How does feeding the output of a system back to its input let it control itself, as a thermostat keeps a room at a steady temperature?
Explain feedback in a control system, distinguish negative from positive feedback, and describe a closed-loop control example
A focused answer to the O-Level Electronics outcome on feedback. Open and closed loops, negative versus positive feedback, and a thermostat as a self-regulating control system.
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What this dot point is asking
SEAB wants you to explain feedback in a control system, to distinguish negative from positive feedback, and to describe a closed-loop control example such as a thermostat. The central insight is that feeding the output of a system back to its input lets the system compare what it is doing with what it should be doing, and correct the difference - which is how a control system regulates itself.
The answer
Open loop and closed loop
A control system can work in two ways:
- An open-loop system acts on its input without checking the result. A toaster on a fixed timer runs for the set time whether the toast is pale or burnt; it has no feedback.
- A closed-loop system measures its output and feeds it back to compare with the desired value, then corrects any difference. A thermostat-controlled heater measures the room temperature and adjusts the heater to match the set value.
Feedback is what turns an open loop into a self-correcting closed loop.
What feedback is
Feedback means taking a measure of the output of a system and returning it to the input, where it is compared with the desired value (the set point). The difference between the desired and actual values is the error, and the system acts to reduce this error. The path that carries the output measurement back to the input is the feedback path.
Negative feedback
In negative feedback, the fed-back signal opposes change: it acts to reduce the error and bring the output back towards the set value. This makes a system stable and self-regulating. A thermostat uses negative feedback: if the room is too cold the heater turns on, and if it is too warm the heater turns off, so the temperature is held steady around the set point. Negative feedback is also what gives an op-amp amplifier its stable, controlled gain.
Positive feedback
In positive feedback, the fed-back signal reinforces change: it pushes the output further in the direction it is already moving. This drives a system rapidly to one extreme rather than holding it steady, which is useful for switching cleanly between two states but unsuitable for steady regulation. Most control systems that must hold a value steady use negative feedback.
Examples in context
Example 1. Cruise control in a car. The driver sets a desired speed, and a sensor feeds the actual speed back for comparison. On a hill the car slows, the error grows, and the controller opens the throttle to restore the set speed; downhill it eases off. This negative-feedback loop holds the speed steady without the driver touching the pedal.
Example 2. The human body. The body keeps its temperature near by negative feedback: if it gets too hot, sweating cools it; if too cold, shivering warms it. The same control principle that governs a thermostat governs living systems, showing how general the idea of feedback is.
Try this
Cue. State what is meant by negative feedback in a control system. The fed-back signal opposes change, reducing the error and bringing the output back towards the set value, making the system stable.
Cue. Give one example of an open-loop control system. A toaster on a fixed timer, which runs for a set time without checking how brown the toast actually is.
Cue. In a thermostat, identify the input, the output and the feedback path. The input is the set temperature, the output is the actual room temperature, and the feedback path is the sensor returning the measured temperature for comparison.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksA thermostat controls a heater to keep a room at a set temperature. Explain how negative feedback keeps the temperature steady, identifying the input, the output and the feedback path.Show worked answer →
The input is the set (desired) temperature; the output is the actual room temperature, produced by the heater. A temperature sensor measures the room and feeds this back to be compared with the set value.
If the room is below the set temperature, the difference turns the heater on, warming the room. As the room reaches the set value, the difference falls and the heater turns off. If the room gets too warm, the heater stays off until it cools again. This negative feedback opposes any change, holding the temperature steady around the set value.
What markers reward: set temperature as input and room temperature as output, the sensor feeding back the actual temperature for comparison, and the feedback opposing change to keep the output near the set value.
Original3 marksState the difference between an open-loop and a closed-loop control system, and give one example of each.Show worked answer →
An open-loop system has no feedback: it acts on its input without checking the result. An example is a toaster timer that runs for a set time regardless of how brown the toast actually is.
A closed-loop system feeds the output back to compare with the desired value and corrects any difference. An example is a thermostat-controlled heater that measures the room temperature and adjusts the heater to match the set value.
What markers reward: open-loop having no feedback and not checking the output, closed-loop using feedback to correct against a target, and a sensible example of each.
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