A current regulator is provided for a two wire sensor and comprises first and second conductors arranged to provide a sensor output current, a first resistance and a current reference coupled across the first and second conductors, and a second resistance and sensor load terminals coupled across the first and second conductors. An amplifier has first and second inputs and an output. The first input is coupled to a first junction between the first resistance and the current reference, the second input is coupled to a second junction between the second resistance and the sensor load terminals, and the output is connected to the second input. The amplifier controls a first voltage at the first junction to be substantially equal to a second voltage at the second junction.
A two-wire sensor (S) is connected to a pole via a first connection line (V1) in which a voltage longitudinal controller (SR) and a current-limiting resistor (R1) are positioned, and via a second connection line (V2) to which the other pole of a power supply source is connected. The output of the voltage longitudinal controller (SR) is connected to the second connection line (V2) via a first limiting diode (D1) and a second limiting diode (D2) which preferably is installed opposite the first connected diode. The joint node of the two limiting diodes (D1, D2) is connected to the control input of the voltage longitudinal controller (SR).
A two-wire sensor (6) is attached to one pole of a voltage distribution source (U) by an initial connecting line (V1) in which a current-limiting resistor (R1) is positioned and is attached to the other pole by a second connecting line (V2). Parallel to the series connection consisting of the two-wire sensor (S) and the current-limiting resistor (R1) is at least one limiting diode (D1). A current regulator (SR) which is attached to the poles of the power source (U) regulates a parallel current (I) which runs parallel to the current through the two-wire sensor (D), so that there is a sum current at the poles of the power source (U) that always corresponds to the measured value delivered by the two-wire sensor (S). The regulating inputs of the current regulator (SR) are connected to the terminals of a current-sensing resistor (R2) positioned in a connecting line (V1). When a HART.RTM.-interface is used with the two-wire sensor (S) a HART.RTM.-resistor (RH) is positioned in one of the two connecting lines (V1).
A generator controller (4) is provided which is able to detect and respond to failure of a generator output voltage sensor, whilst being able to distinguish voltage sensor failure from output voltage collapse due to the presence of an overload or short circuit condition. The presence of a voltage sensor failure is indicated by the measured output voltage being below a first voltage threshold whilst the output current is also below a current threshold.
