RU178894U1 - SMALL RESISTANCE METER - Google Patents

Abstract

The utility model relates to electrical engineering, in particular to the field of measuring electrical resistance to direct current and can be used to control the transient resistances of electric switches, disconnectors, welded and detachable joints, and in other cases when it is required to measure electrical resistances of small size. The technical result consists in simplifying the design and increasing reliability when measuring low resistances with high accuracy. The low resistance meter contains a stabilized measuring current source, an analog-to-digital converter, a microcomputer and an indicator, as well as two current and two potential clamps for connecting the measured resistance. Current clamps are connected to the outputs of a stabilized measuring current source, and potential clamps are connected to the inputs of an analog-to-digital converter, two resistors and two threshold devices are additionally introduced. 2 ill.

Description

The utility model relates to electrical engineering, namely, to the field of measuring electrical resistance to direct current and can be used to control the transient resistances of electrical switches, disconnectors, plug-in connections, and in other cases when small electrical resistances are required.

A known low resistance meter (patent RU No. 2173858, publ. September 20, 2001), the principle of which is based on a comparison of the voltage drop across the measured resistance and the reference resistor (shunt) when the same measuring current passes through them. It contains a stabilized current source, an analog-to-digital converter (ADC), a current direction switch, a digital indicator, as well as two current and two potential clamps for connecting the measured resistance, a precision ADC, a controller, and a shunt.

The disadvantage of this device is the high complexity and, as a consequence, the high cost. Another disadvantage is the relatively long measurement time due to the double measurement of resistance at measuring currents opposite in direction, which is necessary to compensate for the zero offset of the measuring circuit and thermo-EMF at the points of connection of the potential clamps to the measured resistance.

The closest in technical essence to the claimed object is an IKS-5 microohmmeter (see technical description URL: http://www.limi.ru/x5/manualiks-5.pdf), the principle of which is based on measuring the voltage drop across the measured resistance when a measuring current of a given value passes through it. The known microohmmeter contains a pulse voltage converter, powered by a storage battery (AB), a current source in the form of a series-type compensating stabilizer, output current clamps TK, input potential clamps PZ connected to the input of an analog-to-digital converter (ADC), a microcomputer, receiving digital codes from the ADC, controlling the inclusion of the current source through the electrical isolation unit and a liquid crystal display (LCD).

The disadvantage of this microohmmeter is its high complexity and, as a consequence, its high cost. Another disadvantage is that there is no indication of contact failure between the potential probes and the measured resistance. In case of violation of one of these contacts, the readings of the device become uncertain, which leads to an incorrect measurement of resistance.

The objective of the claimed utility model is to create a reliable meter of low resistance with high accuracy and relatively low cost.

The task is achieved by such a configuration of the measuring circuit, which allows you to abandon the electrical isolation unit and a pulse voltage converter, as well as by monitoring the voltage at the potential terminals, which makes it possible to identify disturbance of contacts between potential probes and the measured resistance.

The feasibility of the utility model is confirmed by the fact that the inventors developed, manufactured and tested the inventive device. Currently, its pilot operation is being successfully implemented.

In FIG. 1 is a structural diagram of the device, and in FIG. 2 is an example implementation of a stabilized measuring current source. The low resistance meter contains (designation in Fig. 1):

1 - stabilized measuring current source,

2 - current clamps (TK),

3 - potential clamps (PZ),

4 - measured resistance

5 - analog-to-digital Converter,

6 - microcomputer,

7 - indicator

8 - power source (battery),

9, 10 - resistors,

11, 12 - threshold devices.

The output of the stabilized measuring current source 1 through current clamps 2 is connected to the terminals of the measured resistance 4, to the terminals of which the ADC inputs are connected through potential clamps 3.

5. The output of the ADC 5 through the control lines / data is connected to a microcomputer

6, which transmits information about the measurement results to indicator 7. All nodes of the device are powered by battery 8. To monitor the status of contacts of potential probes 3 between them and a common point, high-resistance resistors 9 and 10 are included, and the inputs of two threshold devices 11 and 12.

The structure of a stabilized measuring current source (Fig. 2) includes the following nodes:

13 - source of reference voltage

14 is a key controlled from a microcomputer 6,

15 - differential amplifier

16 - transistor,

17 - measuring resistor, 2 - current clamps.

The measuring current, which when the key is closed 14 flows through the current clamps 2, is determined by the formula:

where Uop is the output voltage of the reference voltage source 13, Ri is the resistance of the measuring resistor 17.

The voltage at the measured resistance Rx is determined by the formula:

The device (Fig. 1) works as follows. In the initial state, clamped current clamps 2 and potential pickup 3 are connected to the measured resistance 4. The stabilized source of the measuring current 1 is turned off and no current flows through the measured resistance 4. At the operator’s command, the microcomputer 6 starts measuring the voltage across the PZ 3. The result of this measurement Uo includes the zero offset voltage of the ADC 5 and the thermo-EMF at the points of connecting 4 potential clamps to the measured resistance 3. The voltage Uo is remembered by the microcomputer 6. Then the micro- Computer 6 turns on the stabilized measuring current source 1 and, after establishing current I through the measured resistance 4, starts measuring the voltage Uп on ПЗ 3 by means of ADC 5 and saves this voltage in the memory of microcomputer 6, and then turns off stabilized first source of measuring current 1. After that, the microcomputer 6 calculates the resistance according to the formula:

In view of (1)

Thus, the influence of zero offset of the ADC and thermo-EMF contacts is eliminated and the measurement of low resistances with high accuracy is ensured.

Resistors 9 and 10 connect the inputs of the ADC 5 with a common point in the circuit. With normal contact between the potential clamps 3 and the terminals of the measured resistance 4 in the measurement mode, when the measuring current flows, the potentials of the ADC inputs 5 are higher than the common point potential by the voltage drop across the measuring resistor 17 of the stabilized measuring current source 1. In this case, the signals at the threshold outputs devices 11 and 12 correspond to the correct measurement. In the case of unreliable contact of the potential probes 3 and the terminals of the measured resistance 4 (due to, for example, contamination of the terminals), the connection of the potential clamp 3 with the measured resistance 4 is broken and the voltage at the corresponding input of the ADC 5 becomes close to zero relative to the common point. In this case, the threshold device 11 or 12 provides the corresponding signal to the input of the microcomputer 6. The microcomputer 6 analyzes this signal and, if this occurs while the stabilized measuring current source 1 is on, gives an error message to the indicator 7. The resistance of resistors 9 and 10 is chosen so large that their shunting effect does not significantly affect the accuracy of resistance measurements in the operating range of the meter.

As a source of reference voltage, ADC, microcomputer and differential amplifier, conventional devices manufactured by the industry in the form of integrated circuits can be used. Integrated comparators can be used as threshold devices, and comparators or ADCs that are part of microcomputer microcircuits can also be used.

Claims (1)

A low resistance meter containing a stabilized measuring current source, an analog-to-digital converter, a microcomputer and an indicator, as well as two current and two potential clamps for connecting the measured resistance, the current clamps being connected to the outputs of the stabilized measuring current source, and the potential clamps being connected to the analog inputs -digital converter, characterized in that two resistors and two threshold devices are introduced into it, the first conclusions of the first and second resistor in are connected respectively to the first and second inputs of the analog-to-digital converter, and the second outputs of the resistors are connected to a common point of the circuit, the inputs of the first and second threshold devices are connected to the first and second inputs of the analog-to-digital converter, and their outputs to the digital inputs of the microcomputer.

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