Patent ID: 7872846

Claim:
A protective circuit comprising: a plurality of intrinsically safe electromagnetic actuators for switching electro-hydraulic valves in underground mining; a power supply approved for underground mining; each of said intrinsically safe electromagnetic actuators comprising: a coil connected to a first potential of the power supply; at least two spaced-apart short-circuit means connected in parallel with the electromagnetic actuator coil for short-circuiting the coil in the event of a reversal of the potential of the coil voltage; a semiconductor switch connected in series between the coil and a second potential of the power supply, said semiconductor switch also connected in series with the short-circuit means between the first and second power supply potentials; a biasing means for providing a bias to the semiconductor switch to render the switch normally conductive; and a detecting circuit comprising a comparator having a pair of input terminals connected in parallel with the coil for detecting a potential reversal across the coil, the comparator having an output terminal coupled to the semiconductor switch to cause the semiconductor switch to become non-conductive to isolate the coil and the short-circuit means from the second potential in the event of a reversal of the potential across the coil.