This article covers the following items about electrical timers:

The five sections of timers
Nine timer operating principles
Five common modes of timer operations
Timer charts

Sections of Electrical Timer

Figure 1 provides a block diagram that shows the five functional sections of a timer. Timers can be divided into the following sections:

Electrical Timer Block Diagram — Figure 1 Timer Block Diagram

Power or energy input
Signal input
Adjustment-mode selection
Decision-timing mode
Output Energy

Input Section: The energy input section receives the electrical energy from a supply source. The needed energy could also come from a mechanical source such as a wound spring.

Signal Input (Start-Initiate) Section: The start, the trigger, or the initiate signal serve the same purpose, they connect an external signal to the internal decision-making section of the timer. A timer may or may not have an external start input. The timer could be built so that when power is applied, it automatically begins its timing function.

Adjustment-Mode Selection Section: Some timers can be adjusted for the amount of time or the mode of operation or both.

Decision-Timing Mode Section: In this section, the timer receives a signal and compares it to its instructions program, makes a decision, and then takes some action, such as energizing an output relay. The decision section receives information (in the form of an electrical signal) from the input section and commands the output section to take some action.

Output Section: This section interfaces with the electrical circuit of the machine controlled by the timer. This is the section that controls both the instant (when provided) and the time-delayed contacts.

Other Section Descriptions: Timers can also be broken down into the following sections:

Timing means: Determines the range and accuracy of the timer.
Starting means: Initiates the timing; can be either automatic or manual.
Setting and control: Actuates the controlled contacts; can be adjusted for desired time intervals, and possibly modes.
Load contacts: Links the timer to the controlled process. May switch a different voltage than the timer.
Indicating means: May be provided to indicate the status of some function of the timer.

Five Common Modes of Electric Timers

First, let us build a level of understanding of the five basic types of timer modes or functions. Timers can accomplish the following:

Compress an event; make it occur in less time.
Expand an event; make it occur over a longer time.
Delay, or shift an event; make it happen at some point in the future.
Make a single event occur repeatedly.
Cause a sequence of events to occur once or repeatedly.

The five most common timer modes of operation are:

On-delay
Off-delay
Single shot
Repeat cycle
Interval

There is no national standard for the names of the various timer modes. This results in many different names used for timers that accomplish the same task. For example, on-delay timers are also known by the following terms: delay-on-make timer, delay-on-operate timer, delay-on pick-up, delay-on-energize, and sadly by many other terms.

Starting a Timer’s Mode of Operation

A manufacturer can use any one of the following methods to start a timer:

Apply power to the timer
Apply-remove a signal to the timer’s initiate terminal
Apply a continuous signal to the initiating terminal
Remove a signal from the timer’s initiate terminal

Depending upon the specific timer, applying or removing the initiate signal may or may not result in the timer resetting. That is, once applied, the initiate signal may or may not be ignored during the timing period.

Nine Types of Timer Operating Principles

The following is a list of the various operating principles used by timer manufacturers:

Spring wound
Motor and gears (time clocks)
Electrothermal
Electromagnetic
Air-pneumatic
Electromagnetic and hydraulic
Electromechanical
Solid state
Photoelectric

Solid State, Microprocessor: Today the workhorses of industry are the logic timers, used in programmable logic controllers (PLC), and the solid-state (SS) electronic, integrated circuit (IC) microprocessor-based microcontrollers. These timers are reliable, small, low-cost, multifaceted, and field adjustable.

Figure 2 Solid State Multimode Programmable Timer with Digital Display and Power on LED

Figure 2 is a drawing of a typical multifunction timer face panel, while Figure 3 is a drawing of a simple two wire single-function adjustable timer. These types of timers are capable of switching faster than an electrical relay and can communicate with plant-wide control systems such as field buses and other digitally based control systems.

Figure 3 Solid State Two-Wire Timer, Adjustable from One to Five Minutes “Cube” Timer

Photoelectric Timer

The photoelectric timer uses an ambient light–powered switch. Light as energy is converted to electrical energy by a photocell. A photocell produces a very small amount of electrical energy; it is enough to operate a small relay. Many times, outdoor security lights use a small photocell timer to turn the light fixture on and off at specific levels of area luminance.

On-Delay Mode Timer

An on-delay timer begins its timing period when power is first applied to it. Its controlled contacts will change position when the timing period has elapsed. When power is applied to the timer, the timing period begins. Should power be removed, the timer resets to zero time. When power is reapplied, it begins the timing period from time zero. This will keep a controlled load from briefly turning on and then off or short cycling.

Off-Delay Timer

An off-delay timer is used when one portion of a machine needs to keep operating after another has stopped. The delay period begins when the timer is turned off. Upon completion of the time period, the controlled contacts change position.

The off-delay timer is also known by the following terms: delay-off timer, delay-on release, delay-on dropout, post-purge delay, and power-off timer.

Some timers do and some do not require a separate signal to begin-initiate-start their timing operations. Almost all timers require electrical power for them to operate. All timers will have a time delay relay (or solid-state equivalent) that controls the load.

When looking at a timing chart, if a start-initiate signal line is not seen, the timer will start when power is applied or removed from the timer. When looking at a timing chart and an initiate or start line is seen, the timer will begin its timing operation when a signal is received or removed from that terminal on the timer.

Interval ON Mode Timer

The words “interval ON” indicate that the output relay’s normally open contact will close immediately when power is applied to the timer. Interval ON timers are also known as ON intervals, pulse shaping timers, single pulse on operate, and bypass timers.

