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An ESR meter is a two-terminal
electronic
Electronic may refer to:
*Electronics, the science of how to control electric energy in semiconductor
* ''Electronics'' (magazine), a defunct American trade journal
*Electronic storage, the storage of data using an electronic device
*Electronic co ...
measuring instrument
A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Es ...
designed and used primarily to measure the
equivalent series resistance (ESR) of real
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of a ...
s; usually without the need to disconnect the capacitor from the circuit it is connected to. Other types of meters used for routine servicing, including normal
capacitance meters, cannot be used to measure a capacitor's ESR, although combined meters are available which measure both ESR and out-of-circuit capacitance. A standard (
DC) milliohmmeter or multimeter cannot be used to measure ESR, because a steady
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
cannot be passed through the capacitor.
Most ESR meters can also be used to measure non-inductive low-value resistances, whether or not associated with a capacitor; this leads to a number of additional applications described below.
Need for ESR measurement
Aluminium
electrolytic capacitor
An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel ...
s have a relatively high ESR that increases with age, heat, and
ripple current; this can cause the equipment using them to malfunction. In older equipment, this tended to cause hum and degraded operation; modern equipment, in particular
switch-mode power supplies
A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
Like ...
, is very sensitive to ESR, and a capacitor with high ESR can cause equipment to malfunction or cause permanent damage requiring repair, typically by causing power supply voltages to become excessively high.
Example of high-ESR capacitors causing voltages to rise in a circuit and destroy components.
High ESR capacitors cause "5V dropping quite low and causing every other voltage to go sky high (and doing things like frying the HDD with upwards of 15V rather than 12V, and frying the tuning agc transistor with upwards of 36V instead of 30V)." Electrolytic capacitors are, nevertheless, very often used because they are inexpensive and have a very high capacitance per unit volume or weight; typically, these capacitors have capacitance from about one microfarad
The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI). It is named after the English physicist Michael Faraday (1791–1867). In SI base uni ...
to tens of thousands of microfarads.
Capacitors with faults leading to high ESR often overheat and thereafter bulge and leak as the electrolyte chemicals decompose into gases, making them somewhat easy to identify visually; however, capacitors that appear visually perfect may still have high ESR, detectable only by measurement.
Precise measurement of ESR is rarely necessary, and any usable meter is adequate for troubleshooting. When precision is required, measurements must be taken under appropriately specified conditions, because ESR varies with frequency, applied voltage, and temperature. A general-purpose ESR meter operating with a fixed frequency and waveform will usually be unsuitable for precise laboratory measurements.
Methods of ESR measurement
Measuring ESR can be done by applying an alternating voltage at a frequency at which the capacitor's reactance is negligible, in a voltage divider
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the i ...
configuration.
It is easy to check ESR well enough for troubleshooting by using an improvised ESR meter comprising a simple square-wave generator and oscilloscope, or a sinewave generator of a few tens of kilohertz and an AC voltmeter, using a known good capacitor for comparison, or by using a little mathematics.
A professional ESR meter is more convenient for checking multiple capacitors in rapid succession.
A standard measurement bridge, and many LCR and Q meters, can also measure ESR accurately, in addition to many other circuit parameters. The dedicated ESR meter is a relatively inexpensive special-purpose instrument of modest accuracy, used mainly to identify capacitors with unacceptably large ESR and sometimes to measure other low resistances; measurements of other parameters cannot be made.
Principles of ESR meter operation
Most ESR meters work by discharging a real electrolytic capacitor (more or less equivalent to an ideal capacitor in series with an unwanted resistance, the ESR) and passing an electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movin ...
through it for a short time, too short for it to charge appreciably. This will produce a volt
The volt (symbol: V) is the unit of electric potential, electric potential difference ( voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
D ...
age across the device equal to the product of the current and the ESR plus a negligible contribution from a small charge in the capacitor; this voltage is measured and its value divided by the current (i.e., the ESR) shown in ohms or milliohms on a digital display or by the position of a pointer on a scale. The process is repeated tens or hundreds of thousands of times a second.
Alternatively an alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
at a frequency high enough that the capacitor's reactance is much less than the ESR can be used. Circuit parameters are usually chosen to give meaningful results for capacitance from about one microfarad up, a range that covers typical aluminium capacitors whose ESR tends to become unacceptably high.
