A transient blocking unit (TBU) having reduced series impedance is provided. A TBU includes two or more depletion mode transistors arranged to provide a low series impedance in normal operation and a high series impedance when the input current exceeds a predetermined threshold. A nonlinear impedance element is included in the TBU that acts as a current limiter having a substantially constant saturation current over a range of applied voltages. This saturation current is selected to be the threshold current of the TBU. When the threshold is exceeded, the voltage developed across the nonlinear impedance element tends to drive the TBU into its high impedance state. When the operating current is below threshold, the TBU series resistance is relatively low because the nonlinear impedance element is in its low-resistance state. The nonlinear impedance element can be a separate circuit element, or it can be integrated with one or more of the TBU transistors.