Through air radar level transmitter with galvanic isolation

1. A loop powered process control instrument comprising:

a control system measuring a process variable and developing a measurement signal representing the process variable;

a two-wire circuit, for connection to a remote power source using a two-wire process loop, for controlling current on the loop in accordance with the measurement signal; and

a power supply with galvanic isolation, connected to the two-wire circuit and the control system to isolate the two-wire circuit from the control system, for receiving power from the two-wire process loop and supplying power to the control system, wherein the power supply comprises a voltage regulator receiving loop power and developing a regulated output voltage, a current switching circuit receiving the regulated output voltage and driving a primary of a transformer, with a secondary of the transformer coupled to the control system, and a voltage shift control circuit including an isolator circuit driven by the control system for controlling voltage supplied by the voltage regulator.

2. The loop powered process instrument of claim 1 wherein the voltage shift control circuit shifts the voltage regulator voltage to a higher value at lower current levels and to a lower value at higher current levels.

3. The loop powered process instrument of claim 1 wherein the isolator circuit comprises an optical coupler controlled by the control system.

4. The loop powered process instrument of claim 1 wherein the voltage supplied by the voltage regulator is set by plural voltage reference devices and an optical coupler controlled by the control system selectively shifts the voltage.

5. The loop powered process instrument of claim 1 wherein the voltage supplied by the voltage regulator is set by plural voltage reference devices and a plurality of optical couplers controlled by the control system selectively shifts the voltage.

6. The loop powered process instrument of claim 1 wherein the current switching circuit drives the primary of the transformer to provide a one-to-one current ratio of transformer primary current to secondary current.

7. The loop powered process instrument of claim 6 wherein the current switching circuit comprises a switching oscillator of a DC-DC converter circuit and the oscillator drives the primary of the transformer.

8. The loop powered process instrument of claim 7 wherein switching frequency of the switching oscillator is about 150 kHz to pass lower frequency communication signals.

9. The loop powered process instrument of claim 1 wherein the control system controls loop current at the secondary of the transformer.

10. The loop powered process instrument of claim 1 further comprising a dual compartment housing and the two-wire, and the power supply are in an explosion proof compartment of the housing and the control system is in an intrinsically safe compartment of the housing.

11. A two-wire transmitter comprising:

a dual compartment housing defining a wiring compartment and a control compartment;

a control system in the control compartment including a signal processing circuit measuring a process variable and developing a measurement signal representing the process variable; and

a two-wire circuit and a power supply in the wiring compartment, the two-wire circuit for connection to a remote power source using a two-wire process loop, for controlling current on the loop in accordance with the measurement signal, the power supply being connected to the two-wire circuit and the control system to isolate the two-wire circuit from the control system, for receiving power from the two-wire process loop and supplying power to the control system, wherein the power supply comprises a voltage regulator receiving loop power and developing a regulated output voltage, a current switching circuit receiving the regulated output voltage and driving a primary of a transformer, with a secondary of the transformer coupled to the control system, and a voltage shift control circuit including an isolator circuit driven by the control system for controlling voltage supplied by the voltage regulator.

12. The two-wire transmitter of claim 11 wherein the voltage shift control circuit shifts the voltage regulator voltage to a. higher value at lower current levels and to a lower value at higher current levels.

13. The two-wire transmitter of claim 11 wherein the isolator circuit comprises an optical coupler controlled by the control system.

14. The two-wire transmitter of claim 11 wherein the wherein the voltage supplied by the voltage regulator is set by plural voltage reference devices and an optical coupler controlled by the control system selectively shifts the voltage.

15. The two-wire transmitter of claim 11 wherein the voltage supplied by the voltage regulator is set by plural voltage reference devices and a plurality of optical couplers controlled by the control system selectively shift the voltage.

16. The two-wire transmitter of claim 11 wherein the current switching circuit drives the primary of the transformer to provide a one-to-one current ratio of transformer primary current to secondary current.

17. The two-wire transmitter of claim 16 wherein the current switching circuit comprises a switching oscillator of a DC-DC converter circuit and the oscillator drives the primary of the transformer.

18. The two-wire transmitter of claim 17 wherein switching frequency of the switching oscillator is about 150 kHz to pass lower frequency communication signals.

19. The two-wire transmitter of claim 11 wherein the control system controls loop current at the secondary of the transformer.

20. The two-wire transmitter of claim 11 wherein the wiring compartment comprises an explosion proof compartment and the control compartment comprises an intrinsically safe compartment.