Wide-range resistance monitoring apparatus and method

• US 4,736,157 A
• Filed: 07/18/1986
• Issued: 04/05/1988
• Est. Priority Date: 07/18/1986
• Status: Expired due to Fees

• Current Assignee: Pinion
• Original Assignee: Voyager Technologies, Inc. (Amphenol Corporation)
• Inventors: Betker, Michael R., Dunphy, Robert H.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Harvey, Jack B.
• Application Number:
  US06/886,885
• Time in Patent Office: 627 Days
• Field of Search: 324/133, 324/62, 324/64, 324/65 R, 324/433, 324/509, 324/510, 324/511, 324/525, 324/555, 324/556, 361/90, 340/649, 340/650, 340/660-664, 340/636, 128/734
• US Class Current: 324/557
• CPC Class Codes: G01R 27/18 Measuring resistance to earth , i.e. line to ground

Abstract

An inexpensive circuit monitors the resistance between a body (e.g., a static dissipative work surface, a tool, etc.) and ground potential, and compares that resistance with high and low resistance values defining a preset resistance range. An operational amplifier is connected to the body and causes current to flow from the body and through the op amp feedback network. A switching arrangement coupled to the operational amplifier alternately switches the amplifier feedback network between low and high resistance values defining lower and upper ends, respectively, of the preset resistance range. A logic circuit connected to receive the op amp output (a) lights a green indicator lamp if the resistance under test fails within the preset resistance range, (b) lights red and green lamps alternately and pulses a buzzer if a measured resistance falls below the lower limit of the resistance range, and (c) lights yellow and green lamps alternately and pulses the buzzer if the measured resistance exceeds the upper level of the resistance range. The resistance range can be relatively large, spanning five orders of magnitude of resistance or more. The circuit is particularly useful for preventing static discharges from destroying CMOS and MOSFET electronic devices.

Claims

1. Apparatus for monitoring the resistance between first and second bodies, comprising:

comparing means, electrically connected to said first and second bodies, for inducing current to flow between said first and second bodies and for producing an output signal whenever the magnitude of said current flow exceeds a reference level;

swithching means, operatively connected to said comparing means and connected to receive a periodic control signal, for alternating said reference level between first and second values in response to said control signal; and

indicator means, connected to receive said output and control signals, for providing a first indication when said current flow is less than said first value and for providing a second indication when said current flow exceeds said second value, wherein;

said comparing means comprises;

operational amplifier means having first and second input terminals and an output terminal for producing an output signal at its output terminal whenever the signal level present on its first input terminal exceeds the signal level present on its second input terminal,

a resistive element electrically connected between said first body and said operational amplifier first input terminal, and

reference voltage level producing means, having a first terminal connected to said second body and a second terminal connected to said operational amplifier second input terminal, for producing a fixed reference voltage across said first and second terminals; and

said switching means comprises;

a first resistor connected between said operational amplifier first input and output terminals,

a second resistor having a first end connected to one of said operational amplifier output terminal and said operational amplifier first input terminal, said resistor having a further end, and

a switching element having a control terminal and first and second switching terminals, said control terminal being connected to receive said control signal, said first switching terminal being connected to said second resistor further end, said second switching terminal being connected to the other one of said operational amplifier output terminal and said operational amplifier first input terminal.

2. Apparatus as in claim 1 wherein:

said switching means sets said reference level to said first value when said control signal is present and sets said reference level to said second value when said control signal is not present, said first reference value being less than said second reference value; and

said indicator means comprises;

a first indicator lamp,

a second indicator lamp,

means, operatively connected to said first indicator lamp and connected to receive said control signal and said output signal, for activating said first indicator lamp whenever said output signal is not present and said control signal is present, and

means, operatively connected to said second indicator lamp and connected to receive said control signal and said output signal, for activating said second indicator lamp whenever said output signal is present and said control signal is not present.

3. Apparatus as in claim 2 wherein said indicator means further includes:

a third indicator lamp; and

means, operatively connected to said third indicator lamp and connected to receive said control signal and said output signal, for activating said third indicator lamp whenever (a) said output signal and said control signal are both present, or (b) said output signal and control signal are both not present.

4. Apparatus as in claim 1 wherein said indicator means includes:

a first transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base connected to receive said output signal, said collector being connected to receive said control signal;

a second transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said first transistor collector; and

a first light emitting diode being connected between a supply voltage and said second transistor collector.

5. Apparatus for monitoring the resistance between first and second bodies, comprising:

comparing means, electrically connected to said first and second bodies, for inducing current to flow between said first and second bodies and for producing an output signal whenever the magnitude of said current flow exceeds a reference level;

switching means, operatively connected to said comparing means and connected to receive a periodic control signal, for alternating said reference level between first and second values in response to said control signal; and

indicator means, connected to receive said output and control signals, for providing a first indication when said current flow is less than said first value and for providing a second indication when said current flow exceeds said second value, said indicator means including;

a first transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base connected to receive said output signal, said collector being connected to receive said control signal;

a second transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said first transistor collector; and

a first light emitting diode being connected between a supply voltage and said second transistor collector;

a third transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to receive said control signal, said collector being connected to receive said output signal;

a fourth transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said third transistor collector; and

a third light emitting diode being connected between a supply voltage and said fourth transistor collector.

