Digital Multimeter
Multimeters are designed and
mass produced for electronics engineers. Even the simplest
and cheapest types may include features which you are not
likely to use. Digital meters give an output in numbers,
usually on a liquid crystal display.
The diagram below shows a
switched range multimeter:
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. If the meter is switched
to 20 V DC, for example, 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, the 20 V DC voltage range is the most
useful. DC ranges are indicated by
on the meter. Sometimes, you will want to measure smaller
voltages, and in this case, the 2 V or 200 mV ranges are
used.
What does DC mean? DC means
direct current. In any circuit which
operates from a steady voltage source, such as a battery,
current flow is always in the same direction. Every
constructional project described in Design Electronics works
in this way.
AC means alternating
current. In an electric lamp connected to the
domestic mains electricity, current flows first one way,
then the other. That is, the current reverses, or
alternates, in direction. With UK mains, the current
reverses 50 times per second.
For safety reasons,
you must NEVER connect a multimeter to the mains supply.
You are not at all likely to
use the AC ranges, indicated by
 ,
on your multimeter.
An alternative style of
multimeter is the autoranging multimeter:
Auto ranging multimeter
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. This type of meter is more expensive, but obviously
much easier to use.
Where are the two meter
probes connected? The black lead is always
connected into the socket marked COM, short for COMMON. The
red lead is
connected into the socket labeled V mA.
The 10A socket is very rarely used. |
