Hand-held ignition voltage tester

• US 6,396,278 B1
• Filed: 09/10/1998
• Issued: 05/28/2002
• Est. Priority Date: 10/07/1996
• Status: Expired due to Fees

• Current Assignee: SPX Corporation
• Original Assignee: SPX Corporation
• Inventors: Makhija, Surender K.
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): Hamdan, Wasseem H.
• Application Number:
  US09/516,598
• Time in Patent Office: 1,356 Days
• Field of Search: 324/402,393,379,72.5,126,395,530
• US Class Current: 324/402
• CPC Class Codes: F02P 2017/006 using a capacitive sensor
• CPC Class Codes: G01R 1/06788 Hand-held or hand-manipulated probes, e.g. for oscilloscopes or for portable test instruments end pieces terminating in a probe H01R11/18
• CPC Class Codes: G01R 15/16 using capacitive devices
• CPC Class Codes: H01T 13/60 of electrical properties

Abstract

A hand-held ignition voltage tester for detecting voltage on a spark plug wire in an ignition system, such as a distributorless ignition system. The tester includes a housing, positive and negative power cables and a capacitive probe for capacitively coupling to the spark plug wire. The capacitive probe generates a voltage signal which is representative of the voltage on the spark plug wire. A plurality of voltage level indicators are mounted to the housing to form a bar graph, with each indicator corresponding to a selected voltage level. A measurement circuit is coupled to the power cables and the capacitive probe for activating the voltage level indicators in response to the voltage signal.

Claims

1. A hand-held ignition voltage tester for detecting voltage on a spark plug wire in a distributorless ignition system, the tester comprising:

a housing;

an input terminal for receiving a voltage signal representative of the voltage on the spark plug wire;

a plurality of light emitting diodes (LEDs) mounted to the housing, each LED corresponding to a selected voltage level;

an input circuit comprising an input coupled to the input terminal to receive the voltage signal, a measurement output and an absolute value circuit coupled between the input and the measurement output; and

a measurement circuit coupled between the measurement output and the LEDs for selectively lighting the LEDs in response to the measurement output.

2. The hand-held ignition voltage tester of claim 1 wherein the plurality of LEDs are mounted to the housing in ascending order of the corresponding voltage levels to form a bar graph.

3. The hand-held ignition voltage tester of claim 2 and further comprising:

an open circuit indicator positioned at a high end of the bar graph and being operated by the measurement circuit to indicate an open circuit condition when the voltage signal exceeds a selected maximum voltage; and

a short circuit indicator positioned at a low end of the bar graph and being operated by the measurement circuit to indicate a shorted condition when the voltage signal is below a selected minimum voltage.

4. The hand-held ignition voltage tester of claim 1 wherein:

the measurement circuit comprises a plurality of comparators, with each comparator having a first input coupled to the measurement output, a second input coupled to a respective reference voltage terminal, and an output coupled to a respective one of the plurality of LEDs.

5. The hand-held ignition voltage tester of claim 4 wherein the input circuit further comprises:

a variable gain amplifier coupled between the input of the input circuit and the absolute value circuit.

6. The hand-held ignition voltage tester of claim 4 wherein the absolute value circuit has an output and the input circuit further comprises:

a measurement capacitor coupled to the output of the absolute value circuit; and

a discharge resistance coupled in parallel with the measurement capacitor.

7. The hand-held ignition voltage tester of claim 6 wherein the input circuit further comprises:

a peak store switch coupled in series with the discharge resistance for selectively disconnecting the discharge resistance from the measurement capacitance.

8. The hand-held ignition voltage tester of claim 4 wherein the absolute value circuit has an output and the input circuit further comprises:

a range amplifier having an input coupled to the output of the absolute value circuit and an output coupled to the first inputs of the plurality of comparators.

9. The hand-held ignition voltage tester of claim 8 wherein the range amplifier comprises:

an operational amplifier having an inverting input, a non-inverting input coupled to the output of the absolute value circuit, and an output coupled to the first inputs of the plurality of comparators;

a feedback resistor coupled between the inverting input and the output of the operational amplifier; and

a range switch coupled in parallel with the feedback resistor.

10. The hand-held ignition voltage tester of claim 4 wherein the plurality of LEDs comprises a first LED corresponding to a selected minimum voltage level and wherein the measurement circuit further comprises:

a diode having an anode coupled to the output of the comparator that is coupled to the first LED and having a cathode;

a reference voltage generator;

an amplifier having a non-inverting input coupled to the reference voltage generator, an inverting input coupled to the cathode and an output; and

a low voltage indicating LED coupled to the output of the amplifier.

11. The hand-held ignition voltage tester of claim 10 wherein the measurement circuit further comprises:

a time delay capacitor coupled to the cathode of the diode and having a capacitance;

a load resistor coupled in parallel with the time delay capacitor and having a resistance; and

wherein the amplifier turns the low voltage indicating LED off when the first LED is on and turns the low voltage indicating LED on after the first LED has been off for a selected time determined by the capacitance of the time delay capacitor and the resistance of the load resistor.

12. The hand-held ignition voltage tester of claim 1 and further comprising a capacitive probe coupled to the input terminal, wherein the capacitive probe comprises:

a conductive probe handle;

a clamping member for clamping to the spark plug wire;

a first insulator attached to the clamping member which insulates the clamping member from the spark plug wire; and

a shielded cable attached to the probe handle and comprising an inner conductor, a second insulator surrounding the inner conductor, a conductive shield surrounding the second insulator, and a third insulator surrounding the conductive shield, wherein the probe handle is grounded to the conductive shield and wherein the inner conductor extends along the clamping member between the clamping member and the first insulator and forms a capacitive pickup, with the conductive shield being stripped from the inner conductor along the clamping member only.

13. The hand-held ignition voltage tester of claim 12 and further comprising:

positive and negative power cables;

a ground terminal coupled to the negative power cable and to the conductive shield; and

a voltage regulator coupled to the positive and negative power cables and having positive and negative supply terminals coupled to the measurement circuit.

14. A hand-held ignition voltage tester for detecting voltage on a spark plug wire in a distributorless ignition system, the tester comprising:

a housing;

an input terminal for receiving a voltage signal representative of the voltage on the spark plug wire;

a plurality of voltage level indicators mounted to the housing to form a bar graph, each indicator corresponding to a selected voltage level;

an absolute value circuit having an input coupled to the input terminal for receiving the voltage signal and having a magnitude output; and

measurement means coupled to the magnitude output and the plurality of voltage level indicators for activating each voltage level indicator for which the magnitude output exceeds the corresponding selected voltage level.

15. The hand-held ignition voltage tester of claim 14 and further comprising:

a low voltage indicator; and

wherein one of the selected voltage levels is a minimum voltage level and wherein the measurement means activates the low voltage indicator when the magnitude output is less than the minimum voltage level.