
- Output configuration: 1 normally open (SPST-NO).
- Rated output current: 25 A at 40 °C.
- Rated output voltage: 230 V AC.
- Output switching voltage range: 19–305 V AC.
- Control voltage range: 4–32 V DC.
- Switching mode: Random turn-on.
- Maximum peak current: 400 A for 10 ms under specified conditions.
- Repetitive peak off-state voltage: 800 V.
- Input-to-output impulse insulation: 6 kV (1.2/50 µs).
- Integrated heat sink and protective housing.
- Silent, spark-free and bounce-free switching.
- Control and load terminals positioned on opposite sides.
- Housing width: 22.5 mm.
- Mounting: 35 mm DIN rail.
- Terminal arrangement: Box-clamp screw terminals.
- Protection classification: IP20.
High-Speed Random Turn-On Switching for Industrial Automation
The Finder 77.21.9.024.8251 uses a semiconductor switching element rather than mechanical contacts to control an AC load. When a suitable 4–32 V DC control signal is applied, the relay activates its TRIAC output, allowing current to flow through the connected load. Because the device uses random turn-on switching, it does not deliberately wait for the AC voltage to cross zero before initiating conduction.
This operating principle makes the relay useful for applications where a short switching response is desirable, including certain motor control and industrial automation circuits. Its integrated heat sink assists with thermal management, while its contactless construction supports frequent switching without mechanical contact wear. The relay must be correctly rated for the load, operating temperature, heat dissipation and required protective devices. As a TRIAC-based SSR, it is intended for AC load switching and is not a substitute for an isolating switch.
Typical Applications
- Industrial automation and electronic load switching.
- PLC-controlled AC loads using compatible DC control outputs.
- Industrial motor control applications requiring random turn-on switching.
- Electrical heating equipment and temperature control systems.
- Industrial machinery and automated production equipment.
- Contactors and solenoid control circuits.
- Conveyor and material handling control systems.
- OEM machinery requiring silent semiconductor switching.
- Industrial electrical control panels and DIN-rail assemblies.
- Applications requiring frequent, bounce-free AC switching.
Technical Specifications
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| Device Type | Solid State Relay | |
|---|---|---|
| Terminal Type | Screw | |
| Mounting Type | DIN rail | |
| Manufacturer | Finder | |
| Series | 77 Series | |
| Model | 77.21.9.024.8251 | |
| Product type | Modular solid state relay | |
| Output technology | TRIAC | |
| Output contact equivalent | 1 NO / SPST-NO | |
| Rated output current | 25 A at 40 °C | |
| Rated output voltage | 230 V AC, 50/60 Hz | |
| Output switching voltage range | 19–305 V AC, 50/60 Hz | |
| Control input voltage | 4–32 V DC | |
| Switching characteristic | Random turn-on | |
| Maximum peak current | 400 A for 10 ms | |
| Repetitive peak off-state voltage | 800 V | |
| Input-to-output insulation | 6 kV (1.2/50 µs) | |
| Off-state leakage current | 1 mA | |
| Ambient operating temperature | −20 to +80 °C | |
| Enclosure width | 22.5 mm | |
| Enclosure dimensions (H × W × D) | 83.3 × 22.5 × 120.3 mm | |
| Mounting | 35 mm DIN rail (EN 60715) | |
| Terminal type | Box-clamp screw terminals | |
| Thermal management | Integrated heat sink | |
| Protection rating | IP20 | |
| Compliance | CE, UKCA and RoHS | |
| Product category | Solid State Relays |
Frequently Asked Questions
What is the Finder 77.21.9.024.8251 used for?
The Finder 77.21.9.024.8251 is a 25 A modular solid state relay used to switch suitable AC electrical loads through a low-voltage DC control signal. Typical applications include industrial automation, heating control and machinery switching.
What control voltage does the Finder 77.21.9.024.8251 require?
The relay accepts a 4–32 V DC control input, making it suitable for appropriately rated electronic controllers, PLC outputs and DC control circuits.
What is the maximum rated load current of this solid state relay?
The Finder 77.21.9.024.8251 has a 25 A rated output current at 40 °C. The permissible load current depends on the operating conditions, thermal management and applicable load characteristics.
What AC voltage range can the Finder 77.21.9.024.8251 switch?
The relay has an AC output switching range of 19–305 V AC at 50/60 Hz, with a nominal output voltage of 230 V AC.
What does random turn-on switching mean?
Random turn-on switching means the relay can initiate conduction when the control signal is applied without deliberately waiting for the next AC voltage zero crossing. This is useful where a short turn-on response is required.
What is the difference between Finder 77.21.9.024.8251 and 77.21.9.024.8250?
The 77.21.9.024.8251 uses random turn-on switching, whereas the 77.21.9.024.8250 uses zero-crossing turn-on switching. Both belong to the Finder 77 Series and provide a 25 A single normally open solid state output.
Does this solid state relay have mechanical contacts?
No. The Finder 77.21.9.024.8251 uses a TRIAC semiconductor output instead of mechanical switching contacts, enabling silent operation without mechanical contact bounce.
Can the Finder 77.21.9.024.8251 be mounted on a DIN rail?
Yes. The relay is designed for 35 mm DIN-rail mounting according to EN 60715 and features a compact 22.5 mm housing width.
Does the Finder 77.21.9.024.8251 have an integrated heat sink?
Yes. The relay incorporates an integrated heat sink to assist with heat dissipation during operation. Suitable ventilation and thermal design remain necessary for the intended load and ambient temperature.
Can this relay switch DC loads?
No. The Finder 77.21.9.024.8251 has a DC control input but an AC TRIAC load output. It should not be used to switch DC loads; a suitable DC-output solid state relay should be selected instead.
Does the Finder 77.21.9.024.8251 provide electrical isolation when switched off?
The relay provides input-to-output insulation, but its semiconductor output has off-state leakage current. It therefore does not provide the same load isolation as a mechanical isolating switch. Appropriate isolation and circuit protection must be provided separately.
Trusted by Industrial Automation and Electrical Engineering Professionals
Finder 77 Series solid state relays are designed for electrical engineers, panel builders, industrial automation specialists and machinery manufacturers requiring compact, silent and high-speed electronic switching with dependable integration into modern industrial control systems.
