The basic relay provided adequate control for automatic control and processing of industrial processing for many years, but as the demand for improvement grew, the limits of the relay became apparent. Mass production industries required improvements that the relay could not compete with in the sense of high speeds, reliability of service and minimum maintenance.

Solid-state circuitry was introduced initially because of its ability to operate as on/off devices and to repeat the operation millions of times, more than a relay could and without more than cursory maintenance. The decision for it to be in either an on-state or an off-state depended on an input signal from an external device called a transducer.

The transducer, in general, provides an input signal to the solid-state device and this, in turn, operates the required device. For example, a thermostat can be manufactured to close its contacts at a specific temperature. Closure of the contacts acts on an electronic component, which can then start an air-conditioning system. When the temperature in the monitored area drops sufficiently, the contacts of the thermostat open, and the air-conditioner stops. Note that the system has only two states: on or off.

In this example, the thermostat replaced the start push-button of the relay circuit, and a solid-state device such as a triac replaced the contacts of the relay. For industrial process control, this on/off method does not provide sufficient accuracy to produce many products. Devices such as the thermostat above cannot operate at the one temperature for both on and off states. This introduces a differential; that is, if the contacts open at 22° C, it is extremely unlikely they will also close at the same temperature. Depending on the circuit, the closing temperature may be 18° C giving a differential of 4° C.

This led to the introduction of the programmable controller. Originally intended to be a replacement for the relay panel, initially, it did little more than manage the on/off sequencing of motors and solenoids. It gradually evolved into the intelligent item of equipment it is today.

The modern programmable logic controller (PLC) has a relatively low cost, and the original program can easily be reprogrammed to monitor something completely new when the manufacturing requirements change. A logic control program is stored in its memory, and this tells the central processing unit when certain events should take place and the sequence in which they will occur. It makes these decisions based on the information it receives from sensors connected to its input terminals combined with user-programmed instructions.

Solid-state reduced-voltage starter

A solid-state reduced-voltage starter for a three-phase motor has a central processor unit with three inputs to receive information from the starting circuit, the line voltage and the line current. This enables it to start and operate the motor efficiently. It always monitors the line current, particularly during the start-up sequence, to limit the amount of current that can flow into the motor at that time. This is often set at about 300% of full-load current. If at any time any one-phase voltage deviates outside set limits or disappears altogether, the motor protection circuit conveys this information to the processor, and the processor takes the required action to protect the motor. Similarly, the motor is also protected against overloading and overheating. Note that there must be an operator on hand to push the necessary buttons for the motor to start and stop.

Programmable logic controllers

The inputs to the PLC receive information from external sources while the PLC outputs send information to the controlled process, whether it is the starting of an electric motor or some other appropriate task.

By introducing a PLC into the starting process for the motor controller, there is no longer any need for an attendant to be on hand to start or stop the motor.

At the heart of the PLC is a decision-making unit called a microprocessor.

Technically, there is a difference between a microcomputer and a microprocessor. A microcomputer contains a microprocessor and various other circuits to store information, provisions to connect it to the process being automated and a clock. The clock provides specific electric pulses at specific times to control the internal processes of the unit.

Microprocessor programs

A group of instructions that enable a microprocessor to perform a specific task is called a program. A program is a step-by-step procedure to solve a problem, initiate certain actions, or manipulate data that it stores within its memory.

The program consists principally of numbers that must be placed in a read-only memory (ROM) within the processor. It is then able to control the process under control repeatedly without variation until it is reprogrammed from an external source.


By definition, a transducer is a device where any variation in energy magnitude of any form is able to reproduce that variation in another measurable form. In this electronic age, this is generally accepted to appear as an electrical voltage even though it may be in millivolts.

One type of transducer is a thermocouple where two dissimilar metals being heated at their junction produce a voltage. For example, a copper/constantan thermocouple with the cold end kept at a constant temperature produces 4.3 mV at 100° C and 14.8 mV at 300° C. A graph of a range of these values is approximately linear over a restricted range and can be used to indicate the temperature on a voltmeter that has been suitably calibrated to read the temperature. Alternatively, the thermocouple can be connected to a PLC and be part of the control process.

Mechanical devices can be used as electrical limit switches to indicate to a PLC that some machine has reached the limit of its position. Pressure switches can be fitted with a pair of contacts to indicate an on/off relationship, or if fitted with certain types of resistive material, they can provide an infinite number of pressure readings to a PLC.

Positive and negative temperature coefficient resistors (thermistors), provided they are suitably calibrated, can also be used as temperature indicators.

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