Abstract: Electronic unit in which a printed circuit board is located within a housing which includes a heat sink. The board is normally urged out of heat conductive relationship with the heat sink. As temperature increases the circuit board and heat sink are caused to move relatively so as to bring the board into heat conductive relationship with the heat sink so as to remove heat from the board. As temperature decreases the board and heat sink are again moved out of heat conductive relationship. Preferably a heat conduction element is provided to conduct heat from the board. This comprises at least one compressible member, e.g. compressible cellular material, which is progressively compressed as temperature increases. Preferably the heat conductive efficiency of the compressible member increases as compression increases.