Low impedance fault detection system and method

• US 4,074,188 A
• Filed: 08/01/1975
• Issued: 02/14/1978
• Est. Priority Date: 08/01/1975
• Status: Expired due to Term

• Current Assignee: Schlumberger Systems & Services Incorporated
• Original Assignee: Testline Instruments Incorporated
• Inventors: Hall, Melvin R., Boatman, Roger M.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/601,172
• Time in Patent Office: 928 Days
• Field of Search: 324/51, 324/52, 324/66, 324/67, 324/72.5, 324/127, 324/133, 324/158 P, 324/149, 324/73 PC, 324/73 R
• US Class Current: 324/537
• CPC Class Codes: G01R 31/281 Specific types of tests or tests for a specific type of fault, e.g. thermal mapping, shorts testing G01R31/2818 takes precedence

Abstract

Isolation of a wired-OR fault on a printed circuit board is accomplished by first powering up the circuit board, then injecting a short pulse on the common buss and detecting the current flow by use of a ferrite core. The ferrite core is sufficiently small that it can be moved about the circuit board to the terminals of the various components that are connected to the current buss in a wired-OR configuration. The change of inductance created by the magnetic field associated current pulse flowing through the terminal of a component sinking the current creates a ringing effect which is amplified and compared against a reference voltage. The output of the comparator circuit is used to create a fault alarm signal.

Claims

1. Electrical circuit test apparatus for isolating a wired-OR fault in an electrical circuit supported on a support base and having a plurality of housings mounted on said support base, each of said housings having a plurality of electrical terminals which serve as electrodes for active elements contained in said housings with electrical terminals on one housing being directly connected to electrical terminals on other housings and with some of said terminals being connected to a common buss in a wired-OR configuration;

said apparatus being characterized by having;

a power supply for producing a voltage corresponding to a particular logic state and for producing a current pulse at the normal operating voltage level;

means connected to said power supply for controlling all active circuits connected in a wired-OR configuration intended to carry said common buss to a particular logic state and for applying said current pulse to said common buss for a first short time interval;

means for producing a sync signal while supplying said current pulse to said common buss;

current sensing means including a probe sufficiently small to selectively respond to current flow in individual electrical terminals on said housings when placed adjacent an individual electrical terminal;

amplifier means connected to said current sensing means;

circuit means to receive the output signal from said amplifier means and to receive the sync signal to be rendered operative for a second time interval less than said first time interval for detecting a current sensed by said current sensing means above a predetermined level; and

means connected to the output of said last mentioned circuit means for producing a unique indication in the event said current level is above said predetermined value, said indication lasting much longer than said first time interval.

2. Apparatus as defined in claim 1 wherein the probe includes a cylindrical ferrite core with a surrounding coil having terminals at opposite ends with the coil terminals being connected to wires in a flexible shielded cable, said cable being connected to a housing including said amplifier means and being part of a tuned input circuit for said amplifier means, said input circuit having a resonant frequency which is affected by the change in inductance of said ferrite core due to the first derivative of the magnetic field surrounding the housing terminal of a circuit component that is sinking current from the common buss.

3. Electrical circuit test apparatus for isolating a wired-OR fault in an electrical circuit employing MOS logic supported on a support base and having a plurality of housings mounted on said support base, each of said housings having dual in-line rows of electrical terminals which serve as electrodes for active elements contained in said housings with electrical terminals on one housing being directly connected to electrical terminals on other housings and with some of said terminals being connected to a common buss in a wired-OR configuration;

said apparatus being characterized by having;

a power supply for producing a voltage corresponding to a particular logic state and for producing a current pulse at the normal operating voltage level;

means connected to said power supply for controlling all active circuits connected in a wired-OR configuration intended to carry said common buss to a particular logic state including one terminal which applies said current pulse to said common buss for a first short time interval less than about 10 μ

seconds;

means for producing a sync signal while supplying said current pulse to said buss;

current sensing means including a probe sufficiently small to selectively respond to current flow in individual electrical terminals on said housings when placed adjacent an individual electrical terminal;

amplifier means connected to said current sensing means;

means for generating a positive and a negative reference voltage;

circuit means including a comparator connected to receive both reference voltages and the output signal from said amplifier means and to be enabled in response to the sync signal to be rendered operative for a second time interval less than about 1 μ

second for providing a unique voltage level if the current sensed by said current sensing means exceeds either reference voltage during said second time interval;

indicator means including a bistable circuit connected to the output of said last mentioned circuit means and set to produce a fault indication in response to said unique voltage level; and

means for resetting said bistable circuit in response to the sync signal accompanying the beginning of the next short time interval.

