Introduction:
Over-voltages can cause significant damage to semiconductor-controlled rectifiers (SCRs). Therefore, it is essential to minimize the effect of over-voltages to ensure the proper functioning and longevity of SCRs. One effective method of minimizing over-voltages is by using varistors.

Explanation:
1. What are SCRs?
SCRs, also known as thyristors, are semiconductor devices widely used in power electronics applications. They are primarily used for rectification, control, and switching of electric power.

2. The Effect of Over-voltages on SCRs:
Over-voltages, which are voltages higher than the rated voltage of the SCR, can lead to various detrimental effects on the device, including:

- Thermal Stress: Over-voltages can cause excessive heat generation, leading to thermal stress on the SCR. This can result in degradation of the SCR's performance, reduced efficiency, and even permanent damage.

- Breakdown: High voltages can cause breakdown in the SCR, leading to the formation of a conducting path between the anode and cathode. This can result in a short circuit and potentially damage the device and the connected circuitry.

- Avalanche Effect: Over-voltages can trigger the avalanche effect, where a rapid increase in current occurs due to the multiplication of charge carriers. This can lead to a rapid increase in temperature and potential device failure.

3. Varistors to Minimize Over-voltages:
Varistors, also known as voltage-dependent resistors (VDRs), are electronic components that have a non-linear voltage-current characteristic. They are specifically designed to protect sensitive electronic devices, such as SCRs, from over-voltages.

When a varistor is connected in parallel with the SCR, it acts as a voltage-sensitive resistor. During normal operating conditions, the varistor presents a high resistance to the current flow, thereby protecting the SCR from excessive voltage. However, when an over-voltage occurs, the varistor's resistance decreases exponentially, providing a low impedance path for the excessive current to flow. This helps to divert the excess voltage and limit it to a safe level, thereby protecting the SCR from damage.

Conclusion:
In conclusion, the effect of over-voltages on SCRs can be minimized by using varistors. Varistors provide a protective mechanism by diverting excessive voltage away from the SCR and limiting it to a safe level. This helps to prevent thermal stress, breakdown, and the avalanche effect, ensuring the proper functioning and longevity of the SCR.