DIFFERENTIAL CAPACITANCE CIRCUITRY FOR DIFFERENTIAL PRESSURE MEASURING INSTRUMENTS

• US 3,688,190 A
• Filed: 09/25/1970
• Issued: 08/29/1972
• Est. Priority Date: 09/25/1970
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Michael J. Blum
• Inventors: Blum, Michael J.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/075,596
• Time in Patent Office: 704 Days
• Field of Search: 324/57,60,61
• US Class Current: 324/679
• CPC Class Codes: G01L 13/025 using diaphragms
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor

Abstract

Capacitance measuring circuitry is disclosed which includes a source of alternating current and a first capacitor having two plates connected on one plate to a first terminal of the alternating current source. A second capacitor is connected on one plate to the same terminal of the alternating current source. A first diode is connected at its anode to the second plate of the first capacitor and a second diode is connected at its cathode to the second plate of the first capacitor. A filter network is connected to the cathode of the first diode and the anode of the second diode with the anode of the second diode being connected to a second terminal of the alternating current source. A third diode is connected at its cathode to the second plate of the second capacitor and a fourth diode is connected at its anode to the second plate of the second capacitor. The anode of the third diode and the cathode of the fourth diode are connected to the filter network and the anode of the third diode is connected to the second terminal of the alternating current source. The circuitry enables precise comparison and measurement of small capacitances such as those encountered with differential pressure measurement instruments.

Claims

1. Capacitance measuring circuitry comprising:

a. a source of alternating current;

b. a first capacitor having two plates, connected on one plate to a first terminal of the alternating current source;

c. a second capacitor having two plates connected on one plate to the same terminal of the alternating current source;

d. a first diode connected at its anode to the second plate of the first capacitor;

e. a second diode connected at its cathode to the second plate of the first capacitor;

f. a filter network connected to the cathode of the first diode and the anode of the second diode, the anode of the second diode being connected to a second terminal of the alternating current source;

g. a third diode connected at its cathode to the second plate of the second capacitor; and

h. a fourth diode connected at its anode to the second plate of the second capacitor;

the anode of the third diode and the cathode of the fourth diode being connected to the filter network and the anode of the third diode being connected to the second terminal of the alternating current source.

2. The capacitance measuring circuitry of claim 1 wherein the filter network includes:

a. a first resistor connected at one terminal to the cathode of the first diode and at a second terminal to the anode of the second diode;

b. a second resistor connected at one terminal to the cathode of the fourth diode and at a second terminal to the anode of the third diode; and

c. a filter capacitor connected between the cathode of the first diode and the cathode of the fourth diode.

3. The capacitance measuring circuitry of claim 1 wherein the filter network includes:

a. a first resistor connected at one terminal to the cathode of the first diode and at a second terminal to the anode of the second diode;

b. a second resistor connected at one terminal to the cathode of the fourth diode and at a second terminal to the anode of the third diode;

c. a first filter capacitor connected between the cathode of the first diode and the anode of the second Diode; and

d. a second filter capacitor connected between the cathode of the fourth diode and the anode of the third diode;

4. The capacitance measuring circuitry of claim 1 wherein the source of alternating current is connected to the rest of the circuitry by a transformer having a single secondary.

5. The capacitance measuring circuitry of claim 1 wherein the source of alternating current is connected to the rest of the circuitry by a transformer having two secondaries, two terminals of which are common and constitute the second terminal of the alternating current source, the first plates of the first and second capacitors being each connected to a separate one of the two remaining terminals.

6. The capacitance measuring circuitry of claim 5 wherein the two secondaries have differing number of turns so that the gains on each side of the circuitry are different.