Abstract: The device includes a thermostat which is potted or secured by epoxy in a recess in the closed end of a generally cup-shaped, dielectric housing. A generally disc-shaped, aluminum alloy casting is secured coaxially against an opening formed in the bottom of the housing, and against a PTC resistor which is positioned in the opening between the casting and the thermostat, and in electrical contact with the latter. A bimetallic coil is mounted in the open end of the housing with its inner end staked to an integral post on the Al casting, and with its free end mounted to unwind or rotate angularly in the housing with changes in ambient temperature. The Al casting functions as a heat sink which can be trimmed, before being secured in the housing, to calibrate the bimetel coil.