GB2273365A

GB2273365A - Hand-held digital multimeter capable of performing isolation resistance measurements

Info

Publication number: GB2273365A
Application number: GB9321347A
Authority: GB
Inventors: Wolfgang Schulze; Burkhardt Krien; Herbert Ziegler; Sabastian Kiefhaber
Original assignee: Metrawatt Gossen GmbH
Current assignee: Metrawatt Gossen GmbH
Priority date: 1992-10-15
Filing date: 1993-10-15
Publication date: 1994-06-15
Anticipated expiration: 2013-10-15
Also published as: GB9321347D0; GB2273365B; FR2697092B1; DE4234789A1; FR2697092A1

Description

X 1

Description

2273365 The invention relates to a digital multimeter, in particular-a hand-held digital multimeter. Multimeters of this type are used to measure current, voltage, resistance and/or capacitance, and comprise a voltage divider circuit connected via switches to the measuring input sockets of the multimeter, which are installed in a casing. Furthermore, an analog/digital convertor is provided, of which the analog inputs are connected to the voltage divider circuit, of which the reference input can be acted upon by a reference voltage source of the multimeter and of which the digital output signals are transmitted to a microprocessor, in order to generate output signals or display driver signals for a digital display device. Finally, a plurality of current measuring shunts is provided, which can be connected selectively to a measuring input socket via a switch arrangement.

So-called isolation resistance measurements on sections of cable cannot usually be performed with normal digital hand-held multimeters of th-is type, and, in this respect, there are known, as further prior art, conventional isolation resistance measuring devices which comprise a separate

14 2 voltage source, generally known as the isolation voltage source, which delivers the necessary measurement voltage of for example 500 V in the case of domestic installations.

Furthermore, in the case of isolation resistance measuring devices, steps are taken to protect the operator, the system and/or the current source, by restricting the measuring current. Measurement is generally performed using currents of up to approximately 1 milliampere, the current restriction of the measuring current for protection of the operator being approximately 1 milliampere.

Claims

The object of the invention is to develop further a digital multimeter

having the features given in the preamble of Claim 1 in such a manner that, using existing multimeter components. it can also be used for performing isolation resistance measurements in a simple manner, in particular in the area of domestic installations. This object is achieved by the characterising features of Claim 1. Advantageous further developments are given in subclaims 2 to 8, and, finally, a process for measuring the isolation voltage of a section of cable is protected by Claim 9.

3 The essence of the invention is regarded as being the inclusion of the components necessary for an isolation resistance measurement in a circuit of a digital multimeter in such a manner, that the multimeter components or subassemblies can be used both in the multimeter operating modes on the one hand, and in the isolation resistance measuring mode on the other hand. For this purpose, an isolation voltage source is located in parallel with the voltage divider circuit of the multimeter and delivers a voltage of approximately 500 volts for measuring the isolation of domestic installations.

The voltage of this isolation voltage source can be applied to the measuring input sockets of the multimeter with the interposition of a measuring resistance and simultaneous switching-off of the reference voltage source, and from there can be introduced into the domestic installation. During the measuring process, the voltage that is now dropped across the measuring resistance of the multimeter circuit is delivered to the reference input of the A/D convertor. At this location, a so-called ratio measurement can be performed, i.e. the ratio constant x Iso-voltage/R x Iso-current 4 (constant x isolation voltage/[measuring resistance x isolation current]) can be determined, which ratio is proportional to the isolation resistance present in the domestic installation.

The microprocessor of the multimeter, which processor is connected to the A/D convertor, then reads out the digital value from the A/D convertor and transforms it into driver signals for a digital display device of the multimeter. The operator can thus perform and read off isolation voltage measurements on the multimeter in a simple manner.

It is particularly advantageous, if different measuring resistors of different values can be switched on selectively, wherein the measuring resistors can be formed by the current measuring shunts provided for the current measurement in multimeter operation. A further structural simplification and a reduction in size of the entire device can be achieved thereby.

Further advantageous measures relating to the integration of the components for the isolation resistance measurement in the multimeter can be 1 found in Claims 4 to 6. Claim 7 relates to a circuit variant which ensures particularly simple handling of the multimeter in measuring operation.

Finally, Claim 9 relates to the process for measuring the isolation resistance of a section of cable, wherein, if the individual process steps are carried out consistently, the isolation resistance of a section of cable can be measured using the subassemblies of a digital hand-held multimeter.

The invention will be explained in further detail, with reference to embodiments, in the drawings, in which:

Figure 1 shows a first elementary circuit diagram of a multimeter comprising components for measuring the isolation resistance; and Figure 2 shows a further elementary circuit diagram of a modified embodiment of a multimeter comprising components for measuring the isolation resistance.

