Temperature control

Here is the circuit of a simple and inexpensive temperature controller which can control temperature  from about 20°C to 200°C with an accuracy of 0.5°C


R1 , R2 , 8.2 K
R5 470 Ohm
R6 22 K
R7 820 Ohm
R3 All 100 Ohm
D1 Zener diode 5.1V 400mW
D2 , D5 , D6 1N4001
D3 , D4 15V 1W
IC1 op-amp 741
 T1 BC147
 X1 18V 
 C1 , C2 Capacitor 470μ 50 V
 R1 RELAY 12V 250Ω
S1 1-POLE, 10 way switch




temperature control
temperature control



The controller has been designed and fabricated using the  op-amp IC 741 and a 1k thermistor as the temperature sensor. It is based on the principle of Wheatstone bridge. The ratio arms R1 and R2 of the bridge are kept fixed (say unity). The voltage across the thermistor, i.e. across the arm R4, is compared with the voltage across the variable arm R3 which is kept fixed for a particular temperature (using the op-amp).

When the bridge is not balanced the output of 741 drives transistor BC147 to conducting state. Thus, the relay is energised and switches the heater on. As the temperature increases, the voltage across the thermistor decreases and when this voltage is equal to the voltage across R3 (i.e. the preset value) the output of 741 becomes zero. The transistor then stops conducting, the relay is de-energised, and the heater is switched off. Band switch S1 gives the different ranges of temperature  and potentiometer VR1 is used for fine adjustments

11 thoughts on “Temperature control”

  1. I am using a temp.controller made by eltech.It contains electronic circuit with ic and 6v relay ,a tranformer230 to6v.This controller i am using on plastic moulding machine where polyacetal copolymer requires precision temp.Company stopped making this product and i am unable to repair it as its ic no. is scrathed.I need such controller.

  2. Thank you very much for sharing this circuit. I attempt to adapt it to variety of homemade plastic-melting devices, operating between 200 and 300 celcius degrees. Would you be so kind as to inform me which components should be replaced, in order to make this circuit operate on such a temperature range?
    I have an extremely limited education in electronics, so any clarification would be greatly appreciated.

    Emilio Leotta.
    Buenos Aires, Argentina.

  3. Can I use the circuit to control temperature up to 250 C centigrade.
    And what modifications should be done to do so.

    Thank you and best regards.

  4. I have several outdoor installations where I will need to turn on a low wattage heater when the temperature drops below a -20 F°. I was wondering since I have a 24vdc power supply, can I eliminate D5, D6, X1 and substitute the bridge with a resistor between S1 and R1 as shown by the dotted line R3.

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