Battery operated capacitance measurement circuit

• US 5,294,889 A
• Filed: 03/27/1992
• Issued: 03/15/1994
• Est. Priority Date: 03/27/1992
• Status: Expired due to Term

• Current Assignee: Tandy Corporation (RS Legacy Corp.)
• Original Assignee: Tandy Corporation (RS Legacy Corp.)
• Inventors: Heep, Jerry J., Curtis, Douglas R.
• Primary Examiner(s): Tokar, Michael J.
• Assistant Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US07/858,857
• Time in Patent Office: 718 Days
• Field of Search: 324/676, 324/677, 324/678, 340/870.37
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitance measuring circuit uses only a DC voltage source to perform the capacitance measurement. For this purpose, the circuit is arranged to charge an unknown capacitor which is being measured from a DC source and to discharge the capacitor through a constant current circuit such that the discharge voltage across the capacitor decreases linearly from its initial value to a predetermined, final value. The measurement circuit monitors the time taken by the capacitor to discharge in order to determine the capacitance value.

Claims

1. Apparatus for measuring a capacitance value of a capacitor, said apparatus comprising:

A) a voltage source;

B) means for charging said capacitor from said voltage source, whereby a voltage is impressed across said capacitor, said voltage having an initial value;

C) means for discharging said capacitor with a current having a magnitude proportional to said voltage, whereby said voltage across said capacitor decreases substantially linearly from said initial value to a predetermined final value; and

D) timing means connected to said capacitor for measuring a time interval taken by said capacitor to discharge to said final voltage value, said time interval indicating said capacitance value.

2. Apparatus for measuring a capacitance value according to claim 1 wherein said discharging means comprises a constant current source.

3. Apparatus for measuring a capacitance value according to claim 2 wherein said constant current source comprises a resistor and means for generating a constant voltage across said resistor.

4. Apparatus for measuring a capacitance value according to claim 3 wherein said constant voltage generating means comprises a reference capacitor, means connected to said charging means for charging said reference capacitor to said initial voltage and means for impressing a second voltage from said charged reference capacitor across said resistor.

5. Apparatus for measuring a capacitance value according to claim 2 further comprising means for connecting said constant current source to said capacitor and means cooperating with said connecting means for starting said timing means when said constant current source is connected to said capacitor.

6. Apparatus for measuring a capacitance value according to claim 5 further comprising means connected to said capacitor and responsive to the voltage on said capacitor for stopping said timing means when a voltage on said capacitor equals said predetermined final voltage.

7. A method for measuring a capacitance value of a capacitor, said method comprising the steps of:

A. charging said capacitor by impressing a voltage across said capacitor at an initial value;

B. discharging said capacitor with a current having a magnitude proportional to said initial voltage value, whereby said voltage across said capacitor decreases substantially linearly from said initial value to a predetermined final value; and

C. measuring a time interval taken by said capacitor to discharge to said final voltage value, said time interval indicating said capacitance value.

8. A method for measuring a capacitance value according to claim 7 wherein step B comprises the step of:

B1. discharging said capacitor with a current having a constant magnitude.

9. A method for measuring a capacitance value according to claim 8 wherein step B1 comprises the steps of:

B1A. generating a constant voltage; and

B1B. impressing said constant voltage across a resistor.

10. A method for measuring a capacitance value according to claim 9 wherein step B1A comprises the steps of:

B1A'"'"'. charging a reference capacitor to said initial voltage; and

B1A". impressing a voltage on said charged reference capacitor across said resistor.

11. A method for measuring a capacitance value according to claim 8 wherein step B further comprises the step of:

B2. connecting said constant current source to said capacitor.

12. A method for measuring a capacitance value according to claim 11 wherein step C further comprises the step of:

C1. starting a timer when said constant current source is connected to said capacitor.

13. A method for measuring a capacitance value according to claim 12 wherein step C further comprises the step of:

C2. stopping said timer when a voltage on said capacitor equals said predetermined final voltage.

14. A capacitance measurement instrument for measuring a capacitance value of a capacitor, said instrument comprising:

A) a voltage source for generating a constant initial value of direct voltage;

B) means connected to said voltage source and said capacitor for charging said capacitor to said initial voltage value;

C) a resistor connected to said capacitor;

D) means connected to said resistor for impressing a constant voltage having a magnitude proportional to said initial voltage across said resistor causing a constant current to discharge said capacitor; and

E) timing means connected to said capacitor for determining a time interval required to discharge said capacitor from said initial voltage value to a predetermined final voltage value, said time interval indicating said capacitance value.

15. The instrument according to claim 14 further comprising means for changing a value of said resistor in order to change said time interval.

