U.S. patent number RE31,606 [Application Number 06/280,430] was granted by the patent office on 1984-06-19 for digital ohmmeter with electrical continuity tester.
This patent grant is currently assigned to Beckman Instruments, Inc.. Invention is credited to John B. Crosby.
United States Patent |
RE31,606 |
Crosby |
June 19, 1984 |
Digital ohmmeter with electrical continuity tester
Abstract
In a digital test instrument such as a digital ohmmeter for
measuring and digitally displaying the resistance of an unknown
circuit element, there is included an electrical continuity tester
coupled to the input of the digital ohmmeter for instantaneously
and digitally indicating electrical continuity. Connection of the
electrical continuity tester to the input of the digital ohmmeter
is achieved in a manner such as to not overload or otherwise affect
the accuracy of the resistance measurement.
Inventors: |
Crosby; John B. (Yorba Linda,
CA) |
Assignee: |
Beckman Instruments, Inc.
(Fullerton, CA)
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Family
ID: |
26960300 |
Appl.
No.: |
06/280,430 |
Filed: |
July 6, 1981 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
961154 |
Nov 16, 1978 |
04228394 |
Oct 14, 1980 |
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Current U.S.
Class: |
324/712; 324/133;
324/99D; 324/549; 324/705 |
Current CPC
Class: |
G01R
31/54 (20200101); G01R 13/02 (20130101); G01R
31/52 (20200101); G01R 27/14 (20130101) |
Current International
Class: |
G01R
31/02 (20060101); G01R 13/00 (20060101); G01R
27/14 (20060101); G01R 13/02 (20060101); G01R
027/02 () |
Field of
Search: |
;324/62,51,99D
;340/347AD |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Khan, Analog Linear-Scale Ohmmeter, IEEE Transactions on
Instrumentation & Measurement, pp. 150-155, Jun. 1975..
|
Primary Examiner: Krawczewicz; Stanley T.
Attorney, Agent or Firm: Steinmeyer; R. J. Mehlhoff; F. L.
Canzoneri; Al A.
Claims
I claim:
1. In a digital test instrument for measuring resistance including
a pair of input terminals adapted to be connected across a circuit
element of unknown resistance, a first reference current or voltage
source operatively couple to said input terminals so that an analog
voltage appears across said input terminals which is proportional
to the resistance of said circuit element, a filter connected to
one of said input terminals for filtering noise in said analog
voltage, an analog-to-digital converter having an input connected
to said filter and an output, a digital display, and driver circuit
means connected between said output of said analog-to-digital
converter and said digital display, the improvement comprising
means for detecting and immediately signalling electrical
continuity across said input terminals comprising:
a single bit analog-to-digital converter having first and second
input terminals, said first input terminal being connected directly
to said one of said input terminals, said converter having a
sufficiently high input impedance not to affect the operation of
said resistance measuring circuitry of said digital test
instrument;
a second source of reference voltage operatively connected to said
second input terminal of said single bit analog-to-digital
converter, said single bit analog-to-digital converter generating a
logical true or logical false signal depending upon whether said
analog voltage is higher or lower than said second reference
voltage;
a digital continuity display; and
driver circuit means connected between said single bit
analog-to-digital converter and said digital continuity display for
instantaneously indicating continuity.
2. In a digital test instrument according to claim 1, the
improvement wherein said single bit analog-to-digital converter
comprises:
a comparator.
3. In a digital test instrument according to claim 2, the
improvement wherein said comparator has an inverting input
connected directly to said one of said input terminals and a
noninverting input connected to said second source of reference
voltage, said comparator generating a logical false signal when
said analog voltage is greater than said second reference
voltage.
4. In a digital test instrument according to claim 3, the
improvement wherein said comparator generates a logical true signal
when said analog voltage is less than said second reference
voltage.
5. In a digital test instrument according to claim 2, 3, or 4, the
improvement wherein said comparator has a MOSFET input stage having
an input impedance in excess of approximately 10.sup.12 ohms.
6. In a digital test instrument according to claim 1, 2, 3, or 4,
the improvement wherein said digital continuity display is part of
said digital display.
