In this work we study in more details the impact of voltage trimming on different characteristics of model thick-film resistors (TFRs). Two series of RuO2-based TFRs with different conducting grain sizes are studied. As the composition of model pastes is completely known, it allows connecting the effects of trimming with the microscopic structure of the resistors.This study focuses on the change of resistivity, thermal coefficient of resistance, piezoresistivity and their post-trim stability. Those characteristics are studied as a function of conducting filler volume fraction. We show that the relative change of conductivity and the change in pezoresistivity due to voltage trimming are both diverging as the critical volume fraction, where the conductor/insulator transition occurs, is approached. It is also shown that voltage trimming changes the critical DC exponent, and can lead to a crossover from a nonuniversal to a universal behavior. We propose a description of voltage trimming which can explain the observed changes in characteristics of the samples. Key words: High-voltage Trimming, Thick- film resistors, Stability