Capacitance meter
Encyclopedia
A capacitance meter is a piece of electronic test equipment
used to measure capacitance
, mainly of discrete capacitor
s. Depending on the sophistication of the meter, it may display the capacitance
only, or it may also measure a number of other parameters such as leakage, equivalent series resistance
(ESR), and inductance
. For most purposes and in most cases the capacitor must be disconnected from circuit
; ESR can usually be measured in circuit.
s which tend to be of high capacitance and to be subject to poor leakage. A multimeter
in a resistance range can detect a short-circuited capacitor (very low resistance) or one with very high leakage (high resistance, but lower than it should be; an ideal capacitor has infinite DC resistance). A crude idea of the capacitance can be derived with an analog multimeter in a high resistance range by observing the needle when first connected; current will flow to charge the capacitor and the needle will "kick" from infinite indicated resistance to a relatively low value, and then drift up to infinity. The amplitude of the kick is an indication of capacitance. Interpreting results requires some experience, or comparison with a good capacitor, and depends upon the particular meter and range used.
s) have a capacitance-measuring function. These usually operate by charging and discharging the capacitor under test
with a known current
and measuring the rate of rise of the resulting voltage
; the slower the rate of rise, the larger the capacitance. DVMs can usually measure capacitances from nanofarads
to a few hundred microfarads, but wider ranges are not unusual.
It is also possible to measure capacitance by passing a known high-frequency alternating current
through the device under test and measuring the resulting volt
age across it (does not work for polarised capacitors).
When troubleshooting circuit problems, a few problems are intermittent
or only show up with the working voltage applied, and are not revealed by measurements with equipment, however sophisticated, which uses low test voltages. Some problems are revealed by using a "freezer" spray and observing the effect on circuit operation. Ultimately, in difficult cases routine replacement of capacitors (relatively cheap components) is easier than arranging measurements of all relevant parameters in working conditions.
Some more specialised instruments measure capacitance over a wide range using the techniques described above, and can also measure other parameters. Low stray
and parasitic capacitance can be measured if a low enough range is available. Leakage current is measured by applying a direct voltage and measuring the current in the normal way.
More sophisticated instruments use other techniques such as inserting the capacitor-under-test into a bridge circuit
. By varying the values of the other legs in the bridge (so as to bring the bridge into balance), the value of the unknown capacitor is determined. This method of indirect uses of measuring capacitance ensures greater precision. The bridge usually can also measure series resistance and inductance. Through the use of Kelvin connection
s and other careful design techniques, these instruments can usually measure capacitors over a range from picofarads to farads. Combined LCR meter
s that can measure inductance, resistance, and capacitance are available.
Bridge circuits do not themselves measure leakage current, but a DC
bias voltage can be applied and the leakage measured directly.
Modern bridge instruments usually include a digital display and, where relevant, some sort of go/no go
testing to allow simple automated use in a production environment. As with all modern instruments, bridges can be interfaced to computer
and other equipment to export readings and allow external control.
will measure the equivalent series resistance
(but not the capacitance) of a capacitor without the need to disconnect it from circuit. Capacitors which cause circuit failure due to high ESR with capacity within tolerance, particularly in switch-mode power supplies, are common.
Electronic test equipment
Electronic test equipment is used to create signals and capture responses from electronic Devices Under Test . In this way, the proper operation of the DUT can be proven or faults in the device can be traced and repaired...
used to measure capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
, mainly of discrete capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s. Depending on the sophistication of the meter, it may display the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
only, or it may also measure a number of other parameters such as leakage, equivalent series resistance
Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated to a very good approximation as ideal capacitors and inductors in series with a resistance; this resistance is defined to be the equivalent...
(ESR), and inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
. For most purposes and in most cases the capacitor must be disconnected from circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
; ESR can usually be measured in circuit.