16. A capacitance measurement instrument for measuring a capacitance value of a capacitor, said instrument comprising:

A) a voltage source for generating a constant initial value of direct voltage;

B) means connected to said capacitor and said voltage source for charging said capacitor to said initial voltage value;

C) a resistor connected to said capacitor;

D) means connected to said resistor for impressing a constant voltage having a magnitude proportional to said initial voltage across said resistor causing a constant current to discharge said capacitor during a time interval required to discharge said capacitor from said initial voltage value to a predetermined final voltage value, said impressing means comprising;

1) a reference capacitor;

2) means connected to said reference capacitor for charging said reference capacitor to said initial voltage value;

3) a voltage follower connected to said resistor to provide a discharge path of said capacitor; and

4) means operable at a beginning of said time interval to connect said reference capacitor to said voltage follower for maintaining a constant voltage across said resistor during said time interval; and

E) timing means connected to said capacitor for determining said time interval, said time interval indicating said capacitance value.

17. Apparatus for measuring a capacitance value of a capacitor as a function of the discharge time of said capacitor using only direct current comprising:

A) a direct current, constant voltage source for generating an initial voltage;

B) a resistor;

C) circuit means connected to said voltage source and said resistor and operable to first charge said capacitor to said initial voltage and subsequently to discharge said capacitor with a constant current discharge through said resistor; and

D) timing means connected to said capacitor for determining the interval required to discharge said capacitor from said initial voltage to a final, predetermined voltage.

18. Apparatus according to claim 17 further comprising a buffer amplifier and a reference capacitor and wherein said circuit means has a first switchable condition for charging both said capacitor and said reference capacitor to said initial voltage and a second switchable condition for connecting said reference capacitor as an input to said buffer amplifier and for connecting said capacitor to discharge through said resistor and through said buffer amplifier so that said buffer amplifier maintains a constant voltage drop across said resistor during said time interval thereby providing constant current discharge.

19. Apparatus according to claim 18 wherein said timing means comprises:

means connected to said circuit means for initiating a timing interval when said circuit means enters said second switchable condition; and

means responsive to a voltage across said capacitor for terminating said timing interval when said voltage equals said predetermined final value.

20. Apparatus according to claims 17, 18 or 19 further comprising means for selectively providing different values for said resistor for altering a length of said discharge interval.

21. A test circuit for measuring a capacitance value of a capacitor using only direct current, said test circuit comprising:

A) a direct current, constant voltage source;

B) a buffer amplifier having an input;

C) a resistor;

D) a reference capacitor;

E) a voltage comparator having an output;

F) a switching circuit operable;

a. in a first switched condition, to connect said voltage source through said buffer amplifier to a node common to said capacitor and to said reference capacitor for charging both said capacitor and said reference capacitor;

b. in a second switched condition,

i. to disconnect said voltage source from said buffer amplifier,

ii. to connect the voltage across said charged reference capacitor from said node to said buffer amplifier input, and

iii. to connect said node through said resistor to the output of said buffer amplifier thereby providing a constant current discharge path through said resistor for said capacitor; and

G) a circuit connecting said voltage comparator to said node for determining when said capacitor being measured is completely discharged.

22. Apparatus according to claim 21 further comprising timing means responsive to said switching circuit changing from said first switched condition to said second switched condition for initiating timing and responsive to said output of said comparator to terminate timing in order to obtain a time interval required to discharge said capacitor.

23. Apparatus according to claims 21 or 22 further comprising means for selecting different resistance values for said resistor.

24. Apparatus according to claim 21 further comprising indicator means for indicating capacitance values in accordance with a length of said time interval measured for discharge of said capacitor.

25. A capacitance measurement instrument for measuring a capacitance value of a capacitor, said instrument comprising:

A) a voltage source for generating a constant initial value of direct voltage;

B) means connected to said capacitor for charging said capacitor to said initial voltage value;

C) a resistor connected to said capacitor;

D) means connected to said capacitor for impressing a constant voltage having a magnitude proportional to said initial voltage across said resistor causing a constant current to discharge said capacitor, said impressing means comprising

E) a reference capacitor;

F) means connected to said reference capacitor for charging said reference capacitor to said initial voltage value;

G) a voltage follower connected to said resistor to provide a discharge path of said capacitor; and

H) timing means connected to said capacitor for determining a time interval required to discharge said capacitor from said initial voltage value to a predetermined final voltage value, said time interval indicating said capacitance value;

I) means operable at a beginning of said time interval to connect said reference capacitor to said voltage follower for maintaining a constant voltage across said resistor during said time interval; and

J) means for changing a value of said resistor in order to change said time interval.