Multimeters

The digital multimeter measures voltage, current, resistance and more. Learn counts and digits, reading accuracy specs, true-RMS, input impedance and loading, burden voltage, 4-wire resistance, and CAT safety ratings.

The digital multimeter (DMM) is the first instrument every engineer picks up: voltage, current, resistance, continuity, diode test, and often capacitance, frequency and temperature, all in one handheld box.

Getting correct readings — and staying safe — requires understanding its accuracy specification, how it loads the circuit, and which measurement categories it's rated for.

An analogy

A multimeter is a Swiss-army knife for electrical measurement: incredibly useful, but you must pick the right blade (function and range) and know its limits.

Using each function

The connection method depends on the quantity.

• Voltage: in parallel across the component.
• Current: break the circuit and put the meter in series; use the correct input jack (mA or A) — the fuses protect the meter.
• Resistance/continuity/diode: circuit de-energised; the meter supplies a small test current.
• Always start on the highest range (or auto-range) when unsure.

Reading an accuracy spec

A spec like ±(0.5 % + 2) means 0.5 % of the reading plus 2 counts of the last digit. On a 6.000 V range reading 5.000 V: 0.025 + 0.002 = ±0.027 V. Counts matter most at low readings — use the lowest range that fits.

True RMS

Average-responding meters measure the rectified average and scale by 1.11 — correct only for sine waves. With distorted waveforms (VFD outputs, SMPS input current) they can read 30–50 % low. True-RMS meters compute the actual RMS within a crest-factor and bandwidth limit.

Loading, burden and 4-wire resistance

A 10 MΩ voltmeter loads high-impedance circuits; the current range's shunt adds burden voltage that can upset low-voltage circuits. For low resistance, 2-wire test-lead resistance (≈ 0.1–0.3 Ω) dominates; 4-wire (Kelvin) measurement uses separate force and sense leads.

Safety categories

CAT ratings describe the transient overvoltages a meter withstands: CAT II (appliances/outlets), CAT III (distribution boards, fixed installations), CAT IV (utility supply, meters, outdoor lines). A CAT III 1000 V meter is required for work on building distribution; fused current inputs with high-breaking-capacity fuses prevent explosive failures.

Formulas

• Uncertainty: ± (% of reading × reading + counts × resolution)
• Voltmeter loading: V_read = V_th · R_in / (R_th + R_in)
• Ammeter burden: V_burden = I × R_shunt
• Resolution: resolution = range / counts

Worked example: Accuracy of a reading

Given: Range 6.000 V (1 mV resolution); Spec ±(0.5 % + 2); Reading 5.000 V

Find: True-value interval.

• 0.5 % × 5.000 = 0.025 V
• 2 counts = 0.002 V
• ±0.027 V → 4.973 V to 5.027 V

Answer: 5.000 ± 0.027 V

Worked example: Voltmeter loading

Given: 10 V across two 1 MΩ resistors in series; Meter R_in = 10 MΩ across the lower resistor

Find: Reading vs true 5 V.

• 1 MΩ ∥ 10 MΩ = 0.909 MΩ
• V_read = 10 × 0.909/(1 + 0.909) = 4.76 V
• Error −4.8 % caused by the meter itself

Answer: 4.76 V

Common mistakes

• Wrong: Leaving the test lead in the A jack and measuring voltage. — Right: That shorts the source through the shunt — blowing the fuse or worse. Check jacks before every measurement.
• Wrong: Measuring resistance in a powered circuit. — Right: De-energise and discharge capacitors first; the meter's test current is tiny and external voltage corrupts the reading.

Applications

• Electrical installation: Verifying supply voltage, continuity of protective conductors and absence of voltage before work (with a proven tester).
• Electronics debugging: Checking rails, bias points, shorts and component values.
• Maintenance: Clamp-meter current, true-RMS voltage on VFDs, and insulation checks (with a dedicated insulation tester).

Key takeaways

• Voltage in parallel, current in series, resistance de-energised.
• Uncertainty = % of reading + counts; use the lowest range that fits.
• True RMS for non-sinusoidal signals; beware loading and burden.
• Match the CAT rating to the installation; 4-wire for low resistance.

Going further

• Bench DMMs and integration time: 6½-digit bench meters trade speed for noise via integration time in power-line cycles (NPLC); integrating over whole cycles rejects 50/60 Hz pickup.
• Thermal EMFs: At µV levels, dissimilar-metal junctions generate thermoelectric voltages; offset-compensated resistance modes reverse the current to cancel them.
• Low-impedance (LoZ) mode: LoZ mode loads the circuit (~3 kΩ) to dissipate 'ghost voltages' capacitively coupled onto de-energised wires, avoiding false live readings.