May 082015
 

Differential Temperature Controller circuit

The Differential Temperature Controller circuit measures the temperature difference between two sensors and activates a relay when the temperature is not the same.

It is also possible to detect a temperature change, even when the two measured temperatures are normally different.

How the Differential Temperature Controller works?

To implement this circuit we use as temperature sensors, two common semiconductor diodes (D1 and D2). The anodes of the diodes are connected to the inputs of the operational amplifier. The op amp works as a comparator, and any temperature difference will cause a low voltage level at its output.

Differential Temperature Controller circuit

The diodes are placed in two different places with the two temperatures we want to compare. If there is a small variation in temperature in any of the two diodes, the voltage at its terminals decreases.

  • If the diode D1 is placed in a location susceptible to a temperature drop, the op amp will have on its output a low voltage level and will activate the relay through the transistor Q1. The transistor may activate a heating or similar.
  • If the diode D2 is placed on a location susceptible to a temperature rise, the op amp will have on its output a low voltage level and will activate the relay through the transistor Q1. The transistor may activate a refrigeration system or fan.

Differential Temperature Controller PCB - An approximate result of the appearance of the final circuit
When the temperature on the two diodes is again the same, the relay will be deactivated. Note that the inclusion of the potentiometer may vary the operation of the circuit.

Note: The circuit can be powered by a 9V battery. You have to switch the relay by one of the same voltage.

List of circuit components

  • 1  741 operational amplifier (IC1)
  • 1  2N3702 PNP bipolar transistor or equivalent (Q1)
  • 2  4.7K resistors (R1,R2)
  • 2  1.2K resistors (R3,R4)
  • 1  2.7K resistor (R5)
  • 1  100K potentiometer (P)
  • 3  1N4001 diodes (D1,D2,D3)
  • 1  12V relay (RL1)

Differential temperature controller suggested PCB

Differential Temperature Controller PCB

An approximate result of the look of the final circuit is shown on second diagram above.

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