Quick Answer: On Delay vs Off Delay Timer
An on delay timer waits after the input signal turns ON before energizing the output. It is also called TON, timer on delay, delay-on-make, atau delay-on-operate.
An off delay timer energizes the output immediately when the input turns ON, then keeps the output ON for a preset time after the input turns OFF. It is also called TOF, timer off delay, delay-on-break, atau delay-on-release.
Dalam satu ayat: on delay controls when a load starts; off delay controls how long a load keeps running after the command is removed.
Pengambilan Utama
- TON = Timer On Delay. The input must stay ON for the preset time before the output turns ON.
- TOF = Timer Off Delay. The output turns ON immediately, then stays ON for the preset time after the input turns OFF.
- guna on delay for motor sequencing, compressor anti-short-cycle delay, contactor start delay, and false-signal filtering.
- guna off delay for cooling fan post-run, pump run-on, ventilation purge, stairwell lighting delay, and controlled shutdown.
- A normal off delay timer may still need its own control power while timing. If the timer must hold output after total power loss, check for a true power-off delay function.
On Delay vs Off Delay Timer Comparison Table
| item | Pemasa Lengah Hidup (TON) | Pemasa Lengah Mati (TOF) |
|---|---|---|
| Basic function | Delays output ON | Delays output OFF |
| Timing starts when | Input turns ON | Input turns OFF |
| Output when input turns ON | Waits during preset time, then turns ON | Turns ON immediately |
| Output when input turns OFF | Turns OFF immediately | Stays ON during preset time |
| Nama-nama umum | Delay-on-make, delay-on-operate, timer on delay | Delay-on-break, delay-on-release, timer off delay |
| PLC term | TON | TOF |
| Tujuan utama | Prevent premature startup | Maintain controlled run-on |
| Typical motor use | Sequenced startup, anti-short-cycle delay | Cooling fan delay, pump run-on |
| Kesilapan biasa | Expecting immediate output | Assuming it works after total power loss |
How an On Delay Timer Works
An on delay timer starts timing when its control input or coil is energized. During the preset time, the output contact stays in its normal state. If the input remains present until the preset time expires, the timed output changes state.
For a relay output, this usually means:
- a timed normally open contact closes after the delay;
- a timed normally closed contact opens after the delay;
- if the input disappears before the delay is complete, the timer resets and the output does not change state.
On Delay Timer Working Sequence
| Langkah | Input | Pemasa | Output |
|---|---|---|---|
| 1 | DIMATIKAN | Reset | DIMATIKAN |
| 2 | Turns ON | Starts counting | Still OFF |
| 3 | Stays ON until preset time | Reaches preset time | Turns ON |
| 4 | Remains ON | Selesai | Stays ON |
| 5 | Turns OFF | Resets | Turns OFF immediately |
This behavior is useful when a machine should not start from a momentary signal, voltage dip, switch bounce, or unstable sensor input.
Common On Delay Applications
- Starting motors one by one to avoid simultaneous inrush current
- Delaying compressor restart after a short stop
- Starting a conveyor only after a sensor signal remains stable
- Delaying a contactor coil after control power returns
- Star-delta transition timing where a dedicated starter timer is not used
- Filtering short false signals from limit switches, float switches, or pressure switches
How an Off Delay Timer Works
An off delay timer changes the output immediately when the input turns ON. The delay happens later, after the input turns OFF. During the off-delay period, the output remains energized. When the preset time expires, the output returns to its normal state.
For a relay output, this usually means:
- a normally open output contact closes immediately when the input is applied;
- the contact remains closed after the input is removed;
- the contact opens only after the preset off-delay time has elapsed.
Off Delay Timer Operation Sequence
| Langkah | Input | Pemasa | Output |
|---|---|---|---|
| 1 | DIMATIKAN | Reset | DIMATIKAN |
| 2 | Turns ON | Not timing yet | Turns ON immediately |
| 3 | Stays ON | Output remains active | Stays ON |
| 4 | Turns OFF | Starts counting | Still ON |
| 5 | Preset time expires | Selesai | Turns OFF |
This behavior is useful when equipment must continue running briefly after the command signal is removed.
