| 摘要 |
1,260,872. Two-wire current transmitters. ROSEMOUNT ENG. CO. May 27, 1969 [May 27, 1968], No.26816/69. Heading G1U. A two-wire circuit transmitter for connection to a remotely-located D. C. source and load 16, 17 to control the load current I t in dependance on an input D.C. signal from a circuit 40 comprises: an arrangement 20 for providing across a resistor 27 a feedback voltage proportional to the voltage across a second resistor 23 in the load current path whilst maintaining D. C. isolation between the two resistors; and a current controller 11 which controls the load current It in dependance on the difference between the input signal and the feedback voltage. The arrangement 20 comprises a storage capacitor 29 in parallel with the resistor 27, a capacitor 39 inter-connecting first ends of the two resistors 23, 27, and a further capacitor 30 which is switched repetitively between the two first and the two second ends of resistors 23, 27 under the control of an oscillator 14. The power required to operate the oscillator 14 is derived from the load current It by a voltage regulator 14, e.g. a Zener diode, in the load current path. Preferably Fig.2 (not shown) oscillator 14 is a multi-vibrator and each of switches 33, 34 is constituted by a pair of f.e.t. 's whose channels are unblocked in alternation by the signal from the multi-vibrator. The feedback signal may be adjusted by varying resistor 27, and is fed in series with the unknown input signal to a chopper 50, also controlled by oscillator 14, whose output is capacitatively coupled to the input of current controller 11, as described in Specification 1, 240, 493. The input signal may be derived from thermocouple 41 in series with a bridge network 42 supplied with a constant D. C. voltage derived from the oscillator output. The bridge network provides an offset voltage, and/or includes a temperature responsive arm to provide cold junction temperature compensation. The input signal may alternatively be derived from a strain gauge bridge or a resistance thermometer bridge, with or without the auxiliary bridge 42. |
