Feb 062017
 

Overtemperature alarm using operational amplifier

On the diagram below you can see how a semiconductor diode is used as a temperature sensing element in an overtemperature alarm using operational amplifier.

On this circuit the Zener diode is connected in series with the resistor R1 so that between the ends of the voltage dividers formed by R2-R3 and R4-D1, there is a constant voltage of 5.6 V. Practically a constant current circulates for each of these dividers.

On this way we have:

– A constant reference voltage between the R2 – R3 junction and the pin 2 of the operational amplifier, and
– A temperature-dependent voltage with a coefficient of -2 mV /°C between the R1 – R3 junction and the pin 3 of the operational amplifier.

Therefore, a differential voltage with a coefficient of – 2 mV/°C appears between pins 2 and 3 of the amplifier. To tune this circuit, the diode D2 is raised to the desired temperature level, and potentiometer R3 is slowly set , just before the relay is activated.

At this time between pins 2 and 3 of the operational amplifier appears a differential voltage of 1 mV, with the voltage at pin 3 below the voltage at pin 2, which activates transistor Q1 and relay.

Overtemperature alarm using operational amplifier

When the temperature drops below the trigger level, the voltage at pin 3 increases above the voltage at pin 1 at about 2 mV/°C. Whereby the transistor Q1 and the relay are de-energized. The circuit has a typical sensitivity of 0.5 °C, and can be used as an overtemperature alarm from below zero temperatures to above the boiling point temperature of the water.

The operation of the circuit can be reversed to act as an insufficient temperature alarm by reversing the connections of pins 2 and 3 of the operational amplifier.

Overtemperature alarm using operational amplifier list of components

  • 1  741 operational amplifier (U1)
  • 1  2N3702 PNP transistor (Q1)
  • 1  5.6 V Zener diode (D1)
  • 2  common semiconductor diodes (D2, D3)
  • 1  12V relay (RLA)
  • 3  1.2k resistors (R1, R5, R6)
  • 2  4.7k resistors (R2, R4)
  • 1  1k potentiometer (R3)


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