6. Apparatus for monitoring the resistance between first and second bodies, comprising:

comparing means, electrically connected to said first and second bodies, for inducing current to flow between said first and second bodies and for producing an output signal whenever the magnitude of said current flow exceeds a reference level;

switching means, operatively connected to said comparing means and connected to receive a periodic control signal, for alternating said reference level between first and second values in response to said control signal; and

indicator means, connected to receive said output and control signals, for providing a first indication when said current flow is less than said first value and for providing a second indication when said current flow exceeds said second value, wherein said indicator means includes;

a first transistor having a base, and emitter and a collector, said emitter being connected to ground potential, said base connected to receive said output signal, said collector being connected to receive said control signal;

a second transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said first transistor collector; and

a first light emitting diode being connected between a supply voltage and said second transistor collector;

a further transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to the collectors of said first and third transistors, said collector being connected to said supply voltage; and

a further light emitting diode being connected between said further transistor collector and emitter.

7. Apparatus for monitoring the resistance between first and second bodies, comprising:

comparing means, electrically connected to said first and second bodies, for inducing current to flow between said first and second bodies and for producing an output signal whenever the magnitude of said current flow exceeds a reference level;

switching means, operatively connected to said comparing means and connected to receive a periodic control signal, for alternating said reference level between first and second values in response to said control signal; and

indicator means, connected to receive said output and control signals, for providing a first indication when said current flow is less than said first value and for providing a second indication when said current flow exceeds said second value, wherein said indicator means includes;

a first transistor having a base, and emitter and a collector, said emitter being connected to ground potential, said base connected to receive said output signal, said collector being connected to receive said control signal;

a second transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said first transistor collector; and

a first light emitting diode being connected between a supply voltage and said second transistor collector;

a further transistor having a base, an emitter and a collector, said emitter being connected to ground potential, said base being connected to said first transistor collector, said collector being connected to said supply voltage; and

a further light emitting diode being connected between said further transistor collector and emitter.

8. Apparatus as in claim 7 wherein said switching means alternately sets said reference level to correspond to a resistance between said first and second bodies of about 750 kilohms and a resistance between said first and second bodies of about 10¹⁰ ohms.

9. Apparatus for monitoring the resistance between first and second bodies, comprising:

comparing means, electrically connected to said first and second bodies, for inducing current to flow between said first and second bodies and for producing an output signal whenever the magnitude of said current to flow exceeds a reference level;

switching means, operatively connected to said comparing means and connected to receive a periodic control signal, for alternating said reference level between first and second values in response to said control signal; and

indicator means, connected to receive said output and control signals, for providing a first indication when said current flow is less than said first value and for providing a second indication when said current flow exceeds said second value, wherein;

said switching means sets said reference level to said first value when said control signal is present and sets said reference level to said second value when said control signal is not present; and

said indicator means includes;

means for producing an acoustical output in response to an electrical signal applied thereto; and

means, connected to receive said control and output signals and electrically connected to said acoustical output producing means, for applying an electrical signal to said acoustic output producing means whenever either (a) said output signal is present and said control signal is not present, or (b) said output signal is not present and said control signal is present.

10. An apparatus for monitoring the electrical resistance between a body and ground potential, said apparatus comprising:

operational amplifier means having first and second input terminals and an output terminal for generating an output signal at its output terminal in response to the relative levels present on its first and second input terminals;

a resistive element electrically connected at one end to said body and electrically connected at its other end to said operational amplifier first input terminal;

means for applying a fixed reference level to said operational amplifier second input terminal;

switching means, operatively connected to said operational amplifier, for alternately connecting a first resistance and a second resistance between said operational amplifier output terminal and first input terminal; and

indicating means, connected to said operational amplifier output terminal, for providing an indication in response to said operational amplifier output signal, said indicating means (a) providing a first indication whenever the resistance between said body and ground potential is less than a value set by said first resistance, (b) providing a second indication whenever the resistance between said body and ground potential is greater than a value set by said second resistance, and (a) providing a third indication whenever the resistance between said body and ground potential is between said first-mentioned and second-mentioned values.

11. An apparatus for monitoring the electrical resistance between a work surface and ground potential, said apparatus comprising:

a lead electrically connected at one end to said work surface and having another end;

operational amplifier means having first and second input terminals and an output terminal for generating an output signal at its output terminal in response to the relative levels present on its first and second input terminals, said first input terminal being electrically connected to the other end of said lead;

means for applying a fixed reference level to said operational amplifier second input terminal;

switching means, operatively connected to said operational amplifier, for alternately connecting a first resistance and a second resistance between said operational amplifier output terminal and first input terminal, said first and second resistances being a function of the upper and lower limits of a preset resistance range;

a further operational amplifier having first and second input terminals and an output terminal, said first input terminal being connected to the output terminal of said first-mentioned operational amplifier;

means for applying a further, fixed reference level to said further operational amplifier second input terminal; and

indicating means, connected to said further operational amplifier output terminal, for providing an indication whenever the resistance between said work surface and ground potential is less than the lower limit of said preset resistance range, and whenever the resistance between said work surface and ground potential is greater than the upper limit of said preset resistance range.