Interpretation of readings
An acceptable ESR value depends upon capacitance (larger capacitors usually have lower ESR), and may be read from a table of "typical" values, or compared with a new component. In principle, the capacitor manufacturer's upper limit specification for ESR can be looked up in a datasheet, but this is usually unnecessary. When a capacitor whose ESR is critical degrades, power dissipation as the ESR increases usually causes a rapid and large runaway increase, so go/no-go measurement is usually good enough as the ESR often rapidly moves from a clearly acceptable to a clearly unacceptable level; an ESR of over a few ohms (less for a large capacitor) is unacceptable.
In a practical circuit, the ESR will be much lower than any other resistance in parallel with the capacitor, so it is not necessary to disconnect the component, and an in-circuit measurement can be made. Practical ESR meters use a voltage too low to switch on any semiconductor junction
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
s that may be present in the circuit; this might present a low "on" impedance that would interfere with measurements.
Limitations
* An ESR meter does not measure the capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized a ...
of a capacitor; the capacitor must be disconnected from the circuit and measured with a capacitance meter (or a multimeter with this capability). Excessive ESR is far more likely to be an identifiable problem with aluminium electrolytics rather than out-of-tolerance capacitance, which is rare in capacitors with acceptable ESR.
* A faulty short-circuited capacitor will incorrectly be identified by an ESR meter as having ideally low ESR, but an ohmmeter or multimeter can easily detect this case, which is much rarer in practice than high ESR. It is possible to connect the test probes to an ESR meter and ohmmeter in parallel to check for both shorts and ESR in one operation; some meters both measure ESR and detect short-circuits.
* ESR may depend upon operating conditions (mainly applied voltage, temperature); a capacitor which has excessive ESR at operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
and voltage may test as good if measured cold and unpowered. Some circuit faults due to such intermittent capacitors can be identified by using freeze spray; if cooling the capacitor restores correct operation, it is faulty.
* An ESR meter can be damaged by connection to a capacitor with significant voltage across it, either because of residual stored charge or in a live circuit. Protective diodes across the input will minimise this risk, but then the meter can no longer be used to measure battery internal resistance.
* When an ESR meter is used as a milliohmmeter, any significant inductance
Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of t ...
present between the test probes will make measurements meaningless. For example, an ESR meter is unsuitable for measuring resistance in transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
windings because of their inductive characteristics. This effect is significant enough that test probes with coiled cords should not be used due to their inductance.
Other uses of ESR meters
An ESR meter is more accurately described as a pulsed or high-frequency AC milliohmmeter (depending upon type), and it can be used to measure any low resistance. An ESR meter with no back-to-back protective diodes across its input can measure the internal resistance
A practical electrical power source which is a linear electric circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resistance of the source. ...
of batteries
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
(many batteries end their useful life largely due to increased internal resistance, rather than low EMF). Depending upon the exact circuit used, an ESR meter may also be used to measure the contact resistance of switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...
es, the resistance of sections of printed circuit
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich stru ...
(PCB) track, etc.
While there are specialised instruments to detect short-circuits between adjacent PCB tracks, an ESR meter is useful because it can measure low resistances while injecting a voltage too low to confuse readings by switching on semiconductor junctions in the circuit. An ESR meter can be used to find short-circuits, even finding which of a group of capacitors or transistors connected in parallel by printed circuit tracks or wires is short-circuited. Many conventional ohmmeters and multimeter
A multimeter is a measuring instrument that can measure multiple electrical properties. A typical multimeter can measure voltage, resistance, and current, in which case it is also known as a volt-ohm-milliammeter (VOM), as the unit is equipped w ...
s are not usable for very low resistances, and those that are often use too high a voltage, risking damage to the circuit being tested.
Tweezer probes are useful when test points are closely spaced, such as in equipment made with surface mount technology
Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referre ...
. The tweezer probes can be held in one hand, leaving the other hand free to steady or manipulate the equipment being tested.
History
The first major device to measure in-circuit ESR was based on Carl W. Vette's [Carl W. Vette (1978)]
"US Patent #4216424: Method and apparatus for testing electrolytic capacitors "
/ref> under the Creative Electronics brand. The Creative Electronics ESR meter was the primary device many used for the duration of the patent. The patent expired in 1998, when many other companies entered the market.
Additional patents extended the original work, including John G. Bachman's 2001 [John G. Bachman (2001)]
"US Patent #6677764: System for protecting electronic test equipment from charged capacitors"
/ref>
See also
* Q meter
* LCR meter
References
{{Reflist
Electronic test equipment