13Level 4

On/off temperature control of an oven

A temperature transmitter gives an analog signal. The program scales it to degrees Celsius and switches the heater on and off with a ±5 °C hysteresis around 180 °C.

Open in the simulator

The simulator is designed for computers (screens 1024 px or wider).

Virtual plant

Shown at rest. In the simulator the plant moves according to your program's outputs.

Inputs and outputs

Address	Name	Description
I0.0	ON	Oven switch
IW0	TEMP_TRANSMITTER	Transmitter analog input: 0–27648 = 0–300 °C (plant)
MD0	TEMPERATURE	Temperature in °C (REAL)
Q0.0	HEATER	Heater contactor

Program

Ladder diagram (LD)

Rung 1 — Scales the transmitter: 0–27648 → 0–300 °C.
Rung 2 — Below 175 °C: heater on.
Rung 3 — Above 185 °C or with the oven off: heater off.

Structured text (ST)

(* On/off temperature control of an oven — converted from Ladder *)

(* Rung 1: Scales the transmitter: 0–27648 → 0–300 °C. *)
TEMPERATURE := (TO_REAL(TEMP_TRANSMITTER) - TO_REAL(0)) * (TO_REAL(300) - TO_REAL(0)) / (TO_REAL(27648) - TO_REAL(0)) + TO_REAL(0);

(* Rung 2: Below 175 °C: heater on. *)
IF ON AND TEMPERATURE < 175.0 THEN HEATER := TRUE; END_IF;

(* Rung 3: Above 185 °C or with the oven off: heater off. *)
IF TEMPERATURE > 185.0 OR NOT ON THEN HEATER := FALSE; END_IF;

Instruction list (IL)

(* On/off temperature control of an oven — converted from Ladder *)

(* Rung 1: Scales the transmitter: 0–27648 → 0–300 °C. *)
  LD    TEMP_TRANSMITTER
  TO_REAL
  SUB(  0
    TO_REAL
  )
  MUL(  300
    TO_REAL
    SUB(  0
      TO_REAL
    )
  )
  DIV(  27648
    TO_REAL
    SUB(  0
      TO_REAL
    )
  )
  ADD(  0
    TO_REAL
  )
  ST    TEMPERATURE

(* Rung 2: Below 175 °C: heater on. *)
  LD    ON
  AND(  TEMPERATURE
    LT    175.0
  )
  S     HEATER

(* Rung 3: Above 185 °C or with the oven off: heater off. *)
  LD    TEMPERATURE
  GT    185.0
  ORN   ON
  R     HEATER

Step by step

1Press ▶ Run (F5) and turn on ON. Rung 1: SCALE converts the transmitter number (0 to 27648) into degrees (0 to 300 °C).
2Rung 2: if the temperature drops below 175 °C, the heater turns on (Set). Rung 3: above 185 °C, or when the oven is switched off, it turns off (Reset).
3Watch the temperature go up and down between the two marks: that band is the hysteresis. Without it, the heater would switch many times per second near the setpoint, wearing out the contactor.
4In the plant, open the oven door: losses increase and the heater stays on longer.

Open in the simulator