Common Off Delay Applications
- Cooling fan post-run after a motor or drive stops
- Pump run-on after a level switch opens
- Exhaust fan purge after a process ends
- Stairwell or corridor lighting delay
- Delayed release of auxiliary contactor circuits
- Keeping a conveyor running briefly to clear product after a stop command
TON vs TOF Timing Diagram
The easiest way to read the difference is to compare the input and output timing.
| Timing point | Input signal | TON output | TOF output |
|---|---|---|---|
| Before input | DIMATIKAN | DIMATIKAN | DIMATIKAN |
| Input turns ON | HIDUP | OFF during preset time | ON immediately |
| Input remains ON | HIDUP | ON after preset time | HIDUP |
| Input turns OFF | DIMATIKAN | OFF immediately | ON during preset time |
| Delay expires | DIMATIKAN | DIMATIKAN | DIMATIKAN |
In the timing diagram, use three rows: Input (IN), Preset Time (PT), dan Output (Q). Untuk TON, the key visual gap is between input ON dan output ON. Untuk TOF, the key visual gap is between input OFF dan output OFF.
This diagram should be treated as the first check before wiring or replacement. If the expected output waveform does not match the machine sequence, the selected timer function is wrong even if the supply voltage and terminal numbers look correct.
TON vs TOF in PLC Logic
In programmable logic controller (PLC) programming, TON dan TOF are common timer function blocks. IEC 61131-3 is the main international standard framework for PLC programming languages and function block concepts, although exact implementation details still depend on the PLC platform.
| PLC term | Maknanya | Typical parameters |
|---|---|---|
| TON | Timer On Delay | IN, PT, Q, ET |
| TOF | Timer Off Delay | IN, PT, Q, ET |
| IN | Input condition | Boolean trigger |
| PT | Masa pratetap | Target delay time |
| ET | Elapsed time | Current accumulated time |
| Q | Output | Timer result |
TON Logic in Plain Language
bila IN becomes true, ET begins counting. Q becomes true only after ET reaches PT. If IN becomes false before the preset time, ET resets and Q remains false.
TOF Logic in Plain Language
bila IN is true, Q becomes true immediately. When IN becomes false, ET begins counting. Q remains true until ET reaches PT, then becomes false.
A PLC timer and a physical timer relay solve similar timing problems, but they are not the same product. A PLC timer is software logic inside a controller. A timer relay is a hardware device with terminals, coil or supply voltage, output contacts, timing range, and contact ratings.
For basic hardware terminology, see VIOX’s guide on what a time relay is.
Timer Relay Symbols and Contact Terms
Timer relay symbols vary by drawing standard, manufacturer, and project legend. Always check the schematic legend and the timer datasheet before wiring.
There are two common ways engineers encounter timer contact descriptions:
- IEC-style wiring and terminal references, such as A1/A2 for supply and 15/16/18 for changeover contacts.
- North American / NEMA-style contact behavior terms, such as NOTC or NCTO, which describe whether a contact is normally open or normally closed and whether the change is timed.
These systems are often mixed in international panel documentation, especially when a machine is designed in one region and serviced in another. Treat the schematic legend and datasheet as the authority, not the abbreviation alone.
In many control drawings, timed contacts may be described by contact-state behavior:
| Contact notation | Maknanya | Penggunaan tipikal |
|---|---|---|
| NOTC | Normally Open, Timed Closed | A contact closes after a delay |
| NCTO | Normally Closed, Timed Open | A contact opens after a delay |
| NOTO | Normally Open, Timed Open | A contact is related to delayed opening behavior |
| NCTC | Normally Closed, Timed Closed | A contact is related to delayed closing behavior |
For teaching purposes, an on-delay timer is often shown as a normally open contact that closes after the preset time. An off-delay timer is often shown by delayed release behavior, where the output returns to normal only after the delay expires.
Symbol notation alone does not tell you the supply voltage, contact rating, timing range, reset behavior, or whether the device is a single-function or multifunction timer. Those details must come from the product datasheet.
Wiring Logic: On Delay vs Off Delay Relay
Most industrial timer relays include:
- supply terminals such as A1/A2;
- timed output contacts such as 15/16/18 atau 25/26/28;
- a timing range selector;
- a function selector on multifunction models;
- sometimes a separate trigger input.
The wiring may look similar, but the timing function changes the result:
| Wiring question | On delay answer | Off delay answer |
|---|---|---|
| When does timing begin? | When the input or coil is energized | When the input or trigger is removed |
| Does the output energize immediately? | Tiada | Ya |
| Does the output drop immediately when input is removed? | Ya | Tiada |
| What should be checked first? | Input stability and delay range | Control power during timing and release behavior |
For more detailed wiring examples, use VIOX’s time delay relay wiring diagram guide.
