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Negative and positive feedback loops are very straightforward. This is a simplified description of their actions:
Something happens –> feedback “NO” –> something stops happening
Something happens –> feedback “YES” –> something keeps happening
The operation of an oven is an easy example of negative feedback acting both ways, which is how it usually acts. That means that there are deviations in two opposing directions. If an oven is set at 220°C, both a decrease and an increase in temperature is a deviation. So if the temperature drops or rises, a sensor picks that up and commands the heater to turn on or off.
This is negative feedback because it returns the system to its original state.
Sometimes completely separate sensors control a rise or fall in temperature. This separation gives a high level of control.
In contrast with negative feedback, positive feedback drives a system further away from its original state, often starting out with small disturbances which cause a factor which further stimulates the disturbance. For example, a panicking sheep will cause more sheep around to get panicked, potentially causing a stampede.
Unlike the sharp control available in negative control loops, positive feedback may end up spiralling itself out of control and self-destructing.