The Siemens 3RF2120-1AA45 is a single-phase SIRIUS 3RF21 solid-state relay designed for contactless switching in applications requiring frequent switching operations. Instead of mechanical contacts, it uses semiconductor switching technology, making it suitable for repetitive AC load control in industrial automation and process applications.
The device has a compact 22.5 mm width, a 20 A performance class, 48–600 V AC load voltage, 4–30 V DC control input, 1200 V blocking voltage, and screw-terminal connections. Siemens identifies it as an active product in the SIRIUS 3RF2 solid-state relay family.
The 3RF2120-1AA45 is part of the Siemens SIRIUS 3RF2 solid-state relay range, developed for applications where frequent switching makes conventional electromechanical switching less suitable.
Because semiconductor switching does not rely on mechanical power contacts, the device is well suited to repetitive switching applications and can help avoid mechanical contact wear associated with conventional relays.
Typical applications include:
Industrial automation
Heating control
Temperature regulation
Resistance heaters
Process control
Packaging machinery
Industrial ovens
Machine-control systems
Production equipment
OEM control panels
The main advantage of the 3RF2120-1AA45 is its solid-state, contactless switching technology.
It uses semiconductor switching rather than mechanical contacts, making it appropriate for loads that need to be switched repeatedly during normal machine operation.
Typical applications include:
Electric heating elements
Industrial heaters
Temperature-control systems
Packaging sealing equipment
Plastic-processing machinery
Industrial ovens
Process-heating equipment
Automated production machinery
The actual suitability depends on the load type, current, ambient conditions and required cooling.
The 3RF2120-1AA45 has a 20 A performance class. Siemens cautions that the actual permissible rated operational current can depend on the connection method and cooling conditions.
This means the 20 A value should not be treated as an unconditional operating current for every installation. Proper thermal management and installation conditions must be considered when selecting the relay.
The relay is designed for an AC operating-voltage range of 48 to 600 V AC. Siemens' product data confirms this range for the main circuit.
This makes the relay suitable for a broad range of industrial AC loads within the specified ratings.
The connected load should always be selected according to the correct voltage, current, utilization category and thermal requirements.
The 3RF2120-1AA45 uses a 4–30 V DC control input.
This makes it suitable for control signals from:
PLC digital outputs
Temperature controllers
Electronic control modules
Automation systems
Industrial controllers
Machine-control circuits
The controlling output must be compatible with the relay's DC input requirements.
The 3RF2120-1AA45 uses zero-point switching. Siemens specifies zero-point switching as the product's switching function.
This means the solid-state relay switches the AC load near the zero crossing of the AC waveform. This can be particularly useful in appropriate resistive-load and heating applications where controlled switching is desirable.
The product has a specified 1200 V blocking voltage.
The blocking-voltage rating provides a semiconductor switching withstand specification for the main circuit and is an important parameter when evaluating the relay for industrial AC applications.
The 3RF2120-1AA45 has a compact 22.5 mm width. Siemens identifies the product as a 22.5 mm single-phase 3RF2 semiconductor relay.
Its compact housing is useful for:
Automation cabinets
Heating-control panels
OEM machinery
Process-control cabinets
Machine-control panels
Industrial electrical assemblies
The 3RF2120-1AA45 uses screw-terminal connections for wiring. Siemens and RS both identify the product with screw terminals.
This provides a conventional connection method for industrial panel builders and maintenance teams.
Solid-state relays generate heat during operation, so cooling conditions are an important part of product selection and installation.
Siemens' product documentation notes that the actual permissible rated operational current can be lower depending on connection and cooling conditions.
For higher-current operation, appropriate thermal design, mounting and heat dissipation should therefore be considered before commissioning.
The 3RF2120-1AA45 can be used for frequent switching of suitable AC loads in a variety of industrial applications.
The relay can control compatible resistive heating elements in industrial heating systems.