7. In a digital test instrument according to claim 1, 2, 3, or 4,
the improvement wherein said driver circuit means connected between
said single bit analog-to-digital converter and said digital
continuity display is the same as said driver circuit means
connected between said analog-to-digital converter and said digital
display. .Iadd. 8. A digital test instrument for measuring
resistance comprising:
a pair of input terminals adapted for connection across a circuit
element of unknown resistance;
means for producing a voltage across the input terminals the value
of which indicates the value of the unknown resistance;
a digital display and means for operating the display to indicate
the value of the unknown resistance, the digital test instrument
exhibiting a measuring time delay before the display indicates the
value of the unknown resistance; and
means for substantially instantaneously detecting and signaling the
presence of electrical continuity across the input terminals
including:
a continuity indicator;
means for energizing the continuity indicator; and
means for selecting whether the continuity indicator is energized
according to whether the voltage across the input terminals is
greater or less than a reference voltage. .Iaddend..Iadd. 9. The
digital test instrument according to claim 8, wherein said
continuity indicator comprises means for displaying a continuity
symbol. .Iaddend..Iadd. 10. The digital test instrument according
to claim 9, wherein said means for displaying a continuity symbol
is part of said digital display. .Iaddend. .Iadd. 11. The digital
test instrument according to claim 9, wherein the means for
selecting whether the continuity indicator is energized comprises a
one-bit analog-to-digital converter. .Iaddend..Iadd. 12. The
digital test instrument according to claim 11, wherein the one-bit
converter has a first input connected directly to one of the input
terminals of the instrument and a second input connected to a
source of the reference voltage, the one-bit converter generating a
logical signal of a first logic state when the voltage across the
input terminals is greater than the reference voltage and of a
second logic state when the voltage across the input terminals is
less than the reference voltage, the logical signal being supplied
to the continuity indicator energizing means. .Iaddend..Iadd. 13. A
digital test instrument according to claim 12, wherein said first
input is an inverting input and said second signal is a
non-inverting input and wherein the one-bit converter generates a
logical false signal when the voltage across the input terminals is
greater than the reference voltage and a logical true signal when
the voltage across the input terminals is less than the reference
voltage. .Iaddend..Iadd. 14. The digital test instrument according
to any of claims 11, 12 or 13, wherein the one-bit converter has an
input impedance in excess of approximately 10.sup.12 ohms.
.Iaddend. .Iadd. 15. A digital test instrument for measuring
resistance including:
a pair of instrument input terminals adapted to be connected across
a circuit element of unknown resistance;
means for producing a voltage across said instrument input
terminals the value of which indicates the value of the unknown
resistance;
an analog-to-digital converter having an input connected to one of
the instrument input terminals and having an output;
a digital display;
driver circuit means connected between the output of the
analog-to-digital converter and the digital display for operating
the display to indicate the value of the unknown resistance;
and
means for detecting and immediately signaling electrical continuity
across the instrument input terminals comprising:
a reference voltage source;
a one-bit analog-to-digital converter having first and second
inputs and an output, the first input of the one-bit converter
being connected to one of the instrument input terminals and the
second input of the one-bit converter being connected to the
reference voltage source, the one-bit analog-to-digital converter
generating at its output a logical signal having either a logical
true or a logical false value depending upon whether said analog
voltage appearing across the instrument input terminals is higher
or lower than the reference voltage;
continuity symbol display means for receiving an energizing signal
and for displaying a continuity symbol in response thereto; and
driver circuit means connected between the output of the one-bit
analog-to-digital converter and the continuity symbol display means
for instantaneously indicating continuity by selectively supplying
the energizing signal to the continuity symbol display depending on
whether the logical value of the logical signal generated by the
one-bit
analog-to-digital converter is true or false. .Iaddend..Iadd. 16.
The digital test instrument according to claim 15, wherein the
continuity symbol display means is part of said digital display.
.Iaddend..Iadd. 17. The digital test instrument of claim 15,
wherein the one-bit converter has an input impedance in excess of
approximately 10.sup.12 ohms. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical test equipment and,
more particularly, to a digital ohmmeter including apparatus for
independently and digitally indicating electrical continuity.