Motor Delay Timer Applications
A motor delay timer can be either on delay or off delay. The right choice depends on whether the motor circuit needs delayed startup, delayed shutdown, or delayed auxiliary control.
| Motor control need | Better function | Sebab |
|---|---|---|
| Start multiple motors in sequence | On delay | Reduces simultaneous starting demand |
| Prevent compressor short cycling | On delay | Blocks immediate restart |
| Delay star-delta transition | On delay or dedicated star-delta timer | Mengawal pemasaan peralihan |
| Keep cooling fan running after motor stop | Off delay | Removes residual heat |
| Keep pump running after level signal clears | Off delay | Clears remaining fluid |
| Keep ventilation running after process shutdown | Off delay | Purges heat, fumes, or air volume |
For motor starter wiring context, see VIOX’s guide on how to wire a time relay for a motor starter.
Do not use a timer to hide a protection problem. If a motor repeatedly trips a breaker, overload relay, or contactor, check the motor current, overload setting, starting method, wiring, contactor rating, and mechanical load before adding time delay.
Contact Ratings: AC-15 and DC-13 Matter
Timer relay contacts are often used to switch contactor coils, solenoids, pilot lights, small fans, valves, or control inputs. These loads do not stress the contact in the same way.
For control circuit devices, IEC 60947-5-1 utilization categories such as AC-15 dan DC-13 are important:
| Kategori | Typical load type | Mengapa ia penting |
|---|---|---|
| AC-15 | AC electromagnetic control loads, such as contactor coils | Coil inrush and inductive turn-off stress the relay contact |
| DC-13 | DC electromagnetic control loads, such as DC solenoids | DC arcs are harder to interrupt because there is no natural AC zero crossing |
If a timer relay contact is only rated for a resistive load, do not assume it can safely switch an inductive load at the same current. Use a contactor or interposing relay when the controlled load exceeds the timer relay contact rating.
For a deeper explanation, see VIOX’s guide on why time relay contacts fail on inductive loads.
Power-Off Delay vs Off Delay: Do Not Confuse Them
A standard off delay timer delays output release after the control signal is removed, but it may still need its own supply power while timing. If the timer loses all power, it may reset immediately.
Gunakan true power-off delay timer only when the datasheet clearly states that the device can maintain output or timing after supply power is removed. Depending on the design, this may require internal energy storage, a separate auxiliary supply, or another control architecture.
This difference matters in fan run-on circuits, emergency control circuits, and shutdown sequences. If the output must remain active during a complete power interruption, confirm the exact power-off delay behavior before selecting the relay.
How to Choose Between On Delay and Off Delay
Use this simple field rule:
- If the machine should wait before starting, choose on delay / TON.
- If the machine should keep running after the command is removed, choose off delay / TOF.
After choosing the timing function, check these specifications:
| Item pemilihan | Perkara yang perlu disahkan |
|---|---|
| Voltan bekalan | AC or DC control voltage at A1/A2 |
| Julat masa | Minimum and maximum preset time |
| Timing accuracy | Required repeatability for the process |
| Jenis sesentuh output | SPDT, DPDT, timed NO/NC behavior |
| Penarafan sesentuh | Resistive rating and inductive load category |
| Kelakuan set semula | What happens if input drops or power is lost |
| Melekap | DIN rail, socket, panel, or PCB |
| Persekitaran | Temperature, humidity, vibration, enclosure protection |
| Piawaian dan dokumentasi | Project-required IEC, UL, or other approval evidence |
For broader product selection, use VIOX’s timer relay selection guide or evaluate available Timer Relay pilihan.
Kesilapan Lazim
Mistake 1: Using Off Delay When You Need Delayed Start
If the load must start only after a preset delay, you need on delay. An off delay timer energizes immediately, which is the opposite behavior.
Mistake 2: Assuming Off Delay Works After Power Loss
Off delay is not always the same as power-off delay. If the timer supply disappears, a standard off-delay relay may reset. Check the datasheet when the application depends on output holding after power loss.
Mistake 3: Ignoring Coil Voltage vs Load Voltage
The timer supply voltage and the switched load voltage are not necessarily the same. A 24 V DC timer may switch an AC load through its output contact only if the output contact is rated for that voltage and load type.
