ESR meter
Encyclopedia
An ESR meter is a two-terminal electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 measuring instrument
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...

 designed and used primarily to 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...

 (ESR) of real 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—the ESR of an ideal capacitor is zero—usually without the need to disconnect the capacitor from the circuit it is connected to. Most ESR meters work by applying voltage pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...

s to the capacitor under test which are too short to appreciably charge it; any voltage appearing across the capacitor is due to ohmic drop
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...

 across the ESR. An alternating voltage
AC
AC or Ac may refer to:* Air conditioning, the mechanical process of cooling air in enclosed spaces* Air conditioner, a machine that produces cool air* Anarcho-capitalism, a form of free-market anarchism...

 at a frequency at which the capacitor's reactance is negligible, in a voltage divider configuration, can also be used.

Other types of meter, including normal capacitance meters, cannot be used to measure capacitor ESR, although a few meters are available which measure both ESR and out-of-circuit capacitance. A normal (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...

) milliohmmeter cannot be used to measure ESR, as a steady direct current cannot be passed through the capacitor.

An ESR meter can always be used to measure low non-inductive 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
Electrolytic 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 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; much modern equipment, in particular switch-mode power supplies, is very sensitive to ESR, and a capacitor with high ESR can cause equipment to malfunction, or destroy it. This type of capacitor is very often used as it is inexpensive and has a very high capacitance per unit volume or weight; typically these capacitors have capacitance from about one microfarad to tens of thousands of microfarads. Increase of ESR is rarely a problem with other types of capacitor.

Accurate measurement is rarely necessary, and any meter is adequate for troubleshooting.

Principles of operation

Most ESR meters work by discharging a real electrolytic capacitor (essentially equivalent to a perfect capacitor in series with an unwanted resistance, the ESR) and passing an electric 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...

 through it for a short time, too short for it to charge appreciably. This will produce a 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 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) displayed on a display or meter calibrated in ohms or milliohms. The process is repeated tens or hundreds of thousands of times a second. Alternatively an 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....

 at a frequency high enough that the capacitor's reactance
Reactance
Reactance is the opposition of a circuit element to a change of electric current or voltage, due to that element's capacitance or inductance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current...

 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, covering the aluminium capacitors whose ESR tends to become unacceptably high. The ESR considered generally acceptable depends upon capacity (higher capacity affords lower ESR), and may be read from a table of typical values or compared with a new component. In principle the manufacturer's upper limit for ESR can be looked up in a datasheet, but this is usually unnecessary. When a capacitor whose ESR is critical degrades, power dissipation through the higher ESR usually causes rapid and large runaway increase, so go/no-go measurement is usually good enough as the ESR rapidly moves from a clearly acceptable to a clearly unacceptable level; an ESR of over a few ohms (less for a large capacitor) being unacceptable.

In a practical circuit, the ESR will be much lower than any other resistance across the capacitor, so it is not necessary to disconnect the component—in-circuit measurement. The meter is designed to have across its terminals a voltage too low to switch on any semiconductor diode that may be present in the circuit.

It is easy to check ESR well enough for troubleshooting with 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 using a little mathematics.

Other uses

An ESR meter is more accurately described as a pulsed or high-frequency AC milliohmmeter (depending upon type), and can be used to measure any low resistance. Depending upon the exact circuit used, it may be used to measure the internal resistance
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 resistance is termed the internal resistance of the source. When the power source delivers current, the measured...

 of batteries
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

 (batteries end their life largely due to increased internal resistance, rather than low EMF
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

. An ESR meter with back-to-back protective diodes across its input cannot be used for batteries), contact resistance of switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

es, the resistance of sections of printed circuit
Printed circuit
Printed circuit may refer to:* Printed circuit board* Printed Circuit Corporation, an electronics manufacturer...

 (PCB) track, etc.

While there are specialised instruments to detect short-circuits between adjacent PCB tracks, an ESR meter can be used as it can measure low resistances and uses 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 ohmmeter
Ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters make low resistance measurements. Megohmmeters measure large values of resistance...

s and 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...

s are not usable for very low resistances; those that are often use too high a voltage.

Tweezer probes are useful when test points are closely spaced as they are held in one hand, leaving the other free.

Limitations

  • An ESR meter does not measure the capacity of a capacitor; the capacitor must be disconnected from the circuit and measured with a capacitance meter
    Capacitance meter
    A capacitance meter is a piece of electronic test equipment used to measure capacitance, mainly of discrete capacitors. 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...

     (or multimeter with this capability. Most do not also measure ESR). Excessive ESR is far more likely to be a troubleshooting problem with aluminium electrolytics than out-of-tolerance capacity, 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; an ohmmeter or multimeter will easily detect this case, much rarer in practice than high ESR. It is possible to connect the test probe
    Test probe
    A test probe is a physical device used to connect electronic test equipment to the device under test . They range from very simple, rugged devices to complex probes that are sophisticated, expensive, and fragile.- Voltage probes :Voltage probes are intended to measure voltages on the DUT...

    s 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 and voltage may test as good if measured cold and unpowered. Some circuit faults due to such capacitors can be identified with freezer spray; if cooling the capacitor restores correct operation, it is faulty.

  • An ESR meter connected to a capacitor with significant voltage across it, either because of stored charge or in a live circuit, may be damaged; protective diodes across the input will minimise this risk (but the meter can no longer be used to measure battery internal resistance).

  • When used as a milliohmmeter any significant 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...

     present between the test probes will make measurements with an ESR meter meaningless. For example, an ESR meter is unsuitable for measuring the resistance of transformer
    Transformer
    A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

    windings. This effect is significant enough that probes with coiled cords should not be used due to their inductance.

External links

  • http://ludens.cl/Electron/esr/esr.html
  • http://kakopa.com/ESR_meter/index.html
  • http://octopus.freeyellow.com/esr.html
  • http://schematronics.blogspot.com/2008/07/esr-meter.html
  • http://members.shaw.ca/swstuff/esrmeter.html
  • http://members.multimania.co.uk/leeedavison/misc/esr/index.html
  • http://kripton2035.free.fr/esr-repository.html
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