Galvanic isolation coupling of current loop

Details Galvanic isolation coupling of current loop Galvanic isolation coupling of current loop

2000-08-04

2001-07-24

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Signal isolator

C250S2140RC

Reexamination Certificate

active

06265724

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a galvanic isolation coupling of a current loop comprising an operational amplifier as a part of the current loop and an optoisolator comprising two receivers.
Current loops are commonly used for conveying measuring information. A constant-current signal passing through the current loop is generated by a measuring sensor and a measuring transmitter, and a variable to be measured can be e.g. temperature or pressure. The constant-current signal has a typical magnitude of 4 . . . 20 mA, whereby the lower limit of the measuring range of the variable to be measured is set for a 4 mA current signal, and correspondingly, the upper limit of the measuring range is set for a 20 mA current signal.
It is often desirable that the current loop which carries the current signal is galvanically isolated from the circuit utilizing measuring information. Measuring information is utilized as control equipment feedback, for instance. Galvanic isolation allows the measuring information to be processed in potential which differs from the current loop, whereby the reliability of the processing improves and the structure of the required couplings is simplified.
In order to get the information of the current signal in the current loop transferred undistorted to an isolated circuit, the isolation coupling should be highly reliable in structure and operation. Distortions occurring during the isolation have been a drawback with prior art isolation couplings of current loops, and as a consequence it has been difficult to utilize the measuring signal in an appropriate manner.
BRIEF DESCRIPTION OF THE INVENTION
The object of the present invention is to provide an isolation coupling of a current loop by which the above drawbacks can be avoided and which enables information transfer of a current signal of the current loop into a circuit galvanically isolated from the current loop in a reliable and accurate manner by using a simple circuit solution. This is achieved with a coupling according to the invention, which is characterized in that the isolation coupling also comprises a resistance that is connected in series as a part of the current loop together with an operational amplifier and a transmitting LED of an optoisolator such that a second pole of the resistance is coupled to a positive voltage feed point of the operational amplifier and the anode of the transmitting LED is coupled to a negative voltage feed point of the operational amplifier, whereby the current loop is closed via the cathode of the transmitting LED, the coupling additionally comprising
a parallel coupling of a zener diode and a capacitor arranged between the positive and the negative voltage feed points of the operational amplifier such that the cathode of the zener diode is coupled to the operational amplifier's positive voltage feed point which is further coupled to the positive input of the operational amplifier,
a photodiode whose anode is coupled to the operational amplifier output and cathode to the cathode of the transmitting LED,
a resistance whose first pole is coupled to a first pole of the resistance and a second pole is coupled to a negative input of the operational amplifier, whereby the cathode of a first receiving PIN diode of the optoisolator is coupled to a negative input of the operational amplifier and the anode is coupled to the cathode of the transmitting LED, and
a circuit which is galvanically isolated from the current loop and which comprises a second receiving PIN diode of the optoisolator.
The invention is based on the idea that an operational amplifier coupling together with an optoisolator comprising two receivers are employed for the galvanic isolation. Thus the second receiving PIN diode of the optoisolator can be used for feedback in the isolation coupling. Due to the feedback, the current of the PIN diode of the galvanically isolated circuit follows closely the current of the current loop.
An advantage of the isolation coupling of the invention is the high accuracy and broad bandwidth achieved thereby in the isolation. In addition, the isolation coupling to be used is simple to implement and has a reliable structure.


REFERENCES:
patent: 4056719 (1977-11-01), Waaben
patent: 4070572 (1978-01-01), Summerhayes
patent: 4479066 (1984-10-01), Embree
patent: 5107202 (1992-04-01), Renda
patent: 5805062 (1998-09-01), Pearlman
patent: 6011359 (2000-01-01), Days

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