Motor control · lesson 8 of 12
On-delay and off-delay timers
A timer relay is a relay whose contacts wait. An on-delay timer waits before acting when you energize it. An off-delay timer acts at once and waits when you de-energize it. Say "the delay is on the way ON" or "the delay is on the way OFF" and you will never mix them up.
On-delay: warn, then start
A conveyor that starts without warning is a hazard, so this circuit sounds a horn for 5 seconds first. Start latches a run relay CR. CR energizes the timer TR and, through TR's timed-open contact, the horn. When the time is up both timed contacts change together: the horn stops and the conveyor starts.
On-delay: warning horn, then the conveyor starts
Turn on JavaScript to press the buttons and run this circuit.
Do this
Press Start: the horn warns for 5 seconds, then the conveyor moves.
Off-delay: run on, then stop
A heater that shuts off with the fan still cold-starting would leave the elements glowing in still air. Here the fan runs off an off-delay timer's contact: it starts the instant the heat is called for, and keeps going for 6 seconds after the call ends.
Off-delay: the fan runs on after the heater stops
Turn on JavaScript to press the buttons and run this circuit.
Do this
A heater with a fan that keeps running after the heat stops, to carry the heat away.
Reading timed contacts on a print
A timed contact is drawn like any contact with a small arc — the "parachute" — over it. The arc shows which way the delay acts, and the letters under it say where the contact ends up: T.C., timed closed, or T.O., timed open. The lab labels each one. The preset (5 s, 6 s) is set on the timer itself and usually noted next to its coil.
Questions people ask
What is the difference between on-delay and off-delay?
An on-delay timer waits after its coil is energized before its timed contacts change, and resets instantly when the coil drops. An off-delay timer changes its contacts instantly when energized, and waits after the coil drops before letting them go back.
What do T.C. and T.O. mean?
Timed Closed and Timed Open — which way the contact moves when the time is up. An on-delay timer has NOTC (normally open, timed closed) and NCTO (normally closed, timed open) contacts; an off-delay timer has NOTO and NCTC.
Do timers have instantaneous contacts too?
Many do — contacts that move the moment the coil energizes, like a plain relay. On a print they are drawn without the timing arc.
Learn one thing a week
Now and then a new diagram, drill, or reference sheet goes up. Leave your email for a short heads-up when it does — no spam, no selling it on, one click to leave.