Several different timer functions are available to meet the many different requirements of time-based circuits and applications. On-delay and off-delay timer functions were the only two-timer functions available when dashpot and synchronous timers were the only timers being used.
When solid-state timers became available, they offered on-delay, off-delay, one-shot, and recycle timer functions.
Solid-state timers today offer dozens of special timer functions in addition to the four basic timer functions because solid-state timer circuits can be easily modified.
Normally several of the special timing functions are combined into one multiple-function timer.
On-Delay Timer Working
An on-delay (delay-on-operate) timer is a device that has a preset time period that must pass after the timer has been energized before any action occurs on the timer contacts.
Once activated, the timer may be used to turn a load on or off, depending on the way the timer contacts are connected into the circuit. The load energizes after the preset time delay when a normally open (NO) timer contact is used. The load de-energizes after the preset time delay when a normally closed (NC) timer contact is used.
An on-delay timer can be designed to open or close a circuit after a preset time delay. On-delay timer contacts do not change position until the set time period passes after the timer receives power. See Figure 1. After the preset time has passed, the timer contacts change position.
In the on-delay timer circuit, the NO contacts close and energize the load. The load remains energized as long as the control switch remains closed. The load de-energizes the second the control switch is opened.
An operational diagram is used to show timer operation. In the operational diagram, the top line shows the position of the control switch and the bottom line shows the condition of the load.
Figure 1. On-delay timer contacts do not change position until the set time period passes after the timer receives power.
- On-Delay (Timed-Closed)
An on-delay (timed- closed) function may be illustrated using two balloons. See Figure 2.
The solenoid plunger forces air out of balloon A, through orifice B, and into balloon C when control switch S1 is closed. Contacts TR1 close, energizing the circuit to the load after balloon C is filled. This energizes the load. The on-delay function will take 5 secs if it takes 5 secs for balloon C to fill.
One-half of an arrow is used to indicate the direction of the time delay of the NO timing contacts in on-delay timers. The half arrow points in the direction of on delay. The operational diagram should be used if an arrow is not used with an on-delay timer.
- On-Delay (Timed-Open)
An on-delay (timed-open) function also may be illustrated using balloons to show how the contacts are forced open after the timing cycle is complete. See Figure 3.
With control switch S1 closed, the solenoid plunger forces air from balloon A through orifice B and into balloon C. After 5 sec, contacts TR1 open the circuit to the load and the load is de-energized. One-half of an arrow is shown in the line diagram.
The arrow indicates that the NC contacts open after the on-delay function has taken place. This pneumatically operated timing function is the way dashpot timers operate.
A synchronous clock timer or solid-state timer could be substituted for the pneumatic timer. A pneumatic timer is the easiest to understand in terms of mechanical and timing operation.
Figure 2. With an on-delay (timed-closed) function, the contacts close after the timing cycle is complete.
Figure 3. With an on-delay (timed-open) function, the contacts open after the timing cycle is complete.
On-Delay Timer Applications
On-delay timers are the most common type of timer in use. For example, an on-delay timer is often used to monitor a medical patient’s breathing. See Figure 4. In this application, the timer is used to sound an alarm if a patient does not take a breath within 10 sec.
The circuit includes a low-pressure switch built into a patient monitoring system. Pressure switches are available that can activate electrical contacts at pressures less than 1 psi.
The circuit is turned on by the ON/OFF switch once the patient is connected to the monitor. If the patient does not take a breath, the timer starts timing and continues timing until the patient takes a breath (which resets the timer) or the timer times out. If the timer times out, the timer contacts close, sounding a warning.
Figure 4. On-delay timers may be used to monitor a medical patient’s breathing.
Off-Delay Timer Working
An off-delay (delay-on-release) timer is a device that does not start its timing function until the power is removed from the timer. See Figure 5.
In this circuit, a control switch is used to apply power to the timer. The timer contacts change immediately and the load energizes when power is first applied to the timer.
The timer contacts remain in the changed position and the time period starts when power is removed from the timer. The timer contacts return to their normal position and the load is de-energized when the set time period expires.
Figure 5. An off-delay (delay-on-release) timer is a device that does not start its timing function until the power is removed from the timer.
- Off-Delay (Timed-Open)
An off-delay (timed-open) contact circuit may be used to continue to provide cooling in a projector once the bulb has been turned off but has not had time to cool down. See Figure 6.
In this circuit, closing switch S1 turns on the projector bulb and activates timer coil TR1. With timer TR1 energized, NO contacts TR1 immediately close, energizing the fan motor, which controls the cooling of the projector.
