The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix are known to be strongly affected by the matrix thermal expansion. High positive temperature coefficient (PTC) effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes (low temperatures) and semicrystalline polymers (higher temperatures, used commercially in resettable fuses). However, waxes become very liquid upon melting, possibly resulting in carbon sedimentation, and tuneability of semicrystalline polymers is limited. In this work, we therefore study composite resistors with a mixed polymer-wax matrix, the polymer imparting high viscosity in the liquid state and a variety of waxes allowing a high tunability of the behaviour. It is shown that the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. Keywords: thick-film resistors, polymer-carbon composites, PTC effect, resettable fuses