4. Apparatus as defined in claim 3 wherein the probe includes a cylindrical ferrite core having terminals at opposite ends with the core terminals being connected to wires in a flexible shielded cable, said cable being connected to a housing including said amplifier means and being part of a tuned input circuit for said amplifier means, said input circuit having a resonant frequency which is affected by the change in inductance of said ferrite core due to the first derivative of the magnetic field surrounding the housing terminal of a circuit component that is sinking current from the common buss.

5. A system for isolating a low impedance fault which causes sinking of electric current through a current carrying element on a printed circuit board comprising:

a hand manipulatable probe carrying an inductor element at a location on the probe so as to permit the positioning of the inductor element adjacent the current carrying element on the printed circuit board, the inductor element producing an electrical signal as a function of the first derivative of the magnetic field associated with varying current flow in the current carrying element on the printed circuit board;

means for generating a time synchronization signal in approximate time coincidence with expected variations in current flow in the current carrying element on the printed circuit board;

means for manually selecting an amplitude reference value; and

circuit means connected to receive said electrical signal produced by said inductor element and to indicate the presence of excursions of said electrical signal only in excess of said selected amplitude reference value and in time coincidence with said synchronization signal whereby false indications in response to spurious signals are generally avoided.

6. The system of claim 5 wherein said circuit means includes means for generating a synchronization pulse having a predetermined duration and related in time of occurrence to an induced change in current flow in the current carrying element on the printed circuit board, and means for enabling said circuit means to indicate the presence of excursions of said electrical signal only during the duration of said synchronization pulse.

7. The system of claim 5 wherein said circuit means includes means for detecting excursions of said electrical signal in excess of a manually selectable reference value and amplifying means for amplifying said electrical signal and wherein said detecting means comprises comparator circuit means for comparing the amplified electrical signal from said amplifying means to positive and negative reference values so as to detect either positive or negative excusions of said electrical signal in excess of said reference values.

8. The system of claim 5 wherein said circuit means contains capacitance and wherein said inductor connected to said circuit means comprises a resonant circuit that produces a ringing of said electrical signal in response to said changing magnetic field.

9. The system of claim 8 wherein said circuit means includes amplifying means for amplifying said electrical signal, and means for detecting positive or negative excursions in excess of an amplitude selected by manipulation of a manually settable reference circuit.

10. A system for isolating a low impedance fault which causes sinking of electric current through a current carrying element on a printed circuit board comprising:

means for supplying a test signal causing time varying current to flow in said current carrying element;

an inductor element on a manually manipulatable probe, the inductor element including a coil wound on a cylindrical ferrite core having a straight longitudinal axis and positionable adjacent to the current carrying element on the printed circuit board to produce an electrical signal in response to changes in magnetic field associated with varying current flow in the current carrying element;

circuit means connected to said inductor element for amplifying said electrical signal and indicating the presence of positive or negative excursions of said electrical signal in excess of a selectable amplitude said circuit means comprising a signal amplifier connected to receive the electrical signal from the inductor element and an amplitude discriminator manually settable to a selected value and connected to said amplifier to actuate said indicating means only when the electrical signal produced by the inductor element exceeds said selected value, said circuit means further including means synchronized to said test signal for rejecting signals that are not in approximate time synchronization with the time variations of current flow caused by said test signal.

11. The system of claim 10 wherein said signal rejecting means comprises means for generating a synchronization pulse having a predetermined duration and related in time of occurrence to changes in current flow in the current carrying element caused by said test signal, and means for enabling said circuit means to indicate the presence of said excursions only during the duration of said synchronization pulse.

12. The system of claim 10 wherein said circuit means contains capacitance and wherein said inductor connected to said circuit means comprises a resonant circuit that produces a ringing of said electrical signal in response to said changing magnetic field.

13. The system of claim 12 wherein said amplitude discriminator includes means for detecting positive or negative excursions in excess of said amplitude value selected by manipulation of a manually settable reference circuit.