Reference is firstly made to Figure 1.

6 The digital multimeter 1 shown schematically in this Figure together with its components is a hand-held multimeter and is used to measure current, voltage, resistance and/or capacitance. For this purpose, it comprises on its casing 2, which is only partly shown, measuring input sockets 3, 31, of which the measuring input socket 3' is connected to one end of a voltage divider circuit 4. The voltage divider circuit 4 is, in turn, connected to the analog inputs 5 of an A/D convertor 6; the reference input 7 of the A/D convertor can be acted upon by a reference voltage source 8.

In order to apply the reference voltage to the reference input 7, a switch 9, which, however, is open in the embodiment shown in Figure 1, is provided between the reference voltage source 8 and the reference input 7.

T he A/D convertor is connected via an output 10 to a microprocessor 11 which generates the output and/or display driver signals for a digital display device 12.

Furthermore, the multimeter 1 shown in Figure 1 comprises a further row of current measuring shunts 13-17. These current measuring shunts can 7 be selected by means of switches 18-22 and can be connected to the measuring input socket 3.

The voltage drop occurring across the current measuring shunts 13-17 when a measurement is performed can be delivered via further switches 23-27 to the reference input 7 of the A/D convertor.

In order to perform isolation resistance measurements, i.e. for-the measurement of a resistance 28 of a line section, an isolation voltage source 29 is arranged parallel to the voltage divider circuit 4, the voltage of which isolation voltage source can be applied to the two measuring input sockets 3, 31 with the interposition of a measuring resistance which is formed by the current measuring shunt 13.

The voltage which is dropped across the measuring resistance 13 is applied-via the closed switches 23 to the reference input 7 of the A/D convertor. The isolation measuring voltage is applied, with the interposition of a current-limiting protective element 30, to the measuring sockets 3,, 31.

The selection of the current measuring shunts 1317 which form the measuring resistances during the 8 isoi-lation voltage measurement is performed by means of an autoranging process, which controls the switches, which are formed as electronic switches 18-22, as well as the further switches 23-27.

A further switch, via which the measuring input socket 3 can be separated from the earth 32 when the isolation voltage measurement is performed, is designated by the reference numeral 31.

The operating method of the elements integrated in the multimeter for isolation resistance measurements, is as follows:

The isolation measuring voltage of the isolation voltage source 29 is applied via the protective element 30 to the measuring input sockets 3, 31, to which the section of cable to be measured is connected via a cable connection of the section of cable which is to be measured. The isolation measuring voltage is broken down via the voltage divider circuit 4, which is also used for the other voltage and resistance measurements of the multimeter, and is fed to the A/D convertor 6.

The current flowing through the external resistance 28 now also flows via the current A 9 measuring shunt 13 that is selected by the closed switch 18. The voltage drop across the current measuring shunt 13 is applied to the reference input 7 of the A/D convertor via the closed switch 23. The A/D convertor performs a ratio measurement K1 x Isolation-voltage/[R (13) x Isolationcurrent] which is proportional to the isolation resistance 28.

The microprocessor 11 of the multimeter reads out the digital value from the A/D convertor 6 and transforms it into signals for the digital display device 12, such that the isolation resistance value can be read directly in digital form on the digital display device.

It is also possible to read out the isolation resistance value determined and other measuring values - via an interface from the digital multimeter and to process, print or display them at a different location.

For different isolation resistance values, the processor 11 selects different current measuring shunts 13-17 by means of opening and closing the electronic switches 18- 22 or 23-27.

In the following, reference is made to Figure 2 of the drawing. In Figure 2, components which also appear in Figure 1 of the drawings are designated by the same reference numerals.

The principle of isolation measurement using the device according to Figure 2 of the drawings differs from that according to Figure 1 in that the switch 31 for separating the measuring input socket 3 from the earth 32 is not necessary. The measuring input socket 3 remains, as is the case in other operating modes of the multimeter, connected securely to the earth potential.

However, in order to achieve a correct current measurement, it is necessary, in the case of the embodiment shown in Figure 2, for the base point 33 of the voltage circuit 4 to be connected to the negative output 35 of the isolation voltage source 29 by means of a changeover switch 34. The reference voltage which is fed into the reference input 7 of the A/D convertor 6, is generated in a current to voltage convertor 36. In this connection, the current to voltage convertor 36 behaves according to the formula U (output) R 1 x I (input), R being the resistance value of the voltage measuring shunt that has just been switched on.

In addition, two further switches 37, 38 are provided. During the isolation resistance measurement, the switch 37 is closed and the switch 38 is opened, in order to switch off the reference voltage source. If the reference voltage source is to be used in other multimeter operating modes, the switch 38 is closed and the switch 37 is opened.

