The MV Transformer is a machine added by Industrial Craft 2. The MV Transformer converts High Voltage (≤512 EU/t) to Medium Voltage (≤128 EU/t), necessary for connecting Low and Medium Voltage machines to an MFSU or any High Voltage generators. (An LV Transformer is still necessary to convert the resulting Medium Voltage to Low Voltage, if desired.) If a Redstone signal is applied to the MV Transformer, the conversion works in reverse, and Medium Voltage will be converted to High Voltage.
The High Voltage side of the MV Transformer is demarcated with three dots, whereas all other sides have only one dot and will output Medium Voltage. This can be changed using a Wrench, an Electric Wrench, or a Prototype OmniWrench. Using the Wrench on the High Voltage side of the MV Transformer will cause the machine to drop as an item instead.
It's recommended that the MV Transformer not be connected to any machinery while its High Voltage face is being rotated, or else the machinery it is connected to has a risk of exploding.