A transient blocking unit (TBU) is a circuit having series-connected transistors that normally conduct current, but automatically switch to a high-impedance current blocking state in response to an over-current condition. Here enhancement mode devices are used in the primary TBU current path, as opposed to the conventional use of depletion mode devices in this context. This approach provides two main advantages. The first advantage is that the dependence of TBU parameters on poorly controlled depletion mode device parameters can be reduced or eliminated. The second advantage is that such TBUs can provide over-voltage protection in addition to over-current protection.