What Is a Temperature Controller? PID Explained
Short answer
- A temperature controller compares a sensor reading with a setpoint and switches a heater; on/off control cycles around the setpoint, while PID adjusts the power for tighter control.
- Set the controller's input type to match the sensor, since a wrong thermocouple type setting is a classic cause of readings that are off by many degrees.
- Over-temperature protection should be an independent limit device, not the controller that regulates the process.
A temperature controller is an instrument that holds a process at a target temperature. It reads a temperature sensor, compares the measurement with the setpoint, and switches an output, usually to a heater but sometimes to a cooling device, to correct any difference. Temperature controllers are used on ovens, extruders, heat sealers, plastic moulding, laboratory equipment and many other thermal processes.
The control loop
The loop has four parts: a sensor measuring the process variable, the controller comparing it to the setpoint, an output device such as a relay or solid state relay switching power, and the heater or other final element acting on the process. The controller continuously adjusts its output to reduce the error between setpoint and measurement.
On/off control
The simplest approach turns the heater fully on below the setpoint and off above it. A small gap called hysteresis stops the output from switching rapidly around the setpoint. On/off control is simple and low cost, and it suits slow processes with tolerance for temperature swings, but the temperature will cycle around the setpoint.
PID control
PID control adjusts the amount of power rather than just switching on or off. It combines three actions:
- Proportional reduces output as the temperature approaches the setpoint, so power is cut back before the target is reached.
- Integral corrects the steady offset that proportional control alone leaves, by accumulating error over time.
- Derivative reacts to the rate of change to limit overshoot on fast processes.
Many controllers offer an auto-tune function that runs a test cycle and sets the PID values for the process. Poorly tuned loops either oscillate or respond sluggishly, so tuning is worth checking whenever the heater, load or insulation changes.
Inputs and outputs
- Inputs. Controllers accept thermocouples, RTDs and often analog signals. The input type has to be set to match the sensor, and a wrong thermocouple type setting is a classic cause of readings that are off by many degrees.
- Relay outputs suit slow cycling and small loads.
- Solid state relay drive outputs switch an SSR quickly for finer control of heaters, so the SSR is rated for the heater current and mounted on a suitable heat sink.
- Analog outputs drive power controllers or valves for proportional control.
- Alarm outputs signal high or low temperature and sensor failure.
Choosing a controller
- Panel size, commonly specified as a fraction of a DIN standard such as 1/16, 1/8 or 1/4 DIN.
- Input type and range to match the sensor.
- Output type and rating for the heater or the SSR.
- Number of loops and zones, alarms, ramp and soak programming, and communications.
- Supply voltage and environmental rating at the panel face.
Further reading on this site: process and temperature controllers; temperature sensors; solid state relays; RTD vs thermocouple; how solid state relays work.
Related tools: thermocouple and RTD selector.
Over-temperature protection should be handled by an independent limit device, not by the controller that regulates the process, so that a failed controller cannot cause a runaway. If you need to match or replace a temperature controller, the panel size, input and output types and supply are the key details, and we can help identify and source a suitable unit.
Need a quote on a specific part?
Request a QuoteFrequently asked questions
What is the difference between a thermostat and a temperature controller?
A thermostat is a simple on/off device with a setpoint, often adjusted by a dial. A temperature controller has a digital display, accepts several sensor types and usually offers PID control, alarms and communications, so it holds temperature much more tightly.
Why is the display reading wrong?
Common causes are an input type setting that does not match the sensor, thermocouple wires connected with reversed polarity, the wrong extension wire type, a damaged sensor, or electrical noise in the sensor wiring. Checking the input configuration against the sensor is the first step.
When should I use a solid state relay output?
When the heater is switched often, as PID control requires, because a mechanical relay wears out with rapid cycling. The SSR must be rated for the heater current and mounted on a heat sink suited to the load.
What is an over-temperature limit controller?
It is a separate device that monitors the same process with its own sensor and opens a contactor or relay if the temperature goes above a safe limit, independently of the main controller. Because the main controller can fail with its output stuck on, an independent limit device is the usual protection against runaway heating on ovens, heaters and similar equipment.