WO2018033709A1

WO2018033709A1 - Improved test apparatus for testing capacitance

Info

Publication number: WO2018033709A1
Application number: PCT/GB2017/052372
Authority: WO
Inventors: Sunil R NIRMALE; Manoj MULAJKAR
Original assignee: Megger Instruments Ltd
Current assignee: Megger Instruments Ltd
Priority date: 2016-08-18
Filing date: 2017-08-11
Publication date: 2018-02-22
Anticipated expiration: 2019-02-18
Also published as: GB201705924D0; GB2553015A

Abstract

Test apparatus (1) for testing a capacitance of an electrical component (2) comprises a first (3) and second (4) terminal for connection to the electrical component (2), a measurement circuit (5) connected to the first (3) and second (4) terminals, a discharge circuit (6) comprising a resistive load (9), the resistive load being connectable to the first (3) and second (4) terminals, and a control circuit (7). The control circuit (7) is configured to cause, on initiation of a measurement of capacitance, the measurement circuit (5) to measure, with the resistive load (9) disconnected from at least one of the first (3) and second (4) terminals, a voltage on the electrical component (2). Dependent on the measured voltage being greater than a first threshold value, the control circuit (7) automatically causes the resistive load (9) to be connected to the first (3) and second (4) terminals and subsequently causes the resistive load (9) to be disconnected from at least one of the first (3) and second (4) terminals. The capacitance of the electrical component is measured with the resistive load (9) disconnected from at least one of the first (3) and second (4) terminals.

Description

Improved Test Apparatus for Testing Capacitance Technical Field

The present invention relates generally to improved test apparatus for testing the capacitance of an electrical component, and more specifically, but not exclusively, to a hand-held digital multi-meter capable of measuring capacitance.

Background

Conventional digital multi-meters may have a selectable capacitance measurement function, for measuring the capacitance of an electrical component connected across the terminals of the multi-meter. Capacitance is typically measured by applying a constant current to the electrical component and measuring the voltage rise as a function of time. Other well-known methods of measuring capacitance may be used. However, the accuracy of the measurement may be impaired if the electrical component comprises a capacitor which is already charged. For example, the capacitor may have a charge remaining from when the device was previously used in a powered-up circuit. For this reason, it is conventional to advise users of the multi-meter to discharge an electrical component before connecting it to the multi-meter for a measurement of capacitance, for example by connecting a resistor across the terminal of the electrical component. For example, a 20 kOhm 5W resistor may be used for this purpose, depending on the values of capacitance and voltage expected. Some digital multi-meters have a user-selectable low impedance mode, typically selectable using a rotary mechanical contact selector knob, which connects a relatively low impedance across the terminals of the multi-meter. This can be used to discharge the electrical component before a capacitance measurement. Sufficient time needs to be allowed to discharge any charge that may be remaining on the electrical component. However, this may be time-consuming and onerous on the user. Summary

In accordance with a first aspect of the present invention, there is provided test apparatus for testing a capacitance of an electrical component, the apparatus comprising:

a first and second terminal for connection to the electrical component; a measurement circuit, the measurement circuit being connected to the first and second terminals;

a discharge circuit comprising a resistive load, the resistive load being connectable across the first and second terminals; and

a control circuit,

wherein the control circuit is configured:

on initiation of a measurement of capacitance, to cause the measurement circuit to measure, with the resistive load disconnected from at least one of the first and second terminals, a voltage on the electrical component;

dependent on the measured voltage being greater than a first threshold value, automatically to cause the resistive load to be connected across the first and second terminals and subsequently to cause the resistive load to be disconnected from at least one of the first and second terminals; and

to measure the capacitance of the electrical component with the resistive load disconnected from at least one of the first and second terminals.

This allows a reduction in the time taken for a capacitance measurement compared with manual discharge of a capacitance in the electrical component. This is because discharge of the electrical component is automatically performed by the test equipment and is only performed if the electrical component is charged to a greater voltage than the first threshold value, which may be chosen to be a voltage sufficiently high to impair the accuracy of the measurement by the test equipment.

In an embodiment of the invention, the causing of the resistive load to be disconnected from at least one of the first and second terminals is dependent on the measured voltage being less than a second threshold value. This ensures that the capacitance of the electrical component is sufficiently discharged for an accurate measurement of capacitance to be performed. The first and second threshold values may be the same.

In an embodiment of the invention, the discharge circuit has an impedance of less than 10 kOhm when the resistive load is connected across the first and second terminals, and may have an impedance of substantially 9.2 kOhm. This may provide a convenient trade-off of discharge time and current handling requirements.

In an embodiment of the invention, the resistive load comprises a positive temperature coefficient resistor.

