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Using a multimeter: measuring voltage, current and continuity

Properties1
tagsglossar, elektronik, werkzeug

2 min read

Using a multimeter: measuring voltage, current and continuity

In short: A multimeter measures voltage, current and resistance and checks continuity; choosing the right socket and measuring mode prevents wrong measurements and damage.

In more detail: The multimeter is the standard tool for checking power supplies, testing cables, measuring batteries, finding faults. Cheap devices (€15 to €30) suffice for hobby measurements on extra-low voltage.

In Depth

Structure and modes

  • COM (black) and VΩmA (red) for voltage, resistance, small currents; 10A socket only for large currents.
  • V⎓ (DC): direct voltage (batteries, power supplies, USB). V~ (AC): alternating voltage (mains: only with experience and a CAT-rated device).
  • Ω: resistance (only de-energised!).
  • Continuity tester (beep): connection yes/no.
  • Diode test: forward voltage (LED, diodes).
  • A: current (measure in series).
  • Choose range or auto-range; start an unknown value from the top (large range).

Step by step

  1. Measure voltage: in parallel with the component/power supply, DC mode: e.g. a 12 V power supply shows 11.8–12.5 V.
  2. Test a battery: new AA ≈ 1.5 V, empty < 1.1 V (check under load), Li-ion 3.0–4.2 V, USB 5 V.
  3. Continuity: test cable/fuse de-energised, beep = ok.
  4. Resistance: component removed or de-energised, fingers not on the tips (distorts).
  5. Current: break the circuit, meter in series (red lead in A socket); afterwards put leads back into the V socket!
  6. Mind polarity: negative sign = swapped.

Typical mistakes and dangers

  • Current measurement in V mode/socket across a voltage source = short circuit (fuse blows, in the worst case danger).
  • Resistance measurement under voltage damages the device.
  • Measuring mains voltage only with a CAT II/III device, insulated probes, one hand, never wet hands; hands off power supply innards (electrolytic capacitors charge to high voltage).
  • Check test leads (cracks), the device’s battery.
  • Accuracy: cheap devices measure small currents and AC RMS values (true RMS) poorly.
  • With power supplies: measure under load, no-load voltage deceives.

Practical examples

  • Defective power supply of a retro device: measure voltage at the plug; polarity (centre +/−) is on the rating plate.
  • Cable break: continuity per wire.
  • Controller/button: continuity when pressed.
  • Roughly check battery condition (resting voltage).
  • USB power supply: 5 V at the socket, current with a USB measuring adapter.
  • Check fuses (continuity).
  • Document (photo/note) and keep the schematic at hand. Alternative: oscilloscope for signals over time.

See also: Electronics basics, Power supplies, Batteries

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