14. The system of claim 10 wherein said inductor element is connected to said circuit means by a cable having a pair of insulated wires in a flexible sheath and having a length of between about two and three feet, the cable having a distributed capacitance that forms a resonant circuit when connected to said inductor element.

15. The system of claim 11 wherein said test signal is supplied by a test signal supply source having a connector for contacting the current carrying element on the printed circuit board, and wherein said synchronization pulse has a duration of less than the duration of the test signal and occurs a predetermined time interval after the occurrence of the test signal.

16. Apparatus as defined in claim 10 wherein the ferrite core and coil have a length of less than about 1/2 inch as measured in the direction of said longitudinal axis.

17. A method of identifying in a circuit the location of a low impedance fault which causes sinking of current from a buss by:

providing a circuit containing a plurality of logic circuit components in separate housings each having a plurality of terminals connected to said circuit with at least two of said terminals being connected to said buss in a wire-OR configuration;

providing apparatus for tracing the current path along said buss including a manually manipulatable probe having an inductor element containing a coil of electrically conductive material having a straight longitudinal axis with a pair of terminals connected to the input of an amplifier circuit including a selectable amplitude value setting means and a synchronization signal providing means that together provide signal amplitude and time discrimination, said coil being adapted to be oriented with its axis perpendicular to the direction of current flow and positioned sufficiently close to the current path so that a current is induced in said coil in response to the first derivative of the magnetic field accompanying a test current pulse in said buss or housing terminal, and an indicator circuit connected to receive the output signal of said amplifier circuit and responsive to the change in inductance of said inductor element when a test current pulse passes through said buss or housing terminal;

applying to said buss successive, short pulses of test current at a voltage not appreciably higher than the voltage present on said buss during normal operation of a circuit including said buss; and

identifying the location of the low impedance fault by sequentially positioning the inductor element at different locations along the path of current flow through said buss and housing terminals thereby to produce an input signal for said amplifier circuit, and actuating the indicator circuit in response to test current pulses resulting in a signal from said amplifier circuit above said selectable amplitude value and in time coincidence with said synchronization signal to ascertain where sinking of the current occurs while monitoring said indicator circuit, the low impedance fault being at the location of the probe along said path of current flow where the indication on said indicator is lost.

18. The method of claim 17 wherein the inductor element includes a cylindrical ferrite core which is coaxial with said coil and which during the occurrence of test current pulse is positioned adjacent the path of the test current path with its major axis oriented to be perpendicular to the direction of current flow at the location under test.

19. The method of claim 18 wherein the inductor element is connected in a resonant circuit and the first derivative of the magnetic field accompanying the test current pulse is sufficiently large to cause a ringing effect which is amplified by said amplifier circuit and the output signal of the amplifier circuit is applied to the indicator.

20. The method of claim 19 further including the steps of rectifying and amplifying the ringing effect signal from said resonant circuit to produce an amplifier output signal;

comparing the amplifier output signal with a reference signal level; and

producing said fault indication in response to a condition when the amplifier output signal level exceeds the reference signal level.

21. The method of claim 20 further including the steps of:

producing a sync signal related to the occurrence of the current pulse and producing a gating signal in response to said sync signal; and

wherein the amplifier output signal level and the reference voltage signal level are compared only during the duration of said gating signal.

22. The method of claim 21 wherein the duration of the gating signal is less than about 1 μ

second and the gating signal is delayed about 6 μ

seconds after the current pulse is applied to said buss.

23. A method of isolating a low impedance fault which causes sinking of electric current through a current carrying element on a printed circuit board comprising the steps of:

positioning an elongated inductor element disposed at the tip of a hand manipulatable probe adjacent the current carrying element on the printed circuit board in perpendicular relation to the current flow in the current carrying element to produce an electrical signal as a function of the first derivative of the magnetic field due to a varying current in the current carrying element;

manually selecting an amplitude reference level;

producing a synchronization signal coinciding in time with expected variations in current in the current carrying element;

indicating the presence of excursions of said electrical signal produced by said inductor element in excess of said selected amplitude value and in time coincidence with said synchronization signal; and

physically moving the probe along the current carrying element while continuing to indicate the presence of said excursions above said amplitude to ascertain the location of the low impedance fault.