A constant voltage rectifier is normally adjusted by changing:

A constant voltage rectifier is designed to maintain a consistent output voltage regardless of variations in load or input voltage. To achieve this, the output voltage must be controlled accurately, which is typically done by adjusting the transformer that supplies power to the rectifier.

Adjusting the secondary transformer taps allows for precise changes to the voltage level outputted by the rectifier. By selecting different taps on the transformer’s secondary winding, the voltage can be increased or decreased, allowing the rectifier to maintain the desired constant voltage under varying operational conditions. This method provides a stable way to set the rectified output voltage to meet the specific requirements of the circuit being powered.

In contrast, options involving series or parallel resistors would not effectively provide a method for adjusting the output voltage of a rectifier to a consistent level. Resistors can introduce voltage drops or variations based on current flow and are not typically used for adjusting the main voltage output of a system. Adjusting primary transformer taps can also alter the voltage but tends to be less practical for fine-tuning the output of a rectifier compared to adjustments made at the secondary level. Thus, modifying the secondary transformer taps is the most effective way to achieve the needed voltage stability in a constant voltage rectifier setup.