2. Description of the Prior Art
Recent years have evidenced a rapid transformation of test
instruments from ones having analog displays to ones having digital
displays. Such instruments include voltmeters, ammeters, ohmmeters,
and multimeters including all of the above functions. Test
instruments having analog displays utilize meters, commonly a
D'Arsonval movement, for providing a quantitative indication of the
value of the unknown input. Digital instruments perform the same
function, but display the output in digital form, commonly using a
liquid crystal display.
A conventional digital test instrument such as an ohmmeter includes
a pair of input terminals adapted to be connected across a circuit
element of unknown resistance, a reference current of voltage
source operatively coupled to the input terminals so that an analog
voltage appears across the input terminals which is proportional to
the resistance of the circuit element, a filter connected to one of
the input terminals for filtering noise in the analog voltage, an
analog-to-digital converter having an input connected to the filter
and an output, a digital display, and driver circuit means
connected between the output of the analog-to-digital converter and
the input of the digital display.
While digital test instruments of the type previously described are
generally regarded as being quantitatively precise and have,
therefore, acquired a significant share of the electrical test
equipment market, their analog counterparts show significant
advantage in applications where trend, rather than absolute,
information is desired. An example of this advantage is in
continuity measurement. In such circumstances, an analog ohmmeter
can be used to quickly determine if electrical continuity exists in
a circuit. Often, the user is less interested in the absolute value
of conduction than he is in measurement speed.
For instance, a telephone service technician might utilize an
analog ohmmeter to quickly check for a shorted wire within a cable
by simply brushing the meter probe along the cable's connection
points. If a short were present, it would be indicated by a
momentary deflection of the meter. If the test points are closely
spaced, the practiced user could check a multiple conductor cable
in less than a second.
Digital ohmmeters, on the other hand, are poorly suited to perform
such tests. They exhibit an inherent delay of from 0.5 to 1.5
seconds, resulting from the precise analog-to-digital conversion
process, which makes rapid continuity indication all but
impossible. Some digital instrument manufacturers, realizing this
difficulty, have incorporated an ancillary electromechanical meter
in their product to achieve the capability for a rapid continuity
indication. Unfortunately, such an addition exacts a high cost for
the indicator and its attendant drive circuitry, as well as
requiring additional panel space for the meter. Also, since an
electromechanical meter is often the only delicate component
incorporated in the instrument, its inclusion tends to compromise
instrument ruggedness and reliability.
Other instrument manufacturers have provided a separate electrical
continuity tester capable of producing a digital output.
Unfortunately, the user now must purchase and use two separate
instruments and this is not only inconvenient but costly.
OBJECTS, FEATURES, AND ADVANTAGES OF THE INVENTION
It is therefore an object of the present invention to solve the
problem of providing a rapid continuity indication in a digital
ohmmeter. It is a feature of the present invention to solve this
problem by incorporating separate continuity test circuitry in a
digital ohmmeter. It is a further feature of the present invention
to provide such circuitry with a digital output. The advantages are
that such additions can be done at an extremely nominal cost
without requiring additional panel space for an extra meter.
Another advantage is that the instrument maintains its ruggedness
and reliability.
It is another object of the present invention to solve the problems
associated with providing separate digital instruments for
measuring resistance and continuity. It is a feature of the present
invention to solve this problem by connecting to the input of a
digital ohmmeter a digital continuity tester having a sufficiently
high input impedance that it does not load or otherwise affect the
accuracy of the readout of the digital ohmmeter. The advantage
associated with this feature is that a single digital test
instrument can be used to measure both electrical resistance and
electrical continuity.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an improved
digital test instrument. More particularly, the present invention
includes a digital ohmmeter in which the resistance measuring
function is augmented by means for sensing and immediately
indicating the presence of a continuous circuit at the input
terminals thereof. This feature is useful when it is desired to
rapidly check a number of circuits for continuity without regard to
the actual value of resistance.
The digital ohmmeter portion of the present invention is of
generally conventional structure and includes a pair of input
terminals adapted to be connected across a circuit element of
unknown resistance, a reference current or voltage source
operatively coupled to the input terminals so that an analog
voltage appears across the input terminals which is proportional to
the resistance of the circuit element, a filter connected to one of
the input terminals for filtering noise in the analog voltage, an
analog-to-digital converter having an input connected to the filter
and an output, a digital display, and driver circuit means
connected between the output of the analog-to-digital converter and
the input of the digital display.
