Chapter 10: Timing and Counting Circuits

Chapter Overview

Chapter 10 will cover the specific requirements for timing and counting circuits. Commercial and industrial manufacturing and processing are all time-and count-controlled by electrical and electronic circuits and systems. Useful information link.

Types of Timers

Timing or the ability to precisely control and synchronize the operation of various electrical circuits and associated equipment is common in everything from residential sprinkler controls to more complex industrial operations. Timers can be categorized into four distinct types including dashpot, synchronous clock, solid-state, and solid-state programmable. The majority of timers are considered to be “stand alone”, meaning that they are typically placed between the input device (pilot switches) and the output or load that is to be controlled. Solid-state programmable timers are unique in that they are included and are inherent to electrical control devices such as PLC’s.

Dashpot timers were the first timers used in industry and can still be found operating in older systems due to their simple and robust construction. Dashpot timers provide time delay functions based on the movement of liquid or air that is allowed to pass through the bellows or bladder associated with this type of antiquated timing device. Most timing control systems have gone away from the use of outdated pneumatics (air) to control the sequence of operation, although dashpot timers can still be found working to this day in older systems.

Synchronous clock timers provide timing delay through the use of an analog clock. This type of timer opens and closes a circuit based on the position of the hands of the clock. The timing delay is facilitated through the position of the clock hands and look and function similar to analog clocks. Synchronous clock timers offer the simplicity required for more rudimentary timing functions while remaining easy to set up and maintain.

Solid-state timers provide more precise timing sequences through the use of solid-state electronic devices. This type of sequence control offers very accurate timing function at an economical cost as they are mass produced and are intended to be easily replaced. The “solid-state” or electronic operation provides smooth reliable activation, but electronic devices are always limited by the amount of current and abuse they can tolerate thereby leaving room for the continued use of more robust electromechanical control.

Solid-state programmable timers come included in more complex control devices such as PLC’s , and offer the advantage of more precise timing and monitoring of the control systems. These types of timers include an interface where it is possible to program and reprogram various timing sequences which allows for the versatility needed in more complex systems working in tandem.

Editing note: insert flash cards here for the four types of timers- Dashpot, synchronous clock, solid state, and programmable.

Timing Functions

Timing functions can be broken down into on-delay, off-delay, one-shot, and recycle timers in order to meet the varied needs for time-based applications. More complex timing devices, such as PLC’s and PLR’s, normally include several specialized functions which facilitates the versatility needed in larger systems.

On-delay timers delay the operation of the circuit when first initiated so that the load will energize a set time after the sequence has begun. They typically include normally open (timed-closed) and normally closed (timed-open) contacts. On-delay timers are denoted on drawings by an arrow pointing up. Off-delay timers delay the operation until the de-energization of the circuit so the load will remain on for a set time after the circuit has been opened. Off-delay timers are denoted by an arrow pointing down.

A ‘one-shot’ timer simply means that the timing sequence can only happen once. One-shot timers are used when the load is to remain on for a set time. A ‘recycle’ timer allows for multiple timing sequences to function together, and are used for applications that require a fixed on and off timing sequences. Multiple-function timers can perform various separate timing sequences and offer the ability to program separate timing functions in more complex operations.