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Using a multimeter – voltage, current, resistance, continuity and diode test

PhysicsElectricity & CircuitsAges 16–17Virtual lab

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Use with my class

A virtual 3½-digit digital multimeter with a range dial and four sockets (10A, mA, COM, VΩ). On five test boards, students measure colour-coded resistors, voltage (in parallel) and current (in series, after removing a link) in a d.c. circuit, the a.c. voltage of a transformer and of a mains socket, check the continuity of a cable, fuses and a switch, and use the diode test to find good and faulty diodes and LEDs. The meter shows OL when over range, warns about the wrong socket, about measuring resistance in a live circuit or an ammeter in parallel, and its fuse blows when too much current flows.

Lesson: Using a multimeter: choosing the function and range, voltmeter in parallel and ammeter in series, measuring resistance, continuity testing, testing diodes and LEDs

What it shows

A digital multimeter combines a voltmeter, an ammeter and an ohmmeter. A voltmeter is connected in parallel and has a very high input resistance (10 MΩ), so it hardly changes the circuit. An ammeter is connected in series and has a very small resistance, so the red lead goes into a separate fused current socket; connected across a supply it becomes a short circuit. To measure resistance the meter passes its own small test current, so the circuit must be switched off. In the diode test a good silicon diode shows about 0.6–0.7 V forward and OL in reverse, and a red LED about 1.8–2 V.

How to use

Choose a Test board, then a Function and a Range (or tap the dial on the meter). Put the red lead in the right socket with Red lead socket, or tap a socket. Drag the probe tips onto the gold test points, or tap a point to place the probe chosen under Tap a test point to place. Remove a link before measuring current, switch the supply off before measuring resistance, and press Record reading to add the display to the table.

Parameters you can change

  • Test board 1. Three resistors, 2. D.c. circuit, 3. A.c. circuit and socket, 4. Diodes and LEDs, 5. Cable, fuses, switch
  • Function (dial) OFF, DCV, ACV, Ω, Diode, Continuity, DCA, ACA
  • Red lead socket 10A, mA, VΩ
  • Mains supply 230 V · 50 Hz, 120 V · 60 Hz
  • Show hints

Questions to explore

  1. Why must the dial start on the highest range when the size of a voltage or current is unknown?
  2. Why does an ammeter connected across a battery blow its fuse while a voltmeter does not?
  3. How can the diode test tell a good diode from an open or a shorted one?

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