This provides protection against high discharge currents.

In an embodiment of the invention, the resistive load is connectable to at least one of the first and second terminals by means of a switch arrangement suitable for providing isolation for at least 1000V.

This provides the capability of discharging capacitors potentially charged to high voltages.

In an embodiment of the invention, the resistive load is connectable to at least one of the first and second terminals by means of a switch arrangement suitable for conducting a current of at least 50 mA.

In an embodiment of the invention, the switch arrangement comprises at least one optically isolated solid state relay.

This provides a convenient implementation of the switch arrangement. In an embodiment of the invention, the control circuit is configured to cause a message to be displayed if the discharge circuit is connected to the first and second terminals.

This informs the user that measurement of capacitance may be delayed while the capacitance is discharged. Further features and advantages of the invention will be apparent from the following description of exemplary embodiments of the invention, which are given by way of example only. Brief Description of the Drawings

Figure 1 is a schematic diagram illustrating test apparatus according to an embodiment of the invention, connected to an electrical component;

Figure 2 is a flow diagram illustrating operational steps of a control circuit of the test apparatus in an embodiment of the invention;

Figure 3 is decision tree illustrating operation of test apparatus in an embodiment of the invention; and

Figure 4 is a schematic diagram illustrating test apparatus in an embodiment of the invention, illustrating an example of components of a discharge circuit.

Detailed Description

By way of example, embodiments of the invention will now be described in the context of a hand-held digital multi-meter capable of measuring capacitance of an electrical component, but it will be understood that embodiments of the invention may relate to other electrical test equipment and that embodiments of the invention are not restricted to use in a hand-held digital multi-meter.

Figure 1 shows test apparatus 1 according to an embodiment of the invention, in this example a hand-held digital multi-meter, connected to an electrical component 2, for testing of the capacitance of the electrical component 2. The multi-meter has a first terminal 3 and a second terminal 4 for connection to electrical component under test. Each terminal may be, for example, a standard 4mm socket or may take any convenient form to provide an electrical connection. The multi-meter also has a measurement circuit 5, the measurement circuit being connected to the first 3 and second 4 terminals. The measurement circuit may not necessarily be directly connected to the first and second terminals, but may be connected via other components, for example an input protection circuit, which may comprise, for example, a series arrangement of one or more positive temperature coefficient resistors and/or high current resistors and/or fuses. The measurement circuit typically has a high input impedance so as not to disturb the circuit under test when it is connected. For example, when acting as a voltmeter, the measurement circuit may typically have an input impedance of 10 MOhm or more. The measurement circuit, when measuring capacitance, may operate as a high input impedance voltmeter connected across the first and second terminals, with a current source arranged to drive a constant current through the electrical component under test. The rate of the rise of the voltage across the terminals with time is an indication of the capacitance of the electrical component under test. Before the measurement of capacitance commences, the electrical component is presented with the high input impedance of the measurement circuit. As a result, any charge on the electrical component will remain, and this residual charge may affect the measurement of capacitance, causing an erroneous result.

To mitigate the effects of any residual charge on the electrical component, in an embodiment of the invention, the multi-meter is provided with a discharge circuit 6 comprising a resistive load 9, the resistive load 9 being connectable across the first 3 and second 4 terminals. The discharge circuit 6 is controlled by a control circuit 7, which may also control the measurement circuit 5. The control circuit may comprise, for example, a microcontroller having program memory containing program code, or the controller may be part of another processor or may be implemented by digital logic or by a programmable gate array. The controller may be arranged to receive inputs from a mode selector switch 9, which may be settable to select a capacitance measurement mode, and/or a specifically auto-discharge capacitance mode.

In an embodiment of the invention, the control circuit is configured, on initiation of a measurement of capacitance, to cause the measurement circuit 5 to measure, with the resistive load 9 disconnected from at least one of the first 3 and second 4 terminals, a voltage on the electrical component. The controller may receive an input from the mode selector switch, which is typically a rotary mechanical multi-throw switch, indicating that a capacitance measurement mode, and/or a specifically auto-discharge capacitance mode has been selected. The input may be a voltage on an appropriate input line.

