A system that controls our electric heater to keep the temperature of a room at some steady level, say, 25oC, is a common example of a regulator system. The set point can be adjusted from time to time that best suits current conditions. The circuit that is shown below is a fully automatic system. It can be easily used for controlling the temperature levels of an
- oven,
- a refrigerator,
- an incubator,
- a room or
- a building.
The temperature sensor shown by R1 is a thermistor. This is a part of a potential divider and the feedback signal is the voltages at the junction point of R1 and R2. In our diagram, this is labeled “t0”. It goes to the inverting input of the operational amplifier IC1, which is connected to play role of a comparator.
The set point voltage “tS” is coming from the wiper terminal of the VR1, and goes to the non-inverting input of the operational amplifier. This makes the circuit the equivalent of the comparator shown in Figure 33.2. The op amp is connected as an open-loop amplifier so has very high gain (A). The output of the op amp is:
A (ts – to)
This is the error in signal. This is because the open-loop gain of the op amp is very high, the error signal in this case swings fully towards the negative or the positive rails, thereby turning the transistor switch fully on or fully off, keeping in view whether to is less than or greater than tS value.
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