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In this technical article, we will cover the basic principles of how a multi-meter works and how to use a multi-meter in your day-to-day affairs.  We will cover this article in relatively basic terms to provide a simple overview on the topic.  Where applicable, we will provide more detailed information on very important topics.

Tools / Resources: Consumables:
  • digital or analog multi-meter
  • short length of wire

If you do not already own a multi-meter, we highly recommend you buy a digital multi-meter - they are very easy to use and provide very precise data.  If you have the option, buy the best multi-meter you can afford - preferably one which can automatically determine what "scale" or multiplier values to use for resistance (Ω) or voltage (V).  If you have any doubts, have a look at the multi-meter we use in this article and ensure you purchase a digital multi-meter with similar functions.

This is your typical full-feature digital multi-meter.  Before you use this tool, it is best to familiarize yourself with the selection dial.

In this case, the area surrounded with a red border is concerned with current in terms of amperage (A) Amperes.  The orange portion is in terms of voltage (V) in Volts.  Finally, the area surrounded by a black border deals with resistance (Ω) in Ohms.

If you are not familiar with the metric system, some of the symbols will look strange.  If you are interested, we have created a "metric primer" to help you make the conversion.

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To begin using the digital multi-meter, you must place the test probes in their proper receptacles.  The common probe (black) is placed into the COM (common) position.

You will notice that the selection dial is pointing within the resistance section - in particular, the dial is currently pointing to the continuity position.  In this position, when continuity (any resistance) is detected, the digital multi-meter will emit an electronic bell sound.

To start this tutorial, we will cover one of the most useful functions of the multi-meter - measuring resistance.

We have placed the live probe (red) into the socket marked A / bell / Ω / diode.  In this case, we are only really concerned with the Ω function which is the Omega symbol for Ohms (resistance).

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With the digital multi-meter in the continuity position, we will turn the digital multi-meter on.

For ease of demonstration, you will notice a symbol in the upper right corner of each picture - this is a close up of the symbol that the selector dial is currently set to.

When the digital multi-meter first powers up, all possible LCD (liquid crystal display) "bits" will show up - this is normal.  Be sure to watch the power-up routine to identify any symbols that will not illuminate - this may effect your readings.

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Once the unit powers up normally, we will close the circuit that the two test probes make by touching the metal leads together.

Because the digital multi-meter is set to continuity mode, as soon as the two metal probes touch each other, the unit immediately makes the electronic tone indicating continuity is observed.

This is a good test procedure to observe before actually using the continuity function - if the multi-meter does not emit the tone, you may have a malfunction in the digital multi-meter.

Before we show a practical application of the continuity test function, we will show you an invaluable set of diagnostic tools.  Shown in figure seven are two wires with shielded alligator clips on either end.  These "jumper cables" are an incredibly useful addition to any toolbox - we highly recommend you buy a full set of these jumper wires.

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Although the simple circuit in this figure appears to be complicated, it is actually quite simple.  Do not be afraid.

We have connected the jumper cables to their colour coded probes respectively and then to one of two possible ends of a speaker cable.  You will notice that at the other end of the speaker cable, the wires are not touching each other.

Because the very small charge produced by the digital multi-meter is not able to "jump" across the exposed wires at the "open" (not-connected) end of the wire, we observe infinite resistance on the multi-meter.  The infinite resistance condition is denoted by 0.L (zero point L) on the display.  No sound is heard.

When we connect the bare ends of the wire together, the small electric charge is able to pass from the live probe (red) to the common probe (black).

As the electric charge passes through the wire, a loss in current is the result of resistance in the wire.  Once the digital multi-meter calculates the difference in current (at a constant voltage), the multi-meter is able to display a resistance value.

When the bare wires are connected, the digital multi-meter emits the continuity sound and displays the circuit's resistance which in this case is 0.6Ω (almost one half an Ohm) which is small.

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Now that we can appreciate how to test for continuity we should now discuss the implications of such a test.

Imagine you are trying to install a new stereo system in your vehicle.  You pull out the factory head-unit (CD player) and discover that none of the wires are labeled - and they are all different!  How do you know what wire connects to which speaker?!  Not a problem!  We will test for continuity.

For starters, we will cut all wires (or install a wiring harness adapter) to free up the wiring harness.  Now, based on the principle that a speaker is just a coil of wire at the end of two wires, we have a complete circuit (unless your speaker is blown or the wire(s) leading to the speaker have been cut somewhere between the head-unit and the speaker).  If we have a complete (closed) circuit, we can test for continuity.

Once you have exposed a small portion of each wire at the head-unit's wiring harness, clip your jumper wire to the common (black) probe and then attach the other end of the jumper wire to one of the wires. 

Note:  Before testing continuity at this point, you would determine which wires (two) provide 12 V to prevent damaging your multi-meter.  We will cover how to test for DC (direct current) voltage later in this article.

