Abstract: A zero crossing detection circuit produces an output signal which changes state to indicate the occurrence of a positive-going zero crossing of an AC input signal. The circuit includes first and second input terminals, a current sensitive switch such as an opto-isolator, first and second current regulators, and a voltage limiter. The first current regulator is connected in series with the current sensitive switch, and the voltage limiter is connected in parallel with the first current regulator and the current sensitive switch. The second current regulator is connected between the first input terminal and the parallel combination of the voltage limiter and the first current regulator and the current sensitive switch. The first current regulator limits current through the current sensitive switch to a first current limit level, and the second current regulator limits current flowing between the first and second input terminals to a second current limit level which is greater than the first current level.