Magnetic excitation circuit for electromagnetic flow meter

A magnetic excitation circuit used as a circuit for providing a magnetic excitation electric current based on a magnetic excitation power supply voltage to a magnetic excitation coil of an electromagnetic flow meter, wherein the magnetic excitation electric current is switched to positive polarity/negative polarity with each positive interval/negative interval that are repeated with a specific period, where, at the times of the beginnings of these positive interval and negative interval the magnetic excitation power supply voltage is switched to a high voltage during a high-voltage interval, and after a specific time interval has elapsed after the starting time, the magnetic excitation power supply voltage is switched to a low voltage over a low-voltage interval, the magnetic excitation circuit comprising:

• first and fourth switching elements that provide a magnetic excitation electric current that is switched to a positive polarity through turning ON during the positive interval and turning OFF during the negative interval;
• second and third switching elements that provide a magnetic excitation electric current that is switched to a negative polarity through turning OFF during the positive interval and turning ON during the negative interval;
• a constant current circuit that controls the magnetic excitation electric current to a specific value;
• voltage storing circuits, which store and output a driving voltage that is charged by a common driving voltage, connected individually between the respective controlling terminals and output terminals of the first through fourth switching elements to enable operation by a driving voltage that is outputted from the respective voltage storing circuits when the first through fourth switching elements are turned ON with the positive interval/negative interval;
• a charging capacitive element that is connected in common to the input terminals of the first and third switching elements, and provides, as the high voltage, a charge voltage obtained by charging by a reverse electromotive force that is produced in the magnetic excitation coil at the time of switching of the polarity of the magnetic excitation electric current;
• and
• a reverse flow preventing diode, wherein
• the anode terminal is connected to the magnetic excitation power supply voltage and the cathode terminal is connected in common to the input terminals of the first and third switching elements, to prevent reverse flow of the reverse electromotive force to the magnetic excitation power supply voltage, and to provide, to the input terminals of the first and third switching elements, the magnetic excitation power supply voltage in response to a drop in the charging voltage.