The next step would be to touch the live (red) probe to all unknown wires until the multi-meter makes the familiar continuity tone.  If your multi-meter does not have the continuity tone, you can simply set the multi-meter to the lowest resistance test setting and watch the display and test each wire until the unit displays a value other than "infinity" or 0.L.  Once you have found continuity, you know that the two wire ends you are currently testing are connected to each other by the speaker.

Now that we can test continuity, let's cover testing for resistance (Ω).

 

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In this figure, you will notice that the symbol in the upper right corner now shows Ω - this indicates that we are testing resistance.

In this circuit, we are testing opposite ends of a speaker wire.  We will pretend that we do not have a continuity test function so we will check the resistance of the wire instead.

The display currently shows 0.LΩ which is infinity - this means that the two test probes are not touching the same wire.

When we switch the common (black) jumper wire to the other bare wire, we observe 0.5Ω.  This indicates that we now have continuity and the wire offers 0.5 Ohms of resistance.

If we know the resistance of a wire, we can determine a few other interesting or perhaps important properties.  These issues will be covered in greater detail in a physics oriented article at a later time.

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As a point of interest, when we connect the two test probes together, we observe 0.3Ω of resistance.  This indicates that there may be some corrosion on the test probes which will cause all readings which we take to be systematically too high by 0.3Ω.

If we were so inclined, to reduce the resistance offered by the test probes, we would sand the metal tips with a very fine sand paper to remove any oxidation (corrosion) and then test the probe resistance again - noting any positive value.

To simplify the idea of continuity (measured resistance) and discontinuity (infinite resistance) we have a very simple circuit shown in figures 13 and 14.

In this image, the test probe is not touching the free end of the 4 gauge wire.  You should expect to see infinite resistance in this situation - denoted by 0.L on the digital multi-meter.

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When we touch the test probe to the free end of the wire, the digital multi-meter immediately shows resistance, which in this case s 0.5Ω.  If we were to clean the test probes and the free ends of the wire, we could expect to see 0.0Ω of resistance which is NOT the same as infinite resistance.  0.0Ω simply indicates that there is resistance but it is too small for the multi-meter to indicate with any accuracy.

Except for "true super-conductors", all matter exhibits some degree of resistance - hence 0.0Ω is technically untrue.

Now for a practical application of testing resistance.  When you service your vehicle, according to the factory manual you should test the spark plug wires to ensure that the proper resistance is observed.  In this example, the digital multi-meter shows 2.796 but this is not simply 2.796Ω !  Just to the right of the numbers, you may notice the following symbol: kΩ  This means kilo-Ohms or 1'000s of Ohms.

Typically, your factory manual will list a certain number of kilo-Ohms (kΩ) per unit length of sparkplug wire.

Factory spec. for a Mazda MX-5 Miata is:

sparkplug wire:  { 16.0kΩ per 1.00m \ 16.0kΩ per 3.28' }

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So now you understand how to test resistance of circuits and how to use the measure of resistance to determine whether or not you have continuity.  Just a quick word on continuity before we go on:  think of continuity as whether or not a wire is continuous.  If the wire is continuous, it can carry electricity from one end to the other - the circuit is closed.  This wire is said to exhibit continuity.  If the wire is discontinuous, it has a break along its length and is unable to conduct electricity from one extreme to the other - the circuit is open.  This wire is said to exhibit discontinuity and has infinite resistance.

Testing the resistance of a circuit comes in very handy when you are testing a circuit and expect to see a certain value - such as in the example of the sparkplug wire resistance.  In terms of car audio, you can test the way in which you have connected two or more subwoofers to determine the over-all resistance.  This is important when you want to "bridge" your subwoofers down to a certain value such as 4Ω.  If you are not sure if you have connected the wires to the subwoofers properly, you can test the two ends which you would attach to the amplifier to determine the total circuit resistance.  If you want to know the math/physics behind resistors in series and parallel, have a look at our article which describes the physics of electricity.

Now we will cover the second most useful function of a digital multi-meter:  testing voltage.

 

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To test for direct current (DC) voltage, we need to change the position of the live (red) probe on the multi-meter.  We put the socket-end of the test probe in the socket marked V for voltage. 

Although the selector is currently set to continuity, we will test the voltage of a battery by changing the selector dial to V DC (Volts direct current).  This can be done with the digital multi-meter turned off, or even when it is turned on.

With the selector dial currently set to V DC, we will touch the two test probes to the terminals of the battery.

When we attach the digital multi-meter, the display shows 12.69V which is good for a battery which is roughly fully charged.

Notice that the live (red) probe is connected to the positive (+) terminal of the battery and the common (black) probe is attached to the negative (-) or ground terminal of the battery.

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If we reverse the terminals that the test probes are connected to, we observe a negative voltage.

The display currently shows -12.69V which simply indicates that the polarity of test probes is reversed.  This comes in handy if you need to know what terminal on a battery is positive - if the number displayed on the digital multi-meter is positive (no negative sign) then the terminals are connected in the proper polarity.  That is, the live (red) probe is touching a positive (+) terminal.  If the number shows up as negative, the reverse is true, the live (red) probe is touching a negative (-) terminal.


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