We formulate a new approach to the characterization of general power rectifiers with either passive or active loads. We introduce 3-D manifolds of an input current THD and an output ripple. We define the axes of these manifolds as normalized (no source frequency dependent) time-constants of the rectifier, which we found to be the most appropriate to a rectifier characterization. The x-axis is one time-constant of the rectifier which is related to the circuit resonance (caused by the input inductance - L and the capacitive load - C), and the y-axis is the other time-constant of the rectifier which is related to the output ripple (caused by the resistive-capacitive load - RC). In addition, we formulate a new characterization for the mentioned parameters, when an active load (e.g. inverter) is connected. In this case, inter-harmonics caused by the active load, must be considered when characterizing the THD of its input current and output ripple. We introduce 3-D (THD) manifolds of an input current with the new defined axes, when the z-axis is defined as one of the inter-modulation harmonics, caused by the active load. Theoretical results are validated through simulations and lab experiments. Experimental results show high correlation to theoretical analysis. These findings could serve as a simple, useful and quick way for a rectifier characterization.