Timing Chart Levels

Timing charts are graphs of the states of individual elements of a timer, such as power, initiate-control signal, and controlled (both instant and time-delayed) contacts. Each of these elements is represented by a line. Time zero is on the left and time increases to the right.

The top line of a timing chart always represents the power to the timer. The second line is for the separate reset function when it is provided. The third is for the gate function. The fourth line is for N.C.

When a set of electrical contacts will pass power they are said to be closed (C). When they are closed with no voltage applied to the relay coil, they are considered to be normally closed contacts (NC). The fifth level line is for N.O. When a set of electrical contacts are open they will not pass power.

The sixth level line is for an LED when one is provided. Timing charts have only the lines required by the specific timer represented by the timing chart.

Top Level Operating Power

Timers require some type of energy input to operate. The top line on a timing chart represents this power to the timer. It is common practice to think of the brains of a timer as the equivalent of an electromagnetic coil that controls the relay contacts even when the timer uses solid-state components.

The words “power to the coil” are used in this work, even though the timer may not have an electromagnetic coil. As a timer can or cannot have power to it, a timing chart indicates both conditions.

Figure 4 Timing Chart Power Level Diagram

A horizontal line is used to indicate the state of power to the timer. A vertical line above the horizontal line indicates that power is applied to the timer. A second horizontal line to the right indicates the passage of time. A second vertical line below the time horizontal line indicates that power has been removed. The second vertical line is followed by yet another horizontal line indicating the passage of time. The space between the two vertical lines indicates the passage of time. The horizontal lines in a timing chart are typically not marked in units of time such as seconds, minutes, or hours.

Start-Initiate Signal Level

Some timers have an internal jumper that passes power to initiate the timer’s action; others have a separate initiate (or start) terminal that must receive an external electrical signal to begin the timing operation. Still, others use external “dry contacts,” where no foreign voltage is required. The closing of these contacts starts the timing operation.

The second level on a timing chart by convention is used to graphically show if the start or initiate signal is on or off, the same way power is shown to be present-absent in the first line.

Figure 5 shows the initiate signal in both the ON and OFF (power passing and not passing) states.

The width of the horizontal space between the two vertical lines indicates the amount of time that has passed. The initiate, or control, the signal may or may not need to be present during the timer’s operating mode.

With some timers, removing the initiate or control signal will cause the timer to “reset.” With others, the removal of the signal has no impact. For still other timers, removal of the start signal stops time; it freezes it at its current value. When the signal returns, the timer starts at the exact place it previously stopped, and then times out. The word “initiate” is used to indicate the starting of the timing action.

With some timers, after the initiate switch has been closed, opening it will not affect the timer. Thus while the initiate switch may start the timer, it may not be able to stop it. That is, the initiate signal may be ignored by the timer. The words “start” and “initiate” can be understood as the same. However, initiate and control cannot be considered to be the same.

Instant and Time-Delayed Contacts Levels

As an optional feature, in addition to the time-delayed contacts, a timer may also have a set of instant or non-delay contacts. As the timer controls both sets of contacts, they could be called controlled contacts. Care should be exercised to communicate what is intended by using specific words. Thus, the words “controlled” contacts and “time-delay” contacts could be used. The timer controls both sets, one is non-delayed, and the other is subject to some amount of time delay. Figure 6 provides a drawing of both options of a timer’s contacts.

Figure 6 Timed Open and Timed Closed Contacts

N.O. = normally open
N.C. = normally closed
T.C. = timed open
T.O. = timed closed

Timers can have more than one set of time-delay contacts. A timing chart will have one line to indicate the state of the timed contacts. It may also have another line indicating the position of the instant non-delay contacts when provided.

It can be confusing when talking about normally closed contacts opening, and normally open contacts closing. It would be less confusing if the contacts were said to change position or change state, or transfer.

Instant contacts change position immediately when power is applied and again instantly when power is removed. The timed contacts are going to change position (transfer) when the timer’s mode of operation calls for them to change position.

The letters T.C. indicate timed closed, and T.O. indicate timed open. Both initials are sometimes used on timing charts.

Additional Timing Chart Levels

Some timers may have an inhibit signal line. This input stops the timer’s timing. When it is removed, the timer starts adding to the previous value. Some timers may have a line for a reset signal. When the reset signal is applied, the timer resets so that it begins a completely new timing operation.

Complete Timing Charts

The discussion of timers has progressed in small steps, and an example of a complete timing chart has not been provided. Figure 7 provides an example of what a typical off-delay timing chart might look like.

Figure 7 Complete Off Delay Timing Chart

The chart has three levels that represent the power, initiate, and timed contacts.

At the top level, power is applied to the timer. The timed contacts do not change state. When the initiate contacts pass power to the timer, the timed contacts remain as they normally were.

Note that when the initiate contacts open, the timed contacts still do not transfer. It is only when the amount of preset time has passed that the time-delay contacts transfer.

Notice that even though power has been removed from the timer, the time-delay contacts do not transfer. It is only when the preset amount of time after the initiate contacts have opened do the time-delayed contacts transfer. That is why it is called an off-delay timer.

When the initiate signal was removed, the timer began to “time out.” When the preset time expired, the time delay contacts transfer.

There are many types of timers, but all timers use the same type of timing chart to communicate their action.

Electrical Timer & Timer Charts