The projector bulb and the cooling fan remain on as long as switch S1 stays closed. When switch S1 is opened, the projector bulb turns off and power is removed from the timer. Contacts TR1 remain closed for a preset off-delay and then open, causing the cooling fan to turn off.
This off-delay, timed-open circuit is generally set to adequately cool the projector equipment before it shuts off. This circuit can also be used for large cooling fan motors when the fan motor in the control circuit is replaced with a motor starter. The motor starter could be used to control any size motor.
Figure 6. An off-delay (timed-open) contact circuit may be used to continue to provide cooling in a projector once the bulb has been turned off but has not had time to cool down.
- Off-Delay (Timed-Closed)
An off-delay (timed-closed) contact circuit may be used to provide a pumping system with backspin protection and surge protection on stopping. See Figure 7.
Surge protection is often necessary when a pump is turned off and a high column of water is stopped by a check valve. The force of the sudden stop may cause surges that operate the pressure switch contacts, subjecting the starter to chatter (open and close).
Backspin is the backward turning of a centrifugal pump when the head of water runs back through the pump just after it has been turned off. Starting the pump during backspin may damage the pump motor.
To minimize any damage resulting from surge and backspin, the pressure switch PS1 closes on low pressure and energizes the control relay CR. The CR contacts close and energize the pump motor starter M1 through the NC contacts TR.
For M1, one set of NO contacts energize the timer and the other set of NO contacts keep the motor energized even after the NC timer contacts open.
When PS1 opens at the set system pressure, M1 cannot be re-energized until the off-delay contacts TR are allowed to time out and reclose the NC contacts, regardless of the number of times the PS1 contacts chatter during backspin pressures.
Figure 7. An off-delay (timed-closed) contact circuit may be used to provide a pumping system with backspin protection and surge protection on stopping.
Off-Delay Timer Applications
Off-delay timers are used in applications that require a load to remain energized even after the input control has been removed, such as in emergency industrial showers. See Figure 8.
In this circuit, the off-delay timer is used to keep the water flowing for 1 min after the pushbutton for the emergency industrial shower is pressed and released.
Figure 8. Off-delay timers are used in applications that require a load to remain energized even after the input control has been removed.
A stand-alone off-delay timer is called an off-delay or delay-on-release timer. However, when an off-delay timer is programmed using a PLC software program, it is programmed as a TOF timer.
Likewise, a stand-alone on-delay timer is called an on-delay or delay-on-operate timer. But when an on-delay timer is programmed using a PLC software program, it is programmed as a TON timer.
After the pushbutton is pressed, the timer contacts close and the solenoid-operated valve starts the flow of water. The water flows even if the pushbutton is released.
A flow switch is used to indicate when water is flowing. The flow switch sounds an alarm that can be used to bring help. The flow switch also sounds the alarm if there is a break in the flow of water at any point downstream from the switch.
Plugging Using Timing Relays
Plugging can also be accomplished by using a timing relay. Normally, the advantage of using a timing relay is a lower cost since the timer is inexpensive and does not have to be connected mechanically to the motor shaft or driven machine.
The disadvantage is that, unlike a plugging switch, the timer does not compensate for a change in the load condition (which affects stopping time) once the timer is preset.
An off-delay timer may be used in applications where the time needed to decelerate a motor is constant and known. See Figure 9.
In this circuit, the NO contacts of the timer are connected into the circuit in the same manner as a plugging switch. The coil of the timer is connected in parallel with the forward starter.
The motor is started and memory is added to the circuit when the start pushbutton is pressed. In addition to energizing the forward starter, the off-delay timer is also energized.
The energizing of the off-delay timer immediately closes the NO timer contacts. The closing of these contacts does not energize the reverse contacts due to the interlocks.
The forward starter and timer coil are de-energized when the stop pushbutton is pressed. The NO timing contact remains held closed for the setting of the timer.
The reversing starter is energized for the period of time set on the timer when the timing contact is held closed. This plugs the motor to a stop.
The timer’s contact must reopen before the motor is actually reversed. The motor reverses direction if the time setting is too long.
Figure 9. An off-delay timer may be used in applications where the time needed to decelerate the motor is constant.
An off-delay timer may also be used for plugging a motor to a stop during emergency stops. See Figure 10.
In this circuit, the timer’s contacts are connected in the same manner as the plugging switch. The motor is started and memory is added to the circuit when the start pushbutton is pressed. The forward starter and timer are de-energized if the stop pushbutton is pressed.
Although the timer’s NO contacts are held closed for the time period set on the timer, the reversing starter is not energized. This is because no power is applied to the reversing starter from L1.
Figure 10. An off-delay timer may also be used for plugging a motor to a stop during emergency stops.
