Abstract

An interface circuit in a current telemetering system includes first and second current loops connected in parallel between a pair of energizing terminals. The first loop includes a voltage controlling transistor, resistors, a current controlling transistor, and a current receiver connected in series between the energizing terminals. The second loop includes the voltage controlling transistor, a resistor, a Zener diode intrinsic safety barrier, a two-wire link, and a current transmitter connected in series between the energizing terminals. A first amplifier senses the currents in the two loops and controls the current controlling transistor to keep the current in the first loop and the receiver proportional to the current in the second loop as established by the transmitter. A second amplifier monitors the voltage at a point in the first loop and controls the voltage controlling transistor so as to keep the voltage at that point constant at a predetermined value. This, in turn, maintains a corresponding constant voltage at a corresponding point in the second loop, such as a point within the barrier. The specific position of that point in the second loop is set, to prevent the undue limiting of the voltage at the transmitter, by giving a corresponding resistance value to the one of the resistors in the first loop which is adjacent to the noted point in the first loop.