JohdotaAn auxiliary relay is an electrically operated switch for control currents. When the coil is energised, the relay pulls in and all its contacts change state at the same time. When the voltage is removed, the relay drops out and the contacts return to rest.
What a relay is for
- One command can control many circuits, because a relay has several contacts.
- The coil circuit and the contact circuits are electrically separate. For example, a 24 V control can switch a 230 V load.
- Relays are used to build logic: self-holding, interlocks and conditions.
Plug-in relay
Plug-in relays are common in control panels. The socket is fixed to a DIN rail and wired, and the relay is pushed into the socket. A faulty relay can be replaced without disconnecting any wires.
Terminals
The coil terminals are A1 and A2. Plug-in relays usually have changeover contacts, each with a common terminal, an NC terminal and an NO terminal. A common numbering is 11 common, 12 NC and 14 NO, and 21–22–24 in the same way for the second contact. The numbering varies between sockets, so always check the numbers on the side of the socket.
In Johdota the relay contacts are separate NO and NC contacts with the same numbering as contactor auxiliary contacts: NO x3–x4 and NC x1–x2.
Remember
The coil voltage and the type of voltage (AC or DC) must be chosen to match the control voltage. Both are marked on the relay.
In your region
Showing notes for
United States, Canada
On ladder diagrams a control relay is marked CR, and its contacts carry the same letters, for example CR1 or 1CR. The coil is a circle. A normally closed contact has a slanted line across it.
Source: Siemens: Basics of Control Components, part 1 (STEP 2000 course) · Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples · Evolution Motion: JIC standard graphic symbols for electrical ladder diagrams
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
How does a relay work?
When the coil A1–A2 is energised, the relay pulls in and all its contacts change state at once. When the voltage goes, the contacts return to rest.
How do you wire a relay?
The coil goes to A1–A2, and its voltage and type (AC or DC) must match the control voltage. The contacts go to the switched circuits, commonly 11 common, 12 NC and 14 NO. Numbering varies between sockets, so check the socket.
What do relay terminals 11, 12 and 14 mean?
They are a changeover contact: 11 common, 12 NC and 14 NO. The second contact is 21–22–24, and the coil is A1–A2.
What is the difference between a relay and a contactor?
The principle is the same. A contactor's main contacts withstand the starting current of a motor and break the load current. An auxiliary relay is made for control currents.
Why is there a diode across a relay coil?
Switching off a DC coil produces a voltage spike. A diode across the coil suppresses it and protects the contact or transistor that drives the relay. On a relay with a diode, A1 is positive.
What is a control relay used for?
A relay has several contacts, so one command can control several circuits. The coil and the contacts are separate circuits, so a 24 V control can switch a 230 V load. Relays also build logic, such as self-holding and interlocks.
Sources
- Finder: 55 series plug-in relays
- Schneider Electric: RXZE2S108M socket, product page and wiring diagram
- Sprecher + Schuh: auxiliary contact terminal markings 13–14 and 21–22
- Siemens: Basics of Control Components, part 1 (STEP 2000 course)
- Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples
- Evolution Motion: JIC standard graphic symbols for electrical ladder diagrams