Multimeters measure the electrical characteristics of components, like capacitance and resistance.

• Continuity
  How to tell if two points are electrically connected
  
  
• Resistance
  How to measure resistance - resistors, potentiometers and sensors
  
  
• Voltage
  How to measure voltage - battery testing, wall-wart adaptor testing, and the terror of mixed AC/DC measurements.
  
  

Continuity?

If two electronic parts are connected with a wire, they are continuous. If they are connected with cotton string, while they are connected, the cotton string is not conductive and therefore there is no continuity.

You can always use a resistance-tester (ohmmeter) to figure out if something is connected because the resistance of wires is very small, less than 100 ohms. However, continuity testers usually have a piezo buzzer which beeps. This makes them very useful when you want to poke at a circuit and need to focus on where the probes are instead of staring at the meter display.

Continuity tests:

• If your soldering is good. If your solder joint is a cold solder connection it will appear connected but actually won't be.
  
  
• If a wire is broken in the middle. Power cords and headphone cables are notorious for breaking inside the shielding, it appears as if the cable is fine but inside the wires are broken.
  
  
• If a solder joint shorts two connections.
  
  

You can only test continuity when the device you're testing is not powered.

Continuity works by poking a little voltage into the circuit and seeing how much current flows.

Before starting, always test to make sure your meter is working: test by brushing the two tips together. If you hear the beep, you are good to go.

If no beep, maybe the battery is low or its not in the right mode.

Continuity is non-directional, you can switch probes and it will be the same.

If you are testing two points in a circuit and there is a (big) capacitor between those points you may hear a quick beep and then quiet. That's because the voltage the meter is applying to the circuit is charging up the capacitor and during that time the meter 'thinks' its continuous (essentially).

Small resistors (under 100 ohms or so) and also all inductors will seem like short circuits to a multimeter because they are very much like wires.

Continuity doesn't mean "short" it just means very very low resistance. For example, if you have a circuit that draws an Amp from a 5V supply, it will appear to be a 5Ω resistor. If you measure that with your meter it will think its a short circuit, but really its just a high-drain circuit.

1. Put your multimeter into the correct mode. Look for the icon that looks sort of like a 'sound wave' Here are three examples. Turn the multimeter knob so that it points to this symbol
   
   
   

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2. Touch and go
   For a majority of multimeters, you're ready to go, just touch the tips of the probes together so that they make a beeping sound!
   
   
   
   This meter is very simple. When the probes are not touching, the display shows "1"
   
   When you touch the tips together, the display changes to a three digit mode (it's displaying resistance) It also emits a beep
   

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   This meter is dual-mode but still very easy to use. Turn the dial to the symbol. When the probes are not touching the display shows "OL" which stands for Open Loop. (Open loop is another way of saying there is no continuity)
   
   When you touch the probes, the soundwave icon shows up in the display (upper right) and it also shows a number. The number is not the resistance, actually...its the voltage (look for the V in the right hand side for Volts). This is because this mode is also a Diode Test (which will be discussed later)
   

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   This meter is triple-mode and requires an extra step to get to the continuity function. Click on the image to get a closer view of the triple-mode. After you dial to this mode you must press the Mode button, the wave icon will then appear in the display.
   
   You can see the wave icon in the top right as expected. This meter also displays OL
   
   Unlike the other meter, this one displays Ohms (see the symbol on the right of the display). The resistance is low (4.7Ohms) but not 0 (the ideal value) because the probes and wires act as resistors. Usually with these sorts of meters they will beep whenever resistance is under 100 ohms or so.
   

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   Probing a PCB

   Here is an example of testing a PCB for continuity.The first test shows that the two points are not connected.
   
   The second test shows that these two points are connected
   

Resistance

• You can only test resistance when the device you're testing is not powered. Resistance testing works by poking a little voltage into the circuit and seeing how much current flows, its perfectly safe for any component but if its powered there is already voltage in the circuit, and you will get incorrect readings
  
  
• You can only test a resistor before it has been soldered/inserted into a circuit. If you measure it in the circuit you will also be measuring everything connected to it. If you try, you will get incorrect readings and that's worse than no reading at all.
  
  
• You can make sure your meter is working well by having a 'reference resistor' to test against. A 1% 1KΩ or 10KΩ resistor is perfect! Low batteries can make your multimeter wonky.
  
  
• Resistance is non-directional, you can switch probes and the reading will be the same.
  
  
• If you have a ranging meter (as most inexpensive ones are), you'll need to keep track of what range you are in. Otherwise, you will get strange readings, like OL or similar, or you may think you're in KΩ when really you're in MΩ.

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As long as it works, it doesn't matter which type you have. But auto-ranging meters are a little slower.
Compare these two videos as I measure a 1KΩ resistor with an autoranging meter:

Which takes about 4 seconds to settle on a final value, and a 10KΩ resistor with a ranging meter:

Which gets the first significant digit instantly, the second digit after 1 second and the final digit after 2.

Expensive autoranging meters, like Fluke 73s, will be super fast so it's not a big deal.

Ranges will almost always be something like 200Ω, 2KΩ, 20KΩ, 200KΩ, 2MΩ, etc. Why the 2s instead of 100, 1K, 10K etc.?
Because the vast majority of resistors are 5%, the resistor values are 5% apart (or so). For example, the "standard" 5% values between 1K and 10K are:

1.0K, 1.1K, 1.2K, 1.3K, 1.5K, 1.6K, 1.8K, 2.0K, 2.2K, 2.4K, 2.7K, 3.0K, 3.3K, 3.6K, 3.9K, 4.3K, 4.7K, 5.1K, 5.6K, 6.2K, 6.8K, 7.5K, 8.2K, 9.1K

There are way more values between 1KΩ and 2KΩ than between 2KΩ and 3KΩ, etc. By picking 2KΩ as your max range, you get the best precision for the most probable values.

Example 1. Testing a resistor

With an auto-ranging meter, its easy, just put the two probes across the resistor and read the number. For example, this 1KΩ 5% resistor is actually 0.988 Kohm.

And this 10KΩ is really 9.80KΩ. Note that the numbers look similar but the decimal point has moved.

This ranged meter requires that you dial in the range. We'll guess that this resistor is under 2KΩ then measure it. We get 0.992 which means its 0.992 KΩ (or, a 1KΩ resistor)

Now testing a different resistor, we will again guess its under 2KΩ. However, this time we get a strange response, a 1. which means out of range. Some meters will display an OL which you may remember from the continuity secion as meaning "open loop" here it means "the measurement is higher than the range"

We try again, changing the range to 20KΩ

It is a 9.82 KΩ resistor (10KΩ)

Its a little clumsier than auto-ranging but if you are pretty sure you know about how big the resistance you are expecting is, its very speedy.

Example 2. Testing a potentiometer

You can test the max-value of a potentiometer by measuring across the two 'ends' as shown here with a rotational 10KΩ pot. To find the 'range' look at the dial.

You can also use a multimeter to tell whether the potentiometer is a linear or logarithmic (audio) pot. When the pot is centered, if the resistance between the wiper and one end is half of the total value, its linear. (I used clips instead of probles to make it easier to take these photos)

This is a 10KΩ linear potentiometer


The minimum resistance of the pot, 0Ω (a short) as expected


Potentiometer centered, about 5KΩ


Maximum value is 9.5KΩ (it should be around