Monday, 8 July 2013

Guide:- Using a Multimeter

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 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 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.

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 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.