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ESR meter

High-end ESR Meter by BK Precision An ESR meter is a two-terminal electronic measuring instrument designed and used primarily to measure the equivalent series resistance (ESR) 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. Most ESR meters work by applying voltage pulses to the capacitor under test which are too short to appreciably charge it; any voltage appearing across the capacitor is due to ohmic drop across the ESR. An alternating voltage 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) 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.

Contents


Need for ESR measurement

Aluminium electrolytic capacitors 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, 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[1]. This type of capacitor is very often used because 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. High and increasing ESR is rarely a problem with other types of capacitor[2].

Capacitors with faults leading to high ESR often bulge and leak, making them easy to identify visually; however, capacitors that appear perfect may have high ESR, detectable only by measurement.

Accurate measurement is rarely necessary, and any meter is adequate for troubleshooting. Where accuracy is required, measurements must be taken under appropriate conditions, as ESR varies with frequency, applied voltage, and temperature. A general-purpose ESR meter operating with a fixed waveform is unlikely to be suitable.

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 through it for a short time, too short for it to charge appreciably. This will produce a voltage 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 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, 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. Practical meters have across their terminals a voltage too low to switch on any semiconductor junctions that may be present in the circuit and present a low "on" impedance which interferes with measurements.

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[3].

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 of batteries (batteries end their life largely due to increased internal resistance, rather than low EMF. An ESR meter with back-to-back protective diodes across its input cannot be used for batteries), contact resistance of switches, the resistance of sections of printed circuit (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 ohmmeters and multimeters 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 (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 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 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 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 windings. This effect is significant enough that probes with coiled cords should not be used due to their inductance.

References

  1. 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)."
  2. Murata: Basics of capacitors, lesson 2 Includes graph showing impedance as a function of frequency for different capacitor types; electrolytics are the only ones with a large component due to ESR

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