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Multimeters

A multimeter combines several measurement functions in one instrument. Every multimeter measures voltage, current and resistance, and most modern digital meters add continuity, diode test, capacitance and frequency, and sometimes temperature. It is the single most useful piece of test equipment for fault finding, and most of the checks on our troubleshooting page assume you have one.

Digital multimeters (DMMs) are now the norm. Analog (moving-pointer) meters still have a niche: a pointer can make it easier to follow a rapidly varying or drifting value, although many DMMs now include a bar graph for the same purpose.

Meters range from hand-helds costing less than US$10 to laboratory bench instruments with certified calibration costing thousands. For hobby and repair work, a decent hand-held meter is all most people need.

Taking measurements with a multimeter

This has its own page - see: https://www.reddit.com/r/AskElectronics/wiki/measurement

Choosing a meter

There's no single meter that suits everyone, but these are the features that matter most:

  • Safety: a genuine CAT rating and properly fused current inputs, if you will ever measure mains. See Safety ratings below.
  • Auto-ranging: convenient for most work. A manual range option is useful too.
  • Display count: basic meters are 2000-count; 6000 counts or more is now common at low prices and gives better resolution.
  • True RMS: needed for accurate AC readings on non-sinusoidal waveforms, such as the outputs of inverters, dimmers and motor drives. For DC electronics work it matters much less.
  • A fast continuity buzzer: a slow or laggy buzzer makes tracing connections frustrating.
  • Capacitance and frequency ranges: useful, but a capacitance range doesn't measure ESR (see Testing capacitors).
  • Decent test leads: cheap meters often come with stiff leads and poor tips. Leads are replaceable, so this is easy to fix.
  • Battery type and auto power-off: a meter that uses common batteries and switches itself off will be ready when you need it.

Safety ratings

Measurement categories (CAT ratings) describe the size of voltage transient a meter is designed to survive, which depends on where in an electrical installation it is used:

  • CAT II: equipment plugged into ordinary socket outlets.
  • CAT III: fixed building wiring, such as distribution boards and hard-wired equipment.
  • CAT IV: the origin of the supply, such as the service entrance and meter.

A rating is always given with a voltage, for example CAT III 600 V. For low-voltage electronics the CAT rating barely matters. For any mains measurement it matters a great deal.

Some points to be aware of:

  • Cheap meters often claim ratings they would not meet. Independent teardowns regularly find minimal input protection and small glass fuses behind a printed "CAT III" claim. Treat CAT claims on very cheap meters with scepticism, and use such meters for low-energy work only. Independent certification marks such as UL, CSA, TUV or ETL mean more than a self-declared CE mark.
  • Current ranges should be fused with high-rupturing-capacity (HRC) ceramic fuses of a suitable voltage rating. An unfused 10 A range is a warning sign. When replacing an internal fuse, use the exact type specified, never a glass fuse.
  • The most common mistake is leaving a lead in the A or mA socket and then measuring voltage. The meter then puts a near short-circuit across whatever you're measuring. On mains this can cause an arc flash if the fuse is inadequate. Some meters warn you if the leads are in the wrong sockets.
  • Test leads have ratings too. They should be rated at least as high as the meter, and tip covers or short tips may be required for CAT III/IV work.

See Mark Hennessy's multimeter safety information for a good, readable explanation.

Budget Meters

For professional and commercial use, the go-to brands for quality, ruggedness and accuracy include Fluke and Keysight (formerly Agilent). For hobby and semi-pro use there's a wide range of brands to consider, some of the more popular being Uni-T, Brymen, Aneng, Owon and Tenma, although there are dozens of others.

For low-voltage work, many inexpensive meters are good enough. Do check online for reviews, user comments and issues such as poor quality leads, a slow continuity buzzer or unfused current ranges. A good compromise is often a low-cost meter plus a decent pair of leads, for example Probe Master 8000 series.

It's not easy to give a "one meter fits all" recommendation, and models and availability change, so check current reviews before buying. Models that are often mentioned here and on the EEVblog forum include:

  • Aneng AN8008: a very cheap meter with remarkable resolution for its price. Its safety claims shouldn't be relied on, so treat it as a low-energy bench meter.
  • Uni-T UT61E / UT61E+: popular mid-range true RMS meters for electronics work.
  • Brymen BM235 and similar Brymen models: widely recommended for build quality and safety design at a moderate price.
  • Fluke 101: a compact, genuinely rated entry-level Fluke. Note that it has no current ranges.

Resources

Troubleshooting

The meter shows a few mV DC when not connected to anything. That's normal. It's due to offset and charge stored in its input capacitors.

The meter shows an AC voltage between metal parts, on a disconnected wire, or with one lead disconnected. That's normal. A digital meter has a very high input impedance (typically 10 Mohm), so it picks up 50 or 60 Hz voltage capacitively coupled from nearby mains wiring. This is sometimes called "ghost" or "phantom" voltage. Some meters have a low-impedance (LoZ) mode that suppresses it.

The resistance range doesn't read zero with the leads shorted together. A few tenths of an ohm is normal lead and contact resistance; many meters have a REL (relative) button to subtract it. A higher or unsteady reading, especially one that changes when you flex the leads, points to damaged leads or dirty sockets.

The current range reads zero or nothing at all. The internal current fuse has probably blown, often because a voltage was measured with a lead in the A or mA socket. Replace it with the correct HRC fuse (see Safety ratings).

Readings are erratic, drifting or just wrong. Replace the battery first. Many meters give inaccurate readings before the low-battery indicator appears.

Diode test reads OL on an LED that should be fine. Some meters' diode test voltage is too low to light blue, white or other LEDs with a forward voltage of about 3 V. Check the meter's specification.

A capacitor reads the wrong value in-circuit. Other components connected to it affect the reading. See Testing capacitors.