Introduction to Industrial Automation

Figure 2.48  Bimetal temperature switch: (a) electric contact is closed at normal temperature, 

and (b) electric contact opens at high temperature.

Heated

surface 

A

mV~ΔΤ

+NiCr

–NiAl

V

out

R

R

+

_

Hot junction

(measuring temperature)

Cold junction

Thermocouple

Figure 2.49  Thermocouple as a sensing element in a thermal sensor or switch.

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Hardware Components for Automation and Process Control 

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61

conducting metals, such as platinum, copper, or nickel. The corresponding temperature switch uses 

an RTD as a sensing element, characterized by a linear positive change in resistance with respect 

to temperature. There are two common forms of construction for RTD sensing elements: Wire-

wound RTD elements are manufactured by winding a small diameter of wire on a ceramic bobbin, 

as shown in Figure 2.50a. Another form is the thin-film RTD sensing element, which consists of a 

very thin layer of metal deposited onto a flat ceramic substrate, as shown in Figure 2.50b.

From the above-mentioned conducting metals, platinum is the most commonly used in RTD 

manufacturing, and the PT100 temperature detector is widely available in the market. It has a 

standard resistance value of 100 Ω at 0 °C and about 140 Ω at 100 °C, and it can cover a tem-

perature range from −200 °C to 850 °C. There are also PT500 and PT1000 RTDs for higher 

measuring resistances.

Since the RTD elements are constrainedly connected to lead wires for extension purposes, 

their line resistances add errors, due to the increased total resistance value. On the other hand, 

since RTD is a resistive element, an electric current is needed to pass through it in order to detect 

the resulting voltage and to have the corresponding temperature. This creates self-heating of the 

resistive wires, causing additional errors. To avoid these errors, the RTD element is connected to a 

Wheatstone bridge, the other branches of which have a compensation action, as shown in Figure 

2.50c. A voltage source excites the bridge, and the indicative voltage across the bridge output is 

proportional to the resistance of the RTD. This circuit is called a “two-wire connection” of the 

RTD sensing element. Better results from an accuracy point of view are given by the “three-wire” 

and “four-wire” connections in combination with a constant current source, connections that are 

not going to be considered further in this chapter. Figure 2.51 shows an external typical form of 

an RTD temperature sensor or switch, whose probe is inserted in the space where the temperature 

is being detected.

There are many other types of temperature sensors and switches, some of which will be men-

tioned briefly here.

The thermistor is similar to the RTD temperature sensor, whose resistance changes with tem-

perature, having either a negative or positive temperature coefficient. The thermistor is made from 

ceramic type semiconductor materials and presents a large resistance variation for a small tem-

perature change.

The infrared temperature sensor is a non-contact electronic detector of thermal radiation. It 

measures the infrared energy emitted by a stationary or moving object as a result of its thermal 

state.

Ceramic bobbin

Lead wires

Platinum wire

Platinum thin-film layer

on a ceramic substrate

R

V

R

R

RTD

Wire-wound

RTD element 

Thin-film

RTD element

Wheatstone bridge

with one RTD branch

(a)

(b)

(c)