Systems and methods for an open circuit current limiter

• US 7,990,162 B2
• Filed: 08/14/2007
• Issued: 08/02/2011
• Est. Priority Date: 08/14/2007
• Status: Active Grant

• Current Assignee: Fluke Corporation (Fortive Corp.)
• Original Assignee: Fluke Corporation (Fortive Corp.)
• Inventors: Eng, Benjamin Jr.
• Primary Examiner(s): Dole; Timothy J
• Application Number:
  US11/838,870
• Publication Number:
  US 20090045825A1
• Time in Patent Office: 1,449 Days
• Field of Search: 324/713, 324/114
• US Class Current: 324/713
• CPC Class Codes: G01R 15/12 Circuits for multi-testers , i.e. multimeters, e.g. for measuring voltage, current, or impedance at will
• CPC Class Codes: G01R 27/08 Measuring resistance by measuring both voltage and current

Abstract

A resistance measuring circuit includes a current generating component, a current control component, and a voltage measurement component. The magnitude of a target resistance can be measured by connecting the target resistance between first and second measurement terminals of the resistance measuring circuit, applying a current generated by the current generating component to the target resistance, and determining the voltage across the target resistance. When no target resistance is connected between the first and second measurement terminals, the current control component controls the current generating component to reduce current consumption of the resistance measuring circuit.

Claims

1. A resistance measuring circuit, comprising:

a first measurement terminal and a second measurement terminal, wherein a target resistance to be measured is coupled to the first measurement terminal and the second measurement terminal;

a current generating component configured to generate a current along a current path passing through a node connected to the first measurement terminal, wherein the current generating component generates a first current responsive to the target resistance;

and

a voltage measuring component configured to measure a voltage drop across the first measurement terminal and the second measurement terminal,

wherein a first voltage drop is measured by the voltage measuring component when the target resistance is present, and when the voltage measuring component measures an increase in the voltage drop from the first voltage drop, an open circuit is established between the first measurement terminal and the second measurement terminal, and the current generating component generates a second current in response, wherein the second current has a lower magnitude than the first current.

2. The resistance measuring circuit of claim 1, further comprising:

a current measuring component configured to measure the magnitude of the first current.

3. The resistance measuring circuit of claim 2, wherein the current measuring component comprises a level shifter circuit configured to amplify and output a voltage drop across a resistor of known resistance located along the current path.

4. The resistance measuring circuit of claim 2, wherein the current generating component comprises a voltage source.

5. The resistance measuring circuit of claim 1 further comprising:

a current control component configured to control the current generating component based on a voltage level of the first measurement terminal, wherein the current control component controls the current generating component by actuating a switch connected along the current path in response to a voltage level of the first measurement terminal.

6. The resistance measuring circuit of claim 5, wherein the switch comprises a field effect transistor (FET).

7. The resistance measuring circuit of claim 5, wherein the switch comprises a transistor that is actuated from a fully-on state when the resistance measuring circuit is in a closed-circuit configuration to a partially-on state when the resistance measuring circuit is in an open-circuit configuration.

8. The resistance measuring circuit of claim 1, wherein the magnitude of the first current is between 5 and 10 mA and the magnitude of the second current is less than 3 mA.

9. The resistance measuring circuit of claim 1, further comprising a battery-powered handheld electrical multimeter.

10. The resistance measuring circuit of claim 1, wherein the second current has a non-zero magnitude.

11. A method of controlling output current in an electronic apparatus capable of measuring electrical resistance, comprising:

generating a first current along a current path to a first measurement terminal, wherein the generating is responsive to a target resistance connected between the first measurement terminal and a second measurement terminal; and

measuring a voltage drop between the first measurement terminal and the second measurement terminal;

reducing the first current to a second current upon measuring an increase in the voltage drop between the first measurement terminal and the second measurement terminal.

12. The method of claim 11, further comprising determining a magnitude of the target resistance based upon the first current and a measured voltage difference between the first measurement terminal and the second measurement terminal when the target resistance is connected.

13. The method of claim 11, wherein the second current has a non-zero magnitude.

14. The method of claim 11, wherein reducing the first current to a second current comprises actuating a switch connected along a current path of the generated current in response to the increase in the voltage drop.

15. The method of claim 11, wherein the first current is between 5 mA and 10 mA, and the second current is less than 3 mA.

16. A resistance measuring circuit, comprising:

a current generating component that generates a first current along a current path to a first measurement terminal, wherein the first current is generated responsive to a target resistance connected between the first measurement terminal and a second measurement terminal;

a voltage measuring component that measures a voltage drop between the first measurement terminal and the second measurement terminal; and

means for reducing the current generated by the current generating component from the first current to a second current upon measuring an increase in the voltage drop between the first measurement terminal and the second measurement terminal.