[en] The stability of controlled rectifiers defines their operation regimes. The different definitions for rectifier instability: harmonic instability, ripple instability and low-frequency instability, apply to the appearance of disordered harmonics as a measure of the instability of the system. The physical cause of instability has not been clarified yet. Two models of controlled rectifiers were built: a static model, which included a bridge rectifier and inductance and ohmic loads, and a dynamic model which included a bridge rectifier and a direct-current motor as load. In the static model the stability of the rectifier firing pulses was examined directly by measuring the intervals between the firing pulses. In the dynamic model the stability of the firing pulses was examined indirectly by measuring the amplitude of the rectified current waves. The processing of the results of both models confirmed the existence of a clear dependence of the deviation in firing angles and current wave amplitudes on the amplification of the control system. This dependence shows clearly the regimes of stability, the instability, and the boundary point between them. Analysis of the results shows that the physical cause of instability is the disturbance accompanying the regulating voltage, which regulated the firing angles. From the obtained results a new definition to the instability is given, namely the instability of firing angles. This definition corresponds to the deviations obtained directly in the static model, and indirectly in the dynamic model, by the effect of firing angles on the direct current wave amplitudes