Method and apparatus for converting a conventional DC multimeter to an AC impedance meter

• US 5,248,934 A
• Filed: 01/10/1992
• Issued: 09/28/1993
• Est. Priority Date: 01/10/1992
• Status: Expired due to Fees

• Current Assignee: Denes K. Roveti
• Original Assignee: Denes K. Roveti
• Inventors: Roveti, Denes K.
• Primary Examiner(s): Karlsen, Ernest F.
• Application Number:
  US07/819,172
• Time in Patent Office: 627 Days
• Field of Search: 324/115, 324/128, 324/430, 324/502, 324/650, 324/691
• US Class Current: 324/115
• CPC Class Codes: G01R 15/125 for digital multimeters

Abstract

An R/Z converter for use with a conventional digital multimeter (DMM) provides enhanced capabilities of the DMM in measuring impedance and other functions where DC measurements are undesirable. The R/Z converter converts the test voltage of a conventional DMM into a square wave that is applied to the device being tested. The voltage from the tested device is seen by the DMM as an average DC value that is proportional to the impedance of the device.

Claims

1. An apparatus for adapting a conventional ohmmeter to measure the impedance of an object comprising:

a converter means connected between the ohmmeter and the object for converting a DC test voltage output from the ohmmeter to an AC test voltage and outputting the AC test voltage to the object being tested, the converter means including;

a polarity reversing switch connected between the ohmmeter and the object, the switch being supplied with the DC test voltage from the ohmmeter,

a means for producing a switching signal with an adjustable frequency, the switching signal being supplied to the polarity reversing switch causing the polarity reversing switch to reverse polarity, thereby outputting the AC test voltage to the object being tested, and

wherein the AC test voltage is outputted to the object being tested at a rate determined by the switching signal, such that current seen by the ohmmeter is proportional to the impedance of the object being tested, thereby allowing the impedance of the object to be read on an ohm scale on the ohmmeter.

2. An apparatus as recited in claim 1 wherein the AC test voltage generated is an open circuit square wave.

3. An apparatus as recited in claim 2, wherein the square wave generated by the converter means has a peak to peak amplitude that is twice the amplitude of the DC test voltage output by the DMM.

4. An apparatus as recited in claim 2, wherein the square wave generated by the converter means has s 50% duty cycle and minimum DC offset.

5. An apparatus as recited in claim 4 wherein the square wave generated has a frequency that is 1.5 times the frequency of an equivalent sine wave impedance desired to be measured.

6. An apparatus as recited in claim 5 wherein the square wave has a frequency of 90 Hertz to measure an equivalent sine wave impedance of 60 Hertz.

7. An apparatus as recited in claim 2, wherein the frequency of the square wave generated by the converting means is at least 100 times the internal R-C time constant of the DMM.

8. An apparatus as recited in claim 1 wherein the parallel impedance R_AC, is measured as ##EQU3##

9. An apparatus as recited in claim 8, wherein the series impedance Z, is measured as the vector quantity:

##EQU4##

10. An apparatus as recited in claim 9 wherein the parallel impedance R_AC, is measured as the vector quantity:

##EQU5##

11. An apparatus as recited in claim 10, wherein the series impedance Z is measured as

12. An apparatus as recited in claim 1 wherein the converter means further comprises

an offset neutralizer means connected between the ohmmeter and the component object being tested for correcting any DC offset caused by stray resistances in the converter means;

and wherein the switching signal means is an oscillator.

13. An apparatus as recited in claim 12 wherein the oscillator further comprises:

two inverters;

a potentiometer to adjust the oscillating signal from 80 to 100 hertz; and

wherein the oscillating signal is a 9 volt unipolar square wave with a 50% duty cycle.

14. An apparatus as recited in claim 1 wherein the polarity reversing switch is comprised of two sets of first and second FETs, and the first FETs are depletion mode FETs and the second FETs are enhancement mode FETs, and only one of each set of FETs is on at a time, and the gates of all FETS are fed by the oscillating signal.

15. An apparatus as recited in claim 1 wherein the conventional ohmmeter is a digital multimeter (DMM).

16. An apparatus as recited in claim 15 wherein the means for producing a switching signal is an oscillator.

17. An apparatus as recited in claim 16 further comprising a shunt capacitance as an integrator at its input for capacitive loads and wherein the frequency of the switching signal is adjustable to adjust the AC test voltage so that when a capacitive/inductive load is connected to the apparatus, the DMM will read an equivalent sinusoidal impedance value on its scale corresponding to the reactive load.

18. An apparatus as recited in claim 16 further comprising a means for determining resonance of a circuit comprising:

a fixed inductive load;

a variable capacitor connected between the R/Z meter and the fixed inductive load.

19. An apparatus as recited in claim 16 wherein the polarity reversing switch converts the DC test voltage to a square wave shaped AC test voltage when the object being tested is a resistive load.

20. An apparatus as recited in claim 16 wherein the polarity reversing switch is an electronic polarity reversing switch.

23. An apparatus as recited in claim 16 wherein the switching signal is a square wave.

24. An apparatus as recited in claim 16 wherein said apparatus further comprises a DC offset correction means connected between the DMM and the object being tested for correcting any stray resistance in the apparatus.

21. An apparatus as recited in 16 wherein the AC test voltage has a 50% duty cycle and the square wave has a peak to peak amplitude twice that of the DC test voltage.

22. An apparatus as recited in 16 wherein the AC test voltage has a peak to peak amplitude twice that of the DC test voltage.