Temperature sensors and thermostats

Ovens, heat treatment, plastic moulding, motors and hydraulic oil all need temperature control. A thermostat gives an on/off signal; thermocouples and Pt100 sensors give the actual temperature to the PLC.

Thermostat (temperature switch)

A thermostat has a contact that switches at a set temperature. Mechanical types use a bimetal strip or a liquid-filled bulb; electronic types have a display and PNP outputs. Like a pressure switch it has a differential, so the heater does not switch on and off every second.

Thermocouple

Two different metals joined at the tip produce a small voltage that depends on the temperature difference between the tip and the cold end (a few tens of µV per °C).

Use the correct compensating cable all the way to the thermocouple input card, never ordinary copper wire.

Pt100 (RTD)

A platinum resistor with 100 Ω at 0 °C, rising about 0.385 Ω per °C. More accurate and stable than a thermocouple, for −200 to about 600 °C. Use 3-wire or 4-wire connection so the cable resistance does not add to the reading.

Connecting to a PLC

• Thermostat: a digital input, like any contact.
• Thermocouple / Pt100: a dedicated temperature input module, or a head-mounted transmitter that converts to 4–20 mA for a normal analog input.

On/off vs PID control

On/off control is simple but the temperature oscillates around the set point (see the animation). A PID controller in the PLC or a separate temperature controller varies the heater power (with a solid-state relay switched by time-proportioning, or a thyristor unit) and holds the temperature within a degree or two.