Simple checks without a true capacitance meter
Some checks can be made without a specialised instrument, particularly on aluminium electrolytic capacitorElectrolytic capacitor
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics"...
s which tend to be of high capacitance and to be subject to poor leakage. A multimeter
Multimeter
A multimeter or a multitester, also known as a VOM , is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance...
in a resistance range can detect a short-circuited capacitor (very low resistance) or one with very high leakage (high resistance, but lower than it should be; an ideal capacitor has infinite DC resistance). A crude idea of the capacitance can be derived with an analog multimeter in a high resistance range by observing the needle when first connected; current will flow to charge the capacitor and the needle will "kick" from infinite indicated resistance to a relatively low value, and then drift up to infinity. The amplitude of the kick is an indication of capacitance. Interpreting results requires some experience, or comparison with a good capacitor, and depends upon the particular meter and range used.
Simple and non-bridge meters
Many DVMs (digital volt meterVoltmeter
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...
s) have a capacitance-measuring function. These usually operate by charging and discharging the capacitor under test
Device under test
Device under test , also known as unit under test , is a term commonly used to refer to a manufactured product undergoing testing.-In semiconductor testing:...
with a known current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
and measuring the rate of rise of the resulting voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
; the slower the rate of rise, the larger the capacitance. DVMs can usually measure capacitances from nanofarads
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
to a few hundred microfarads, but wider ranges are not unusual.
It is also possible to measure capacitance by passing a known high-frequency alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
through the device under test and measuring the resulting volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
age across it (does not work for polarised capacitors).
When troubleshooting circuit problems, a few problems are intermittent
Intermittent Fault
An intermittent fault, often called simply an "intermittent", is a malfunction of a device or system that occurs at intervals, usually irregular, in a device or system that functions normally at other times. Intermittent faults are common to all branches of technology, including computer software...
or only show up with the working voltage applied, and are not revealed by measurements with equipment, however sophisticated, which uses low test voltages. Some problems are revealed by using a "freezer" spray and observing the effect on circuit operation. Ultimately, in difficult cases routine replacement of capacitors (relatively cheap components) is easier than arranging measurements of all relevant parameters in working conditions.
Some more specialised instruments measure capacitance over a wide range using the techniques described above, and can also measure other parameters. Low stray
Stray
Stray may refer to:* A feral domestic animal; see also estray* Areas of open grassland in Harrogate and Redcar, England* Strays of York; areas of open grassland in York- Music :* Stray , an album by Aztec Camera...
and parasitic capacitance can be measured if a low enough range is available. Leakage current is measured by applying a direct voltage and measuring the current in the normal way.
Bridges
Bridge circuit
A bridge circuit is a type of electrical circuit in which two circuit branches are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate...
. By varying the values of the other legs in the bridge (so as to bring the bridge into balance), the value of the unknown capacitor is determined. This method of indirect uses of measuring capacitance ensures greater precision. The bridge usually can also measure series resistance and inductance. Through the use of Kelvin connection
Four-terminal sensing
Four-terminal sensing , 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal sensing...
s and other careful design techniques, these instruments can usually measure capacitors over a range from picofarads to farads. Combined LCR meter
LCR meter
A LCR meter is a piece of electronic test equipment used to measure the inductance, capacitance and, resistance of a component. In the usual versions of this instrument these quantities are not measured directly, but determined from a measurement of impedance...
s that can measure inductance, resistance, and capacitance are available.
Bridge circuits do not themselves measure leakage current, but a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
bias voltage can be applied and the leakage measured directly.
Modern bridge instruments usually include a digital display and, where relevant, some sort of go/no go
Go/no go
In general go/no go testing refers to a pass/fail test principle using two boundary conditions.The test is passed only when the Go condition is met and also the No go condition fails....
testing to allow simple automated use in a production environment. As with all modern instruments, bridges can be interfaced to computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
and other equipment to export readings and allow external control.
Equivalent series resistance
While not a capacitance meter as such, a dedicated ESR meterESR meter
An ESR meter is a two-terminal electronic measuring instrument designed and used primarily to measure the equivalent series resistance of real capacitors—the ESR of an ideal capacitor is zero—usually without the need to disconnect the capacitor from the circuit it is connected to...
will measure the equivalent series resistance
Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated to a very good approximation as ideal capacitors and inductors in series with a resistance; this resistance is defined to be the equivalent...
(but not the capacitance) of a capacitor without the need to disconnect it from circuit. Capacitors which cause circuit failure due to high ESR with capacity within tolerance, particularly in switch-mode power supplies, are common.