If the measured voltage across the terminals of the multi-meter is greater than a first threshold value, the controller 7 automatically causes the resistive load 9 to be connected to the first 3 and second 4 terminals and subsequently to cause the resistive load 9 to be disconnected from at least one of the first and second terminals. The subsequent disconnection may be on expiry of a time out period arranged to be sufficient for the expected charge on the electrical component to be discharged. Alternatively or in addition, the controller 7 may cause the resistive load to be disconnected if the measured voltage falls below a second threshold value. The first and second threshold values may be the same, or close, but there may be value in making the second threshold lower than the first in terms of introducing hysteresis in the control process and reducing jitter. Typically the first threshold value may be selected to be a voltage value that would be expected to cause a significant measurement error. The precise value of the first threshold value is dependent on the characteristics of the measurement circuit and the capacitance measurement method used, and also on the requirements for measurement accuracy. The values of the first and second thresholds are also dependent on the requirements on measurement time, as a lower second threshold in particular may cause an increase in measurement time. As an example, the first voltage threshold may be 100 mV or greater and the second voltage threshold may be 50 mV or less, but these values are in no way limiting and higher or lower voltage threshold values may be used according to the characteristics of the measurement circuit and the requirements of the test mode.

After the resistive load has been disconnected by the controller, the controller may then instruct the measurement circuit to measure the capacitance of the electrical component, using conventional techniques for measuring capacitance.

As shown in Figure 1, the multi-meter may have a display 8, controlled by the control circuit. The control circuit may be configured to cause a message to be displayed if the discharge circuit is connected to the first and second terminals, which informs the user that measurement of capacitance may be delayed while the capacitance is discharged.

The multi-meter in this embodiment of the invention has the advantage that it may perform a quick and efficient capacitance measurement, following a selection of the appropriate capacitance measurement mode by the operator. The discharge of the electrical component is automatically performed by the test equipment and is only performed if the electrical component is charged by to a greater voltage than the first threshold value, which may be chosen to be a voltage sufficiently high to impair the accuracy of the measurement by the test equipment.

Figure 2 is a flow diagram illustrating an example of operational steps of the control circuit 7 of the test apparatus in an embodiment of the invention.

At step S2.1, the controller detects that the mode selector switch 20 has been set to measure capacitance. If two or more capacitance measurement modes are offered, then the controller detects that an auto-discharge (Auto-Lo Z) mode has been selected. The controller may detect a change in the setting of the mode selector switch 20, and initiate the following steps dependent on the detection of the change.

At step S2.2, the controller 7 sends a signal to the measurement circuit 5 to cause it to measure a voltage on the electrical component 2, with the discharge circuit 6 configured to disconnect the resistive load 9 from at least one terminal of the test equipment. The signal may be a voltage on a control line or may be a digital command, for example.

At step S2.3, if the controller 7 determines that the measured voltage is greater than a first threshold value, the controller 7 causes the discharge circuit 6 to be configured to connect the resistive load 9 across the first 3 and second 4 terminals and subsequently to cause the discharge circuit 6 to be configured to disconnect the resistive load 9 from at least one of the first and second terminals.

At step S2.4, the controller 7 measures the capacitance of the electrical component 2 and configures the discharge circuit 6 to disconnect the resistive load 9 from at least one of the first and second terminals. Figure 3 is a decision tree illustrating operation of the controller of the test apparatus in an embodiment of the invention. At 10, if a capacitance measurement mode is selected, then at 11, the controller determines whether a low impedance mode is selected, such as the automatic discharge mode. If it is, at 12 the controller determines whether the electrical component under test, in this example a capacitor, is charged, for example by measuring its voltage and determining whether it is greater than the first voltage threshold value. If it is, then at 13 the capacitor is discharged by the discharge circuit. At 14, the controller determines whether the capacitor is discharged, by instructing the measurement circuity to measure the voltage across it, which is the voltage across the first and second terminals, and determining if the voltage is less than the second voltage threshold value. If it is, at 15, the controller causes the discharge circuit to disconnect the resistive load, and instructs the measurement circuit to start the capacitance measurement. On completion of the capacitance measurement, the measured capacitance value is displayed, at 16.

If at step 11, the controller determines that a low impedance mode such as the automatic discharge mode is not selected, then the controller determines at 18 whether or not a live circuit flag is set, that is to say whether a high voltage has been detected. If it has, then at 19 a high voltage warning message is displayed on the display of the multi-meter. If not, then, at 15, the capacitance measurement is performed with the resistive load disconnected, and then at 16 the controller causes the measured capacitance value to be displayed.

If, at step 12, the controller detects that the capacitor is not charged to above the first voltage threshold, then the controller causes the measurement circuit to measure the capacitance without discharging the capacitor at step 15, and then the measured capacitance is displayed at step 16.

If, at step 14, the voltage on the capacitor is measured and it is determined that the capacitor has not been discharged to below the second voltage threshold value, and if at step 17 it is determined that a timer for a discharge time has not expired, then discharge of the capacitor continues at step 13. If, at step 17, it is determined that the timer for a discharge time has expired, and if it is determined at step 18 that the live circuit flag is not set, then the discharge circuit is controlled to disconnect the resistive load and the measurement of capacitance is performed at step 15, and the measured capacitance value is displayed at step 16. If it is determined that the live circuit flag is set at step 18, the measurement of capacitance is not performed, and the high voltage warning is displayed at step 19.

