A
multimeter is an essential tool for anyone who does anything using
electronics. Within this guide we will share all the ins and outs of
using a multimeter and you can also expand view technical information
about how the readings are taken.
DC Voltage
DC
Voltage is one of the most common uses of a multimeter. This is used
for anything from checking the voltage at a certain part of a circuit to
testing a battery.
DC Voltage is displayed a few different ways on multimeters but usually either VDC or DCV is used.
Selecting
a scale - The scale that you select will read up to the stated voltage
(A scale of 20V would read from -20V to 20V). Most multimeters give you a
choice of scales which you can use. An average multimeter may contain
the following scales 200m, 2000m, 20, 200 and 1000. The scale that you
choose for your reading will need to cover the voltage range that you
are using but the lower scales will provide you with a higher level of
accuracy.
For example if you were testing a 1.5V AA battery you
would be best using the 2000m scale. This scale will read in mV
(millivolts) so 1.5V would be displayed as 1500. This scale will display
a maximum of 2000 which would be 2V.
If however you were testing a
9V PP3 battery you would need to use a higher scale. The 20 scale which
would read up to 20V would be the scale to use.
Polarity - The
polarity isn't usually the greatest issue with DC Voltage as if you get
the test leads the wrong way round all that will happen is that the
screen will display a minus symbol and the voltage.
Resistance
Resistance
is a really useful function of a multimeter for a great many reasons.
We often use a multimeter on the resistance scale to check the value of a
resistor or as an easy method for checking continuity (Some multimeters
have a continuity function).
Selecting a scale - The scale that
you select for the resistance will have an effect on the accuracy of
your readings. The scales will read from 0ohms to the value of the scale
that you have selected. You should aim to use a scale where the
expected resistance is between the scale below and the scale value.
Range - Use for resistors between200ohm - 0-200ohm
2000ohm - 200ohm to 2000ohm
20k - 2k to 20k
200k - 20k to 200k
2000k - 200k to 2000k
AC Voltage
AC Voltage is the measurement scale that you would use if you are testing mains electrical equipment and supplies.
I
cannot emphasise how important it is to be very careful while doing
this. Mains electricity can kill you, don't touch any of the metal parts
of you test leads while testing this. If your test leads or multimeter
have any signs of damage DO NOT USE THEM TO TEST AC VOLTAGE.
Testing
AC voltage is done in a very similar way to testing DC voltage. With AC
polarity is not an issue as the polarity is changing 50 or 60 times per
second depending upon where you are in the world.
AC voltage is
indicated on a multimeter by one of the following the letters AC, the
letters ACV or a wavy line (Looks like the letter S on its side). *The
wavy line is actually to represent a sine wave.
Choosing the right scale
With
AC it is very rare that you will have more than two scales on your
multimeter. As with all other settings on the multimeter the value that
you choose is the maximum value.
For UK power which is between 220
and 240V you would want a value over 250V. On the multimeter I have in
front of me the scale would be 750V.
For US power which is between
110 and 120V you would want a value over 130V. On the multimeter that I
have in front of me that would be 200V.
Current
Current
is something that you may from time to time need to measure but it is a
bit more tedious than all of the other measurements that the multimeter
takes as the multimeter needs to be in series with the equipment rather
than in parallel.
To test current you may find a set of crocodile clip leads useful as you have to break into the circuit.
The
majority of lower to mid range multimeters will only test current on DC
but higher end ones may have AC current as a feature. Current is
indicated on a multimeter with either the letter A or the letters DCA.
If you cant see either of these look for the numbers with A or mA after
them as they will be a current scale.
When measuring current it is
important to remember that the connections will be fused. One of the
connectors will probably be fused at a higher rating than the other. If
you have a rough idea of the current that the device will be drawing you
may be able to use the lower rating connection for higher accuracy. If
you don't know the current rating it would be advisable to start off
with the higher rated connection.
To use the higher rated
connection swap your red lead into the connector and leave the black
lead plugged into the black connector.
Choosing the scale
The
multimeter that I have in front of me uses a 10A fuse and the lead is
plugged into that socket. To use that scale I switch the dial on the
multimeter to 10A in the DCA area.
If the reading is zero or below
the stated value of the lower fuse, in my case 200mA (0.2A) I would
swap the red lead back and start to reduce the scale until I got a
useful reading.
hFE
This is a scale for
testing transistors. In all honesty I have never actually used this
feature as I usually take the stated value on the data sheet of the
transistor as a given. I am just going to give you the basics of this
scale.
Every transistor PNP or NPN has a rating called hFE which is the current gain of the transistor.
To
test a transistor for its hFE value using your multimeter work out if
you have a NPN or PNP transistor. Identify the pins (C = Collector, B =
Base, E = Emitter) and put them into the holes in the test socket. The
socket for testing for hFE will be round with several holes in it. Each
of the holes will be labeled to tell you which pin is which. Switch on
the multimeter and the value will be displayed.
Frequency
Frequency
is a feature that some multimeters have on them which can be quite
handy when working with timers, oscillators and radio circuits.
The frequency will be displayed in Hz.
Frequency
is sometimes indicated on the multimeter by a square wave symbol
however some multimeters include a square wave generator so it is
essential to check the instructions of your multimeter before using it.
Audible Continuity/Diode
Some
multimeters have audible continuity on them which is indicated by a
series of curved lines to indicate a sound wave. This is a handy feature
to have when testing tracks on circuit boards as if there is a beep
there is a connection, if there isn't there is no connection.
This is often combined with a diode tester which will allow you to work out the polarity of a diode.
