Which device is used to convert AC to DC?

A rectifier is the device specifically designed to convert alternating current (AC) into direct current (DC). This process is fundamental in many electrical applications where DC is required to power devices, charge batteries, or for other specific circuit configurations.

Rectifiers work by allowing current to flow only in one direction, effectively "clipping" the AC waveform to create a pulsating DC output. This is typically achieved using semiconductor components, such as diodes, which permit current to pass when forward-biased while blocking it in the reverse direction.

In contrast, transformers are used to change the voltage levels of AC but do not convert AC to DC. Capacitors, while they can be involved in smoothing out the ripple in the output of a rectifier, do not directly convert AC to DC. Inverters, on the other hand, perform the opposite function by converting DC back to AC. Thus, the rectifier is the correct answer as it directly serves the purpose of AC to DC conversion.