Figure 4 is a schematic diagram illustrating test apparatus in an embodiment of the invention, illustrating an example of components of the discharge circuit 6.

In this example, one of the terminals, in this example the first terminal 3, is connected to the measurement circuit 5 by a protection circuit comprising a positive temperature coefficient (PTC) resistor 21 and a resistor 22, which is typically a high Wattage resistor, for example 3 W capacity or higher. The protection circuit is shown in simplified form, and may comprise additional PTC resistors and fuses. The protection circuit protects the measurement circuit from high currents, for example due to high external voltages or due to fault conditions. The PTC resistor will increase its resistance with temperature, limiting the flow of current.

The protection circuit may form part of the discharge circuit 6 which may be connected across the terminals of the multi-meter 3, 4, as shown in Figure 4. The impedance of the discharge circuit is the series combination of the protection circuit, the PTC resistor 23 and the switch arrangement, which may comprise at least one optically isolated solid state relay, and in the example illustrated comprises Opto MOS relays 24, 25. The Opto MOS relays are optically-coupled solid state relays. Using two suitably specified relays in series may suitable for providing the switching arrangement with a rating of at least 1000V for isolation. This may be beneficial if the meter is to be rated for measuring high voltages, and gives the capability of discharging capacitors charge to a high voltage. Rating the switch arrangement for conducting a current of at least 50 mA also helps provide the capability of discharging capacitors potentially charged to high voltages. In the arrangement shown in Figure 4, the resistive load 9 is made up of the series combination of the two PTC resistors 21, 23 and resistor 22. The series combination of devices in the discharge circuit 6 typically has an impedance of less than 10 kOhm when the Opto MOS relays are set to conduct, and an impedance of substantially 9.2 kOhm has been found to be particularly beneficial in an embodiment of the invention, providing a convenient trade-off of discharge time and current handling requirements.

As shown in Figure 4, the discharge circuit is under control of the control circuit 7, and the measurement circuit is in communication with the measurement circuit 5. The controller may receive inputs from the mode selector switch 20. The controller may also receive inputs from a selector switch or button 26, which may be provided on the control panel of the multi-meter to select a low impedance measurement mode, not necessarily only of capacitance, but of for example DC and AC voltage also. When the controller detects that the low impedance mode is selected, for example by a user operating the button 26, the controller causes the discharge circuit to connect the resistive load across the first and second terminals. This may be performed when the mode selector switch 20 is set to select a measurement of DC or AC voltage. Performing this measurement with the resistive load connected may prevent spurious measurements caused by coupling of "ghost" voltages from other circuit parts that are not under test. Connecting the load resistance across the terminals under control of the controller using solid state relays, as opposed to using contacts of a mechanical switch rotary to connect the load resistance, has the advantage or increased reliability and providing a more intuitive user interface.

The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. Test apparatus for testing a capacitance of an electrical component, the apparatus comprising:

a discharge circuit comprising a resistive load, the resistive load being connectable to the first and second terminals; and

a control circuit,

wherein the control circuit is configured:

dependent on the measured voltage being greater than a first threshold value, automatically to cause the resistive load to be connected to the first and second terminals and subsequently to cause the resistive load to be disconnected from at least one of the first and second terminals; and

2. Test apparatus according to claim 1, wherein causing the resistive load to be disconnected from at least one of the first and second terminals is dependent on the measured voltage being less than a second threshold value.

3. Test apparatus according to claim 1 or claim 2, wherein the discharge circuit has an impedance of less than 10 kOhm when the resistive load is connected across the first and second terminals.

4. Test apparatus according to claim 4, wherein the discharge circuit has an impedance of substantially 9.2 kOhm when the resistive load is connected across the first and second terminals.

5. Test apparatus according to any preceding claim, wherein the resistive load comprises a positive temperature coefficient resistor.

6. Test apparatus according to any preceding claim, wherein the resistive load is connectable to at least one of the first and second terminals by means of a switch arrangement suitable for providing isolation for at least 1000V.

7. Test apparatus according to any preceding claim, wherein the resistive load is connectable to at least one of the first and second terminals by means of a switch arrangement suitable for conducting a current of at least 50 mA.

8. Test apparatus according to claim 6 or claim 7, wherein the switch arrangement comprises at least one optically isolated solid state relay.

9. Test apparatus according to any preceding claim, wherein the control circuit is configured to cause a message to be displayed if the discharge circuit is connected to the first and second terminals.