The electrical continuity tester portion of the present invention
includes a high input impedance, single bit, analog-to-digital
converter connected to the one of the input terminals and a
reference voltage source for providing a logical true or false
signal depending upon whether the analog voltage is higher or lower
than the reference voltage. The output of the converter is coupled
to the driver circuit means and the digital display is provided
with a unique symbol, preferably a Greek omega symbol, incorporated
as part of the liquid crystal digital display for indicating
continuity. In use as a continuity tester, the omega symbol will be
energized within the response time of the analog-to-digital
converter and the display, normally less than 100 ms.
Other objects, features, and attendant advantages of the present
invention will become apparent to those skilled in the art from a
reading of the following detailed description of the preferred
embodiment constructed in accordance therewith, taken in
conjunction with the accompanying drawings wherein like numerals
designate like parts in several figures and wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram of a digital test instrument
constructed in accordance with the teachings of the present
invention;
FIG. 2 shows the display output of the instrument of FIG. 1 during
a steady state open circuit condition;
FIG. 3 shows the display output of the instrument of FIG. 1 during
the initial period of a closed circuit condition; and
FIG. 4 shows the display output of the instrument of FIG. 1 during
a steady state closed circuit condition.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and, more particularly, to FIG. 1
thereof, there is shown a simplified block diagram of a digital
test instrument, generally designated 10, constructed in accordance
with the teachings of the present invention. Specifically,
instrument 10 includes a digital ohmmeter and a digital electrical
continuity tester. The ohmmeter portion of instrument 10 includes a
plurality of weighted current sources 11, three being shown by way
of example, each connected in series with a switch 12, three
switches being shown. The multiple current sources 11 and the
multiple switches 12 comprise the range selection circuitry of a
multi-range ohmmeter. Thus, all of sources 11 and switches 12 are
connected in parallel between a voltage source +V.sub.1 and a first
input terminal 13 of instrument 10. Instrument 10 includes a second
input terminal 14 which may be connected to ground 15.
Depending upon which of switches 12 is closed, a selected current
flows through an unknown resistor 16 when the opposite ends thereof
are connected to input terminals 13 and 14. This produces a
positive voltage at terminal 13 which is in direct proportion to
the absolute value of unknown resistance 16. Other methods for
obtaining a voltage proportional to an unknown resistance are, of
course, possible and are well known to those skilled in the
art.
Instrument 10 includes a conventional analog-to-digital converter
20 for converting an analog voltage appearing at its input to a
digital representation thereof at its output. The analog voltage at
terminal 13 is connected to the input of analog-to-digital
converter 20 via an input filter 21 which typically consists of a
resistor 22 connected between input terminal 13 and the input of
converter 20 and a capacitor 23 connected between the input of
converter 20 and ground 15. Filter 21 is typical of the type of
filters used in contemporary digital meters and is needed to assure
the stability of the displayed reading. However, it is the same
filter which contributes to the overall response delay of the
digital ohmmeter portion of instrument 10, as will be explained
more fully hereinafter.
Instrument 10 includes a digital readout 25 which is preferably a
twisted nematic liquid crystal display of the type well known to
those skilled in the art, including three multiple segment display
elements 26, each display element 26 including seven bars capable
of displaying the numbers 1 through 9. Readout 25 is driven by
conventional decoder/driver circuitry 27 connected between the
output of converter 20 and the input of readout 25. Decoder/driver
27 analyzes the output of converter 20 and converts it to a form
suitable for driving the multiple segments of display elements 26.
All of the above portions of digital test instrument 10 are known
to those skilled in the art.
Instrument 10 further includes circuitry, generally designated 30,
to detect and immediately signal electrical continuity across
terminals 13 and 14. Specifically, circuitry 30 includes a single
bit analog-to-digital converter 31 which senses the voltage across
unknown resistor 16 directly, without a filter or other delay
producing means. Specifically, converter 31 is preferably a
comparator having an extremely high input impedance stage, such as
a MOSFET input stage, which typically has an input impedance in
excess of 10.sup.12 ohms. Comparator 31 has an inverting input 32
which is connected directly to input terminal 13. A reference
voltage +V.sub.2 is applied to the non-inverting input 33 of
comparator 31.
