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Relay Control Systems

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete control system is one designed to operate on Boolean (“on” or “off”) signals supplied by discrete sensors such as process switches. A form of discrete control taught in every introductory course on digital electronics involves the use of circuits called logic gates. These circuits input one or more Boolean signals, and output a Boolean signal according to a simple rule such as “AND” or “OR”:




Industrial control systems rarely utilize logic gates in a direct fashion for discrete control systems, although the fundamental concepts of “AND,” “OR,” and other gate types are universally applied. Instead, control functions are either implemented using electromechanical relays and/or with programmable digital devices such as PLCs (Programmable Logic Controllers). This chapter focuses on the practical use of both technologies for industrial discrete control.


Control relays

An electromechanical relay is an electrical switch actuated by an electromagnet coil. As switching devices, they exhibit simple “on” and “off” behavior with no intermediate states. The electronic schematic symbol for a simple single-pole, single-throw (SPST) relay is shown here:


SPST Relay Normally Open Contact

A coil of wire wrapped around a laminated iron core provides the magnetic field necessary to actuate the switch mechanism. This particular relay is equipped with normally open (NO) switch contacts, which means the switch will be in the open (off) state when the relay coil is de-energized. Recall that the “normal” status of a switch is the condition of minimum stimulus. A relay switch contact will be in its “normal” status when its coil is not energized. A single-pole, single-throw relay with a normally-closed (NC) switch contact would be represented in an electronic schematic like this:


SPST Relay Normally Contact

In the electrical control world, the labels “Form-A” and “Form-B” are synonymous with “normally open” and “normally closed” contact status. Thus, we could have labeled the SPST relay contacts as “Form-A” and “Form-B,” respectively:


SPST Relay Contact showing normally open and normally closed

An extension of this theme is the single-pole, double-throw (SPDT) relay contact, otherwise known as a “Form-C” contact. This design of switch provides both a normally-open and normally-closed contact set in one unit, actuated by the electromagnet coil:


SPDT Relay Form C Contact


A further extension of this theme is the double-pole, double-throw (DPDT) relay contact. This design of switch provides two sets of Form-C contacts in one unit, simultaneously actuated by the electromagnet coil:


DPDT Relay

DPDT relays are some of the most common found in industry, due to their versatility. Each Form-C contact set offers a choice of either normally-open or normally-closed contacts, and the two sets (two “poles”) are electrically isolated from each other so they may be used in different circuits.

A common package for industrial relays is the so-called ice cube relay, named for its clear plastic case allowing inspection of the working elements. These relays plug into multi-pin base sockets for easy removal and replacement in case of failure. A DPDT “ice cube” relay is shown in the following photographs, ready to be plugged into its base (left) and with the plastic cover removed to expose both sets of Form-C contacts (right):


Electrical Relay with cover opened to show its contacts

These relays connect to the socket with eight pins: three for each of the two Form-C contact set, plus two more pins for the coil connections. Due to the pin count (8), this style of relay base is often referred to as an octal base.

A closer view of one Form-C contact shows how the moving metal “leaf” contacts one of two stationary points, the actual point of contact being made by a silver-coated “button” at the end of the leaf. The following photographs show one Form-C contact in both positions:


Relay Form C Contacts

Industrial control relays usually have connection diagrams drawn somewhere on the outer shell to indicate which pins connect to which elements inside the relay. The style of these diagrams may vary somewhat, even between relays of identical function. Take for instance the diagrams shown here, photographed on three different brands of DPDT relay:


Relay Diagrams shown on its cover

Bear in mind that these three relays are identical in their essential function (DPDT switching), despite differences in physical size and contact ratings (voltage and current capacities). Only two of the three diagrams shown use the same symbols to represent contacts, and all three use unique symbols to represent the coil.


Relay circuits

Electromechanical relays may be connected together to perform logic and control functions, acting as logic elements much like digital gates (AND, OR, etc.). A very common form of schematic diagram showing the interconnection of relays to perform these functions is called a ladder diagram. In a “ladder” diagram, the two poles of the power source are drawn as vertical rails of a ladder, with horizontal “rungs” showing the switch contacts, relay contacts, relay coils, and final control elements (lamps, solenoid coils, motors) drawn in between the power rails.

Ladder diagrams differ from regular schematic diagrams of the sort common to electronics technicians primarily in the strict orientation of the wiring: vertical power “rails” and horizontal control “rungs.” Symbols also differ a bit from common electronics notation: relay coils are drawn as circles, with relay contacts drawn in a way that resembles capacitors:

Ladder Diagram Symbols
Another notable convention in relay circuits and their ladder diagrams is that each and every wire in the circuit is labeled with a number corresponding to common connection points. That is, wires connected together always bear the same number: the common number designates a condition of electrical commonality (all points bearing the same number are equipotential to each other). Wire numbers only change when the connection passes through a switch or other device capable of dropping voltage.

An actual ladder diagram of a relay-based motor control system is shown here, complete with red-line edits showing modifications to the circuit made by an industrial electrician:


Actual Ladder Diagram Showing Edits




Summers, Wilford I. and Croft, Terrell, American Electrician’s Handbook, Eleventh Edition, McGraw-Hill Book Company, New York, NY, 1987.

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