FIELD DEVICE
A switch is provided in a supply line for an operating power supply from an operating power supply generating circuit to a main circuit, An electric current detecting circuit and a comparison evaluating circuit are provided as a controlling circuit portion for controlling the ON/OFF state of the switch. The electric current detecting circuit inputs a voltage in accordance with the value of an electric current supplied from an higher-level system, and inverts and amplifies the voltage to produce an output voltage. The comparison evaluating circuit inputs the output voltage from the electric current detecting circuit, compares the output voltage to a reference voltage, and turns the switch ON if the output voltage is higher than the reference voltage. The reference voltage is set in advance in accordance with a lower limit electric current value for the electric current that must be supplied for the main circuit to operate properly.
1 . A field device comprising:
an operating power supply generating circuit generating an operating power supply from an electric current that is supplied through a pair of electric wires from a higher-level system;
a main circuit, which includes a calculation processing portion, which operates by receiving a supply of an operating power supply from the operating power supply generating circuit;
a switch provided in the supply line for the operating power supply to the main circuit;
an electric current detecting circuit detecting, as a present electric current value, a value of an electric current that is supplied through the pair of electric wires; and
a comparison evaluating circuit comparing a value in accordance with the present electric current value that is detected by the electric current detecting circuit and a comparison evaluation value that is set in advance as a value in accordance with a lower limit electric current value for the electric current that must be supplied for the main circuit to operate properly, to close the switch when the value in accordance with the present electric current value is higher than the comparison evaluation value, and to open the switch if the value in accordance with the present electric current value is lower than the comparison evaluation value.
2 . The field device as set forth in claim 1 , wherein:
the comparison evaluating circuit has a first comparison evaluation value and a second. comparison evaluation value, which is set so as to be a specific value lower than the first comparison evaluation value, as comparison evaluation values, to close the switch when the value in accordance with the present electric current value rises above the first comparison evaluation value, and to open the switch when the value in accordance with the present electric current value falls below the second comparison evaluation value.
3 . The field device as set forth in claim 1 , wherein:
the calculation processing portion of the main circuit controls a degree of opening of a regulator valve in accordance with a value of an electric current that is supplied through the pair of electric wires.