Mistake 4: Switching Inductive Loads Without Suppression
Contactor coils, solenoids, and valves can create voltage spikes when de-energized. Depending on the circuit, use a suitable RC snubber, MOV, flyback diode, or interface relay according to the equipment design.
Mistake 5: Replacing a Timer Without Checking the Timing Diagram
Two timer relays can share the same voltage and pin layout but behave differently. Always compare the timing diagram, reset condition, trigger input, contact state, and selected function before replacement.
Mistake 6: Using a Timer to Fix a Mechanical or Protection Fault
A delay timer can sequence equipment, but it should not cover up overload trips, overheating, unstable voltage, contactor chatter, or an incorrectly sized motor protection device.
Manufacturer’s Engineering Note
For a timer relay manufacturer, the critical checks are not only the timing knob and display. A reliable timer relay must be verified for timing repeatability, contact operation, terminal security, insulation performance, and load suitability before it reaches a control panel. In B2B sourcing, ask for the timing function diagram, contact rating under resistive and inductive loads, operating temperature range, and applicable standard documentation instead of selecting only by voltage and time range.
For contactor coils, solenoids, and other electromagnetic loads, the most important practical detail is the contact duty. A relay that performs well on a small resistive load may fail early on an inductive AC-15 or DC-13 control load if the application is not checked correctly.
Soalan Lazim
Why is a timing diagram more reliable than the timer name?
Timer names are often translated differently across manufacturers, but the timing diagram shows the actual logic. Compare the input waveform and output waveform first. If the output waits before turning ON, it is on-delay behavior. If the output waits before turning OFF, it is off-delay behavior.
Which timer function is better for motor cooling fan delay?
Use an off delay timer. A cooling fan should run when the motor runs and continue for a set time after the motor stops. An on delay timer would delay fan startup and stop immediately when the command is removed, which is usually the wrong behavior for post-run cooling.
Can one multifunction timer relay provide both on delay and off delay?
Yes, many multifunction timer relays can provide both functions. The selected mode must be confirmed before wiring and commissioning because the same terminals may behave differently in different modes.
Why does an off delay timer sometimes fail to hold after power loss?
Many off-delay timers need control power during the timing period. When the supply disappears completely, the internal timing circuit may reset. If the output must remain active after supply loss, specify a true power-off delay timer or provide an auxiliary control supply.
What contact rating should I check for contactor coils, solenoids, or valves?
For control coils and solenoids, check inductive control ratings such as AC-15 for AC loads or DC-13 for DC loads where applicable. Do not rely only on the resistive amp rating printed on the timer.
When should I use a timer relay instead of a PLC TON or TOF instruction?
Use a timer relay when the function must work in a hardwired control circuit, a small standalone panel, or a retrofit where no PLC is available. Use a PLC TON or TOF instruction when the machine already has a PLC and the timing logic must be integrated with program conditions, alarms, interlocks, or HMI settings.
Are IEC terminal numbers and NEMA-style timer contact names the same thing?
No. IEC terminal numbers such as A1/A2 and 15/16/18 identify supply and contact terminals. NEMA-style terms such as NOTC or NCTO describe timed contact behavior. A drawing may use both, so always read the schematic legend together with the product datasheet.
What should I check when replacing an old on-delay or off-delay relay?
Check the supply voltage, timing function, timing range, terminal layout, contact configuration, reset behavior, contact rating, mounting style, and whether the original circuit uses a separate trigger input. Do not replace a relay only because the socket or pin count looks similar.
Why does a timer relay output chatter or reset before the preset time?
Common causes include unstable control voltage, switch bounce, sensor noise, wrong function mode, loose terminals, electromagnetic interference from contactors or variable frequency drives, or an under-rated power supply. Check the input signal with a meter or oscilloscope before replacing the timer.
What is the safest way to confirm whether a timer is TON or TOF in the field?
De-energize and inspect the datasheet first. During controlled testing, apply the rated control voltage and observe whether the output changes after input energization or after input removal. The measured output sequence should match the printed timing diagram.
Kesimpulan
Pilih on delay / TON when a load should wait before starting. Choose off delay / TOF when a load should keep running after the command is removed.
For real control panels, the best timer is not selected by the name alone. Confirm the timing diagram, supply voltage, contact rating, load type, reset behavior, wiring terminals, and whether the application requires a true power-off delay function.