It can be switched by a PLC or temperature controller to regulate suitable heating loads.
Frequent semiconductor switching can be useful for heating and sealing applications.
The relay can be incorporated into suitable heating-control systems used in plastic-processing machinery.
Its zero-point switching and contactless operation make it suitable for appropriate automated process-control applications.
The 3RF2120-1AA45 is suitable for OEM machine manufacturers, electrical panel builders, automation integrators and industrial maintenance professionals.
Its combination of 20 A performance class, 48–600 V AC load range, 4–30 V DC control input, zero-point switching and compact 22.5 mm housing makes it suitable for frequent AC switching applications.
Siemens SIRIUS 3RF21 solid-state relay
Article number 3RF2120-1AA45
Single-phase semiconductor relay
20 A performance class
48–600 V AC load voltage
4–30 V DC control input
Zero-point switching
1200 V blocking voltage
22.5 mm width
Screw terminals
Contactless semiconductor switching
Suitable for frequent switching
Suitable for industrial automation
Suitable for heating and process-control applications
Contactless switching: Eliminates mechanical power-contact wear.
Frequent switching capability: Suitable for applications requiring repeated switching operations.
Wide AC voltage range: Supports compatible loads from 48 to 600 V AC.
4–30 V DC input: Works with suitable PLC and electronic control outputs.
Zero-point switching: Provides controlled switching of compatible AC loads.
Compact 22.5 mm design: Helps optimize panel space.
Long switching life: Semiconductor operation avoids mechanical contact erosion.
Industrial suitability: Designed for automation, heating and process-control applications.
The Siemens 3RF2120-1AA45 is a practical choice for applications where an AC load must be switched frequently and without mechanical power contacts.
Its 20 A performance class, 48–600 V AC load range, 4–30 V DC control input and zero-point switching make it suitable for compatible industrial heating and automation applications.
The compact 22.5 mm width also helps save control-panel space, while the screw-terminal design supports conventional industrial wiring.
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It is a Siemens SIRIUS 3RF21 single-phase solid-state relay designed for frequent, contactless AC switching.
It has a 20 A performance class. The actual permissible current depends on cooling and installation conditions.
It supports 48–600 V AC.
The control input is 4–30 V DC.
Yes. The device uses zero-point switching.
The specified blocking voltage is 1200 V.
The device has a 22.5 mm width.
Yes. The relay uses screw-terminal connections.
It is suitable for frequent AC switching applications, especially compatible heating, temperature-control, process-control and industrial automation systems.
You can explore the Siemens industrial electrical and automation product range at SB Smart for solid-state relays, contactors, control components and related products.
| Specification | Details |
|---|---|
| Product Name | Solid State Relay |
| Brand | Siemens |
| Series | SIRIUS 3RF2 |
| Model Number | 3RF2120-1AA45 |
| SKU | 3RF2120-1AA45 |
| Product Type | Semiconductor Relay / Solid State Relay |
| Switching Technology | Contactless Semiconductor Switching |
| Phase | 1-Phase |
| Rated Operational Current | 20 A |
| Contact Configuration | 1NO |
| Number of Poles | 1 Pole |
| Load Voltage Range | 48 – 600 V AC |
| Control Voltage Range | 4 – 30 V DC |
| Switching Method | Zero Voltage Switching |
| Frequency | 50/60 Hz |
| Blocking Voltage | 1200 V |
| Housing Width | 22.5 mm |
| Dimensions (H × W × D) | 85 × 22.5 × 48 mm |
| Connection Type | Screw Terminals |
| Mounting Type | Panel Mount |
| Protection Rating | IP20 |
| Status Indication | LED |
| Operating Temperature | -25°C to +60°C |
| Application | Frequent Switching Applications |
| Suitable Loads | Heaters, Ovens, Temperature Controllers |
| Thermal Management | External Heatsink Recommended |
| Product Family | Siemens SIRIUS Solid State Switching Devices |
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