The four major categories of timers are dashpot, synchronous clock, solid-state, and solid-state programmable. See Figure 1. Dashpot, synchronous clock and solid-state timers are stand-alone timers.
Stand-alone timers are physically connected between the input device (limit switch, etc.) and the output device (solenoid, etc.) controlled by the timer.
Solid-state programmable timers are timing functions that are included in electrical control devices such as programmable logic relays (PLRs).
Figure 1. The four major categories of timers are dashpot, synchronous clock, solid-state, and solid-state programmable.
Dashpot timers are the oldest type of industrial timers. Dashpot timers can be found on old equipment but are rarely used in new installations.
Synchronous clock timers have been installed in millions of control applications and are still specified in some new applications.
Solid-state timers are the most common stand-alone timer used in control applications today.
However, in most electrical systems that include PLRs, the internal solid-state programmable timers of the PLRs can be used to replace any stand-alone timer. Each timer device accomplishes its task in a different way, but all timers have the common ability to introduce some degree of time delay into a control circuit.
A dashpot timer is a timer that provides time delay by controlling how rapidly air or liquid is allowed to pass into or out of a container through an orifice (opening) that is either fixed or variable in diameter. See Figure 2.
For example, if the piston of a hand-operated tire pump is forced down, the piston moves down rapidly if the valve opening is unrestricted. However, if the valve opening is restricted, the travel time of the piston increases. The smaller the opening is, the longer the travel time.
Figure 2. A dashpot timer provides time delay by controlling how rapidly air or liquid is allowed to pass into or out of a container.
Synchronous Clock Timers
A synchronous clock timer is a timer that opens and closes a circuit depending on the position of the hands of a clock. See Figure 3.
Synchronous clock timers may have one or more contacts through which the circuit may be opened or closed.
Figure 3. A synchronous clock timer opens and closes a circuit depending on the position of the hands of a clock.
The time delay is provided by the speed at which the clock hands move around the perimeter of the face of the clock. For example, the contacts can close once every 12 hr. A synchronous clock motor operates the timer.
Synchronous clock motors are AC-operated and maintain their speed based on the frequency of the AC power line that feeds them. Synchronous clock timers are accurate timers because power companies regulate the line frequency within strict tolerances.
A solid-state timer is a timer with a time delay that is provided by solid-state electronic devices enclosed within the timing device. See Figure 4.
A solid-state timing circuit provides a very accurate timing function at the most economical cost. Solid-state timers can control timing functions ranging from a fraction of a second to hundreds of hours.
Most solid-state timers are designed as plug-in modules for quick replacement. Solid-state timers can replace dashpot and synchronous timers in most applications.
Solid-state timers are less susceptible to outside environmental conditions because they, like relay coils, are often encapsulated in epoxy resin for protection. However, because they are encapsulated and therefore impossible to repair, they are normally discarded when they fail since they cost less than other timers.
Figure 4. A solid-state timer has a time delay provided by solid-state electronic devices enclosed within the timing device
Solid-State Programmable Timers
A solid-state programmable timer is a timer that is programmed within a programmable logic relay (PLR) or another programmable logic device (PLD).
A programmable logic relay (PLR) is a solid-state control device that includes internal relays, timers, counters, and other control functions that can be programmed and reprogrammed to automatically control small residential, commercial, and industrial circuits. A PLR includes an input, output, and programming section.
The programming section of a PLR is used to program internal relays, timers, and counters that can be programmed into a control circuit without the need for external components like timers and counters.
PLRs typically include at least 12 timers. Each timer can be programmed for a specific function and time setting based on the application requirements.
PLRs often also include real-time clocks that allow the timer to be set on specific days and times to operate the circuit loads.
The input section of a PLR is where the circuit’s digital (ON/OFF, OPEN/CLOSED, etc.) switches are connected. The input section often includes an analog (0 VDC to 10 VDC, 4 mA to 20 mA, etc.) input section.
The output section is where the circuit’s loads (lamps, solenoids, etc.) are connected. The output section provides either mechanical switches or solid-state output contacts. See Figure 5.
Figure 5. A programmable logic relay can be programmed for specific functions and time settings based on the application.