An input circuit (30) for an I/O module (14e) for an industrial controller or the like provides a shunt regulator (38) for precisely controlling the maximum current through the LED (42) of an optical isolator (44). Substantial improvement in the current regulation decreases power dissipation in the optical isolator element. A low voltage shunt circuit (70) prevents leakage from the shunt regulator from activating the optical isolator at low voltages allowing more sensitive optical isolators providing additional power dissipation savings.