JohdotaA timer relay changes the state of its contacts after a set delay. Modern timer relays are electronic DIN-rail devices. On a multifunction relay, the function is chosen with a switch and the time with a dial.
On-delay
When the supply A1–A2 is switched on, the contacts change state only after the set time. When the supply is switched off, the contacts return at once. If the supply is switched off before the time has run out, nothing happens, and the next time the timing starts from the beginning.
Off-delay with a control input
The supply A1–A2 is on all the time, and the relay is controlled through the control input B1:
- When B1 is energised, the contacts change state at once.
- When B1 is de-energised, the timing starts.
- After the set time, the contacts return.
A common mistake is to wire the push-button into the relay supply. The relay then has no operating voltage during the delay, and the delay does not work. The off-delay timer relay in Johdota is this type, with a control input.
There are also timer relays where the off-delay starts when the supply is switched off, without a separate control input. They are called true off-delay relays.
Contacts
A timer relay usually has a changeover contact where 15 is the common terminal, 16 NC and 18 NO. At rest 15–16 is closed, and when the relay has operated, 15–18 is closed.
Uses
- delayed start and start-up warning (on-delay)
- stair lighting and fan run-on (off-delay)
- the changeover time in star-delta starting, for which there are also dedicated star-delta timer relays.
In your region
Showing notes for
United States, Canada
On ladder diagrams a timer relay is marked TR, and its contacts carry the same letters.
Timed contacts are named by what they do. On an on-delay timer, NOTC is normally open, timed closed, and NCTO normally closed, timed open. On an off-delay timer the contacts are NOTO and NCTC. The mark under the contact is different for on-delay and off-delay contacts.
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 timer relay work?
It changes the state of its contacts after a set delay. On a multifunction relay, a switch selects the function and a dial sets the time.
What is an on-delay timer relay?
When the supply A1–A2 comes on, the contacts change over after the set time. When it goes off, they return at once, and if it goes off before the time is up, nothing happens.
What is an off-delay timer relay?
The contacts change over at once when the control signal comes, and timing starts when it goes. With a control input, A1–A2 stays on and the signal goes to B1. True off-delay relays time from loss of supply instead.
How do you wire a timer relay?
Supply to A1–A2 and, on an off-delay relay with a control input, the control signal to B1. The delayed contact is a changeover: 15 common, 16 NC and 18 NO. A common mistake is to wire the push-button into the supply instead of B1.