If the emergency stop pushbutton is pressed, the forward starter and timer are de-energized and the reversing starter is energized.
The energizing of the reversing starter adds memory to the circuit and stops the motor. The opening of the timing contacts de-energizes the reversing starter and removes the memory.
A comparison chart may be used to compare the operation of on-delay and off-delay timing functions and contacts. See Figure 11.
To help compare timer functions, instantaneous relay contacts are also included. Some manufacturers also use abbreviations in their catalogs to describe the type of contacts used.
Figure 11. A comparison chart may be used to compare the operation of on-delay and off-delay timing functions and contacts.
One-Shot Timer Working
A one-shot (interval) timer is a device in which the contacts change position immediately and remain changed for the set period of time after the timer has received power. See Figure 12. After the set period of time has passed, the contacts return to their normal position.
One-shot timers are used in applications in which a load is ON for only a set period of time.
One-shot timer applications include coin-operated games, dryers, car washes, and other machines.
One-shot timer functions have not been available as long as on-delay and off-delay timing functions because the one-shot timer function became available only when solid-state timers became available. For this reason, and the fact that so many other timer functions are now available, no standard symbol was established for any other timer contacts except on-delay and off-delay.
Today, the symbol for basic NO and NC contacts, along with the timer type and/or operational diagram, are used with all timers that are not on-delay or off-delay.
Figure 12. A one-shot (interval) timer has contacts that change position and remain changed for the set period of time after the timer has received power.
One-Shot Timer Applications
One-shot timers are used in applications that require a fixed-time output for a set period of time. For example, a one-shot timer can be used to control the amount of time that plastic wrap is wound around a pallet of cartons. See Figure 13.
In this application, a photoelectric switch detects a pallet entering the plastic wrap machine. The photoelectric switch energizes the one-shot timer.
The one-shot timer contacts close, starting the wrapping process. The wrapping process continues for the setting of the timer. A second photoelectric switch could be used to detect that the plastic wrap is actually being applied. This can help indicate a tear in the plastic or an empty roll.
Figure 13. One-shot timers are used in applications that require a fixed-time output for a set period of time.
Recycle Timer Working
A recycle timer is a device in which the contacts cycle open and closed repeatedly once the timer has received power. The cycling of the contacts continues until power is removed from the timer. See Figure 14.
In a recycle timer circuit, the closing of the control switch starts the cycling function. The load continues to turn on and off at regular time intervals as long as the control switch is closed. The cycling function stops when the control switch is opened.
Recycle timers may be symmetrical or asymmetrical.
A symmetrical recycle timer is a timer that operates with equal on and off time periods.
An asymmetrical recycle timer is a timer that has independent adjustments for the on and off time periods. Asymmetrical timers always have two different time adjustments.
Figure 14. A recycle timer is a device in which the contacts cycle open and closed repeatedly once the timer has received power.
Recycle Timer Applications
Recycle timers are used in applications that require a fixed on and off time period. For example, a recycle timer can be used to automatically keep a product mixed. See Figure 15.
In this application, power is applied to the timer when the three-position selector switch is placed in the automatic position. The timer starts recycling for as long as the selector switch is in the automatic position.
The recycle timer turns the mixing motor on and off at the set time. An asymmetrical timer works best for this application.
the on-time period (mixer motor is ON) is set less than the off-time period (mixer motor is OFF). For example, the timer may be set to mix the product for 5 min every 2 hr.
Figure 15. Recycle timers are used in applications that require a fixed on and off time period.
Multifunction Timer Working
On-delay, off-delay, one-shot, and recycle timers are considered mono-function timers. That is, they perform only one timing function, such as on-delay or off-delay.
Multifunction timers are solid-state timers that can perform many different timing functions. Multifunction timers are normally programmed for different timing functions by the placement of dual in-line package (DIP) switches located on the timer. See Figure 16.
In this timer, four DIP switches are used to set the timer range and function.
The first two DIP switches set the time range from 0.8 sec to 60 min. The last two DIP switches set the timer function.
The timer can be set for an on-delay, one-shot, or recycle timer function. The recycle timer function can be set to start with the off-time period occurring first or the on-time period occurring first.
Figure 16. A multifunction timer may use the placement of DIP switches to determine the type of timing function and timer setting.
In addition to some (or all) of the basic timing functions, many multifunction timers can also be programmed for special timing functions. See Figure 17.
In this multifunction timer, many timing functions can be programmed with a time range from 0.15 sec to 220 hr. This timer includes standard timing functions such as an on-delay (program setting 1) and special timing functions such as a combination of both on-delay and off-delay (program setting 5).
Figure 17. Multifunction timers can be programmed for many timing functions with various time ranges.