Furthermore, a device for measuring capacitance is designated with the reference numeral 39 and can be used in this case for determining the cable length. The processor 11 multiplies the result from the capacitance measuring circuit 39 with a constant that is typical for the type of cable, in farad/meters, and displays the cable length in metres on the digital display device 12 when the cable length has been calculated.

12 LIST OF REFERENCE NUMERALS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Multimeter Casing Measuring input sockets Voltage divider circuit Analog inputs A/D convertor Reference input Reference voltage source Switch Output microprocessor Digital display device Current measuring shunt Current measuring shunt Current measuring shunt Current measuring shunt Current measuring shunt Switches Switches Switches Switches Switches Further switches Further switches Further switches Further switches Further switches Resistance Isolation voltage source Protective element Further switches Earth Base Point Changeover switch 13 36 37 38 39 outputs Current voltage convertor Switches Switches Device for measuring capacity 1 14 CLAIMS 1. Digital multimeter (1), in particular a hand-held multimeter for measuring current, voltage, resistance and/or capacitance, comprising - a voltage divider circuit (4) connected to measuring input sockets (3, 31) of the multimeter (1); - an A/D convertor (6) -- of which the analog inputs (5) are connected to the voltage divider circuit (4); -- of which the reference input (7) can be acted upon by a reference voltage source (8); and -- of which the digital output signals can be transmitted to a microprocessor (11) to generate output and/or display driver signals for a digital display device (12); and - a plurality of current measuring shunts (1317); f 1 -- which can be selectively connected to a measuring input socket (31) by means of at least one switch (18-22); characterised in that:

- for performing isolation resistance measurements, an isolation voltage source (29) is disposed in parallel with the voltage divider circuit (4); -- the voltage of which isolation voltage source can be supplied to the measuring input sockets (3, 3') with the interposition of a measuring resistance (current measuring shunts 13-17), and the reference source (8) is switched off at the same time; wherein -- the voltage. which is dropped across the measuring resistor (current measuring shunts 13-17) can be applied to the reference input (7) of the A/D convertor (6).
2. Multimeter according to Claim 1, characterised in that a plurality of measuring 16 resistors (current measuring shunts 13-17) of different values can be switched in selectively.
3. Multimeter according to Claim 1 or Claim 2, characterised in that the plurality of measuring resistors is formed by the current measuring shunts (13-17) provided for the measurement of the current.
4. Multimeter according to any one of the preceding claims, characterised in that a switch (31) is disposed between the earth (32) and the measuring input (31) which can be connected to the measuring resistors (current measuring shunts 1317).
5. Multimeter according to any one of the preceding claims, characterised in that the selection of the measuring resistors (current measuring shunts 13-17) is performed by means of the processor (11), in the so- called autoranging process for example.
6. Multimeter according to any one of the preceding claims, characterised in that the isolation measuring voltage is applied to the measuring sockets (3, 31) with the interposition of a current-limiting protective element (30).

4- 17
7. Multimeter according to any one of the preceding claims, characterised in that a changeover switch (34) is disposed at the base point (33) of the voltage divider circuit (4), which changeover switch, in a first operating position, connects the base point to the earth (32), and, in a second operating position, connects the base point to the negative output (35) of the isolation voltage source (29), and to an input of a current to voltage convertor (36), the other input of which is connected to the earth (32) in the second operating position, and the output of which generates the reference voltage for the reference input (7) of the A/D convertor (6).
8. Multimeter according to Claim 7, characterised in that the reference voltage existing at the output of the current voltage convertor (36) is proportional to the resistance of the current measuring shunt (13-16) multiplied by the input current.

Method for measuring the isolation resistance of a section of cable, characterised by the combination of the following features:

18 the application of a digital multimeter for measuring current, voltage, resistance and/or capacitance, comprising a voltage divider circuit connected to the measuring input sockets of the multimeter, an A/D convertor, connected at the input side to the voltage divider circuit and at the output side, for the generation of output and d isplay driver signals, to a processor, and the reference input of which can be acted upon by means of a reference voltage source, wherein the multimeter comprises a plurality of current measuring shunts, which can be selectively connected to the measuring input sockets, characterised by the following process steps:

- provision of an isolation voltage source parallel to the voltage divider circuit of the multimeter; switching off the multimeter reference voltage source; - feeding the voltage of the isolation resistance voltage source to the measuring input sockets; - connecting of the measuring input sockets to the section of cable which is to be measured; 19 - registering the voltage which is dropped across the activated measuring resistance (current measuring shunt of the multimeter); - feeding the voltage to the reference input of the A/D convertor; - performing a ratio measurement