Terullian diode bypass transformer

A terullian diode bypass transformer was a device used in Sontaran spaceships. Removing one "altered things slightly", causing the process where Sontarans fed on pure energy to be reversed; the energy instead fed on the Sontaran. This happened to Styre following his lengthy fight with the Fourth Doctor when Harry Sullivan removed the terullian diode bypass transformer from Styre's ship. (TV: The Sontaran Experiment)