In-circuit current measurement

• US 5,386,188 A
• Filed: 01/15/1993
• Issued: 01/31/1995
• Est. Priority Date: 01/15/1993
• Status: Expired due to Term

• Current Assignee: Keithley Instruments, Inc.
• Original Assignee: Keithley Instruments, Inc.
• Inventors: Banaska, John G., Reindel, Kenneth A., Minneman, Michael, Creque, Andy J., Atwell, Michael, Bish, Gary K.
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): DO, DIEP N
• Application Number:
  US08/005,227
• Time in Patent Office: 746 Days
• Field of Search: 324/691, 324/704, 324/713, 324/715, 324/719, 324/126, 364/482, 364/483
• US Class Current: 324/126
• CPC Class Codes: G01R 1/203 Resistors used for electric measuring, e.g. decade resistors standards, resistors for comparators, series resistors, shunts resistors in general H01C; microwave or radiowave terminations H01P1/26; coupling devices H01R
• CPC Class Codes: G01R 19/0092 measuring current only all subgroups of G01R19/00 take precedence
• CPC Class Codes: G01R 27/16 Measuring impedance of element or network through which a current is passing from another source, e.g. cable, power line
• CPC Class Codes: G01R 31/2818 using test structures on, or modifications of, the card under test, made for the purpose of testing, e.g. additional components or connectors G01R31/2805 takes precedence; printed circuits having, e.g. symbols, test patterns or visualisation means H05K1/0266

Abstract

A current in a circuit is measured without breaking the circuit. A relatively low resistance element in the circuit such as a component lead is chosen. A current is forced through the element and the voltage drop measured. Another current is forced through the element and the voltage drop measured. The values of these currents and voltages are used to determine the original current in the circuit.

Claims

1. A method for measuring current I₀ in a circuit, comprising:

selecting an element in said circuit having a low resistance with respect to a remainder of the circuit, said element remaining connected in said circuit;

connecting a current source in parallel with said element, said current source having an output current and a current control input for controlling said output current;

connecting a voltage measuring device in parallel with both said element and said current source, said voltage measuring device having a voltage measurement output for providing a voltage measurement across said element and a voltage measurement control input for controlling said voltage measurement;

providing a control device connected to said current control input and said voltage measurement control input;

providing a calculation device having a calculation control input connected to said control device;

driving a first current I₁ through the element with said current source and measuring a first voltage V₁ across the element with said measuring device, all in response to said control device, said first current being at or near zero;

driving a second current I₂ through the element with said current source and measuring a second voltage V₂ across the element with said measuring device, all in response to said control device, said second current being greater than said first current;

calculating a value for the current I₀ with said calculation device in response to said control device according to ##EQU8## providing the calculated value to a user or to an automated test system in response to said control device.

2. A method according to claim 1, wherein all of said currents are direct current.

3. A method according to claim 1, wherein said element is a printed circuit trace or a portion thereof.

4. A method according to claim 1, wherein said element is a component lead or a portion thereof.

5. A method according to claim 1, wherein said element is a wire or a portion thereof.

6. A method according to claim 1, wherein said element has a resistance of less than 10 ohms.

7. A method according to claim 6, wherein V₁ is less than 0.2 volts.

8. A method for measuring current I₀ in a circuit, comprising:

selecting an element in said circuit having a low resistance with respect to a remainder of the circuit, said element remaining connected in said circuit;

connecting a current source in parallel with said element, said current source having an output current and a current control input for controlling said output current;

connecting a voltage measuring device in parallel with both said element and said current source, said voltage measuring device having a voltage measurement output for providing a voltage measurement across said element and a voltage measurement control input for controlling said voltage measurement;

providing a control device connected to said current control input and said voltage measurement control input;

providing a calculation device having a calculation control input connected to said control device;

measuring an initial voltage V₀ across the element in response to said measuring device;

driving a first current I₁ through the element with said current source and measuring a first voltage V₁ across the element with said measuring device, all in response to said control device;

driving a second current I₂ through the element with said current source and measuring a second voltage V₂ across the element with said measuring device, all in response to said control device, said second current being greater than said first current;

calculating a value for the current I₀ with said calculation device in response to said control device according to ##EQU9## providing the calculated value to a user or to an automated test system in response to said control device.

9. A method according to claim 8, wherein all of said currents are direct currents.

10. A method according to claim 8, wherein said element is a printed circuit trace or a portion thereof.

11. A method according to claim 8, wherein said element is a component lead or a portion thereof.

12. A method according to claim 8, wherein said element is a wire or a portion thereof.

13. A method according to claim 8, wherein said element has a resistance of less than 10 ohms.

14. A method according to claim 13, wherein V₀ is less than 0.2 volts.