It will be seen that for all unknown voltages more positive than
reference voltage +V.sub.2, the output of comparator 31 on line 37
will be low, a logical "false". Also, for unknown voltages less
than positive than reference voltage +V.sub.2, the output of
comparator 31 on line 37 will be high, a logical "true".
The output of comparator 31 on line 37 is connected to
decoder/driver circuitry 27 which drives a special symbol 38
included in readout 25. According to the preferred embodiment of
the present invention, symbol 38 comprises a Greek .OMEGA. symbol
which is incorporated as part of the twisted nematic liquid crystal
structure in display 25. Decoder/driver 27 utilizes the output of
comparator 31 to energize or de-energize .OMEGA. symbol 38.
In operation, for an open circuit condition at input terminals 13
and 14 of instrument 10, the voltage appearing at input filter 21
and comparator 31 will be limited only by the compliance of the
selected constant current generator and will, in any event, be
greater than reference voltage +V.sub.2, causing a logical false
level to exist at output 37 of comparator 31. As a result, .OMEGA.
symbol 38 is not enabled in display 25, the only reading displayed
at this time being an over-range display supplied by converter 20.
This is shown in FIG. 2 where an over-range signal is indicated by
the maximum reading "999" on elements 26. This is further verified
by the absence of continuity indicator symbol 38. An alternative to
displaying the reading "999" would be to display the reading "OL"
as a shorthand for over-load.
With reference voltage +V.sub.2 being set to some value related to
that required for a full scale indication from the ohmmeter section
of instrument 10, a rapidly falling unknown input drop, such as
that resulting from input terminals 13 and 14 being shorted
together during a measurement, will cause the output of comparator
31 to immediately switch to a logical true state when the unknown
voltage instantaneously reaches the approximate comparison level at
input 33. Within the response time of comparator 31 and display 25,
normally less than 100 ms, and long before the output of converter
20 would respond to the unknown input drop, .OMEGA. symbol 38 will
be energized in display 25, giving a rapid indication of the
continuity condition. These circumstances are shown in FIG. 3. At
this point in time, the measuring system of instrument 10 has not
yet had sufficient time to measure and display a resistance change,
but .OMEGA. symbol 38 is energized. Depending on the specific
design of instrument 10, the behaviour of display elements 26 prior
to reaching a steady reading may differ from what is shown in FIG.
3.
Subsequently, as input filter 21 and analog-to-digital converter 20
settle, the numerical portion of display 25 would come out of
overload and read out the quantitative value of unknown resistor
16. This condition is shown in FIG. 4 wherein sufficient time,
approximately 0.5 to 1.5 seconds, has passed to permit a resistance
measurement to be completed and its value displayed by display
elements 26. Continuity indicator 38 continues to be illuminated as
long as a closed circuit condition exists at input terminals 13 and
14.
It can therefore be seen that the present invention does indeed
solve the problem of providing a rapid continuity indication in a
digital ohmmeter. This problem is solved by incorporating separate
continuity test circuitry 30 in digital ohmmeter 10. Continuity
test circuitry 30 has a digital display 38 so that a separate
digital display is not required therefor. Circuitry 30 is added to
instrument 10 at an extremely nominal cost, without requiring
additional panel space for an extra meter. Furthermore, instrument
10 maintains its ruggedness and reliability.
It is also seen that the present invention solves the problems
associated with providing separate digital instruments for
measuring resistance and continuity. Digital continuity test
circuitry 30 has a sufficiently high input impedance that it does
not load or otherwise affect the accuracy of the readout of the
digital measurement portion of instrument 10. As a result,
instrument 10 can be used to measure both electrical resistance and
electrical continuity.
While the invention has been described with respect to the
preferred physical embodiment constructed in accordance therewith,
it will be apparent to those skilled in the art that various
modifications and improvements may be made without departing from
the scope and spirit of the invention. Accordingly, it is to be
understood that the invention is not to be limited by the specific
illustrative embodiment, but only by the scope of the appended
claims.
* * * * *