The circuit is designed to produce a temperature monitoring scheme using an operational amplifier and a relay with triggering procedures.
In electronics engineering, operational amplifiers or op-amps was developed to provide a gain block whose performance is totally predictable from unit to unit and perfectly defined by the characteristics of an external feedback network in the design of analog computers. Amplifiers are created to increase the amplitude of a signal, normally a voltage or a current. Basically, an op-amp is a type differential amplifier that has a greater input impedance and minimal output impedance, that is commonly driven by a dual polarity power supply ranging from 5 Volts to 15 Volts.
The circuit contains a relay that is triggered by the op-amp (UA741 or UA341) circuit upon reaching the predetermined temperature without the presence of a hysteresis within the circuitry. Hysteresis is the result of a delay in making changes such as reducing or rising power, which can cause the relay to respond quickly after the change. The circuit also employs a NTC thermistor with 47K ohm resistance. The adjustment of the potentiometer balances the circuit and will be altered with a swift variation in temperature then will boost the relay. This will exchange the position of thermistor and potentiometer, thus producing a cold warning.
During the calibration of the device at room temperature, half the supply voltage will enter the op-amp while modifying the potentiometer to permit the function of relay, while the thermistor operates at needed temperature.Placing the device on hot environment, a multimeter can be used to read the thermistor resistance as it decreases. Then set the preset at current temperature to start the trigger on the circuit. The loss of relay power may be a result of temperature drop, producing a relay chatter or bad contact.