Thermal stability and Thermal Runaway:

Thermal stability refers to the ability of a system to maintain a constant temperature under various operating conditions. In the context of electrical engineering, it specifically refers to the stability of electronic devices and circuits when subjected to heat generated during operation.

On the other hand, thermal runaway is a phenomenon that occurs when the heat generated within a system exceeds its ability to dissipate or control it. This leads to a rapid increase in temperature, which can result in the destruction of the system or even cause a fire or explosion.

Uncontrolled Positive Feedback:

Uncontrolled positive feedback is a situation where a change in a particular variable leads to an amplification of that change, which further increases the variable, creating a self-reinforcing loop. In the case of thermal stability, uncontrolled positive feedback can lead to thermal runaway.

When there is an increase in temperature within a system, it can cause an increase in the rate of heat generation. This, in turn, raises the temperature further, leading to even higher heat generation. The process continues, causing an exponential rise in temperature, which is known as thermal runaway.

Importance of Controlled Positive Feedback:

Controlled positive feedback, on the other hand, is designed to prevent thermal runaway. It involves the use of feedback mechanisms and control systems to detect and limit the temperature rise within a system.

When the temperature exceeds a certain threshold, the control system intervenes by activating cooling mechanisms or reducing the power input, thereby preventing further temperature rise. This controlled positive feedback helps to maintain thermal stability and protect the system from damage.

Correct Answer:

Option 'A' - an uncontrolled positive feedback is the correct answer because uncontrolled positive feedback can lead to thermal runaway, which is detrimental to thermal stability. It causes an uncontrolled increase in temperature, leading to system failure or damage.

By identifying and implementing controlled positive feedback mechanisms, engineers can prevent thermal runaway and ensure thermal stability in electronic devices and circuits.