The PLC-BSC-120UC/21-21/H (1012014) is a 14 mm DIN rail-mounted relay base that accepts compatible plug-in electromechanical relays or solid-state relays. It is supplied without the relay itself, allowing users to select the appropriate relay based on their application requirements. The base is designed for 120 V AC/DC control circuits and includes built-in circuitry such as an LED status indicator and bridge rectifier.

Key Features

Compact 14 mm Design

The narrow 14 mm width allows more relay interfaces to be installed inside electrical panels while minimizing DIN rail space.

DIN Rail Mounting

The relay base snaps onto a standard NS 35/7.5 DIN rail, making installation quick and straightforward.

Screw Terminal Connections

Field wiring is completed using reliable screw terminals, supporting conductor sizes from 0.14 mm² to 2.5 mm² (AWG 26–14).

Dual Changeover Contacts

The base supports relays with 2 changeover (DPDT) contacts, allowing one relay to control multiple circuits or perform changeover switching.

LED Status Indicator

An integrated LED provides visual confirmation when the relay coil is energized, making troubleshooting easier.

Universal Control Voltage

Designed for 120 V AC/DC control voltage, the base includes an internal bridge rectifier that enables operation with either AC or DC control signals.

Understanding A1 and A2 (Relay Coil Terminals)

A1 and A2 are the relay coil terminals.

When the rated control voltage (120 V AC/DC for this relay base) is applied across these terminals, the relay coil energizes and changes the state of the relay contacts. The PLC-BSC-120UC/21-21/H base includes an LED indicator and an internal bridge rectifier, allowing it to accept either AC or DC control voltage of the specified rating.

Example

120 V AC/DC Supply

L/+  ─────────── A1
                  │
               Relay Coil
                  │
N/-  ─────────── A2

When voltage is applied between A1 and A2:

  • The relay coil energizes.
  • The LED turns ON.
  • The relay contacts change position.

Understanding 11, 12, and 14 (Relay Contact Terminals)

These terminals belong to one changeover (SPDT/Form C) contact.

TerminalFunctionNormal Condition
11Common (COM)Connected to either 12 or 14
12Normally Closed (NC)Connected to 11 when the coil is OFF
14Normally Open (NO)Connected to 11 when the coil is ON

Relay De-energized (Coil OFF)

      12 (NC)
        │
11 -----●
        │
      14 (NO)

Connection:
11 ↔ 12
11 ✕ 14

Current flows from 11 to 12, while 14 remains open.

Relay Energized (Coil ON)

      12 (NC)

11 -----●
        │
      14 (NO)

Connection:
11 ✕ 12
11 ↔ 14

Current now flows from 11 to 14, while the connection to 12 opens.

Common Applications

The PLC-BSC-120UC/21-21/H relay base is commonly used in:

  • PLC output interfacing
  • DCS control panels
  • Motor control centers (MCCs)
  • Pump control systems
  • HVAC automation
  • Water and wastewater treatment plants
  • Packaging machines
  • Conveyor systems
  • Industrial process automation