Topic Menu

sing a Digital Multimeter

Even the simplest and cheapest types of multimeter may include features which you are not likely to use, as they have been designed and mass produced for electronics engineers.

Digital meters give a numeric output, usually on a liquid crystal display.

There are two types that you may come across in practical sessions:

The switched range multimeter:

The central knob has lots of positions and you must choose which one is appropriate for the measurement you want to make.

You have to connect a black probe lead into the socket marked COM (which is short for COMMON - as it is the connection that has to be made for whatever use the meter is being employed for).

A red probe lead is connected into the socket labelled V=ΩmA.

(The 10 ADC socket is very rarely used).

Measuring DC voltages

What does DC stand for?

- no, not Detective Comics - DC stands for direct current.

In any circuit which operates from a steady voltage source, such as a battery, current flow is always in the same direction. At GCSE most circuits you will work with will be DC circuits.

There are several DC ranges within the DC section of the meter - indicated by (a flat, steady voltage line).

If the meter is switched to 20 V DC, then 20 V is the maximum voltage which can be measured, This is sometimes called 20 V fsd, where fsd is short for full scale deflection.

For circuits with power supplies of up to 20 V, which includes all the circuits you are likely to build at GCSE level, the 20 V DC voltage range is the most useful.

Sometimes, you will want to measure smaller voltages, and in this case, the 2 V or 200 mV ranges are used so you can get a more sensitive reading.

Measuring AC voltages

What does AC stand for?

AC stands for alternating current.

In any appliance connected to the domestic mains electricity, current flows first one way, then the opposite through its circuitry - the current reverses, or alternates, in direction.

In the States the current reverses 50 times per second - it has a frequency of 60 Hz; whereas UK mains has a frequency of 50 Hz, the current reverses 50 times per second.

For safety reasons, you must NEVER connect a multimeter to the mains supply.

AC is indicated by , on your multimeter but you are not likely to use the AC ranges at GCSE Level.

The autoranging multimeter:

This is a simpler form - the central knob has fewer positions, and all you need to do is to switch it to the quantity you want to measure.

Once switched to V, the meter automatically adjusts its range to give a meaningful reading, and the display includes the unit of measurement, V or mV.