A converter assembly was a technology used aboard Constitution-class starships for power generation.
In 2266, the starship USS Enterprise was forced to extend its shields around a small class J starship which it had pursued into an asteroid belt. This over-expenditure of power burned out the lithium crystal circuits at the heart of the ship's power, and caused significant damage to the converter assembly, making the institution of a bypass circuit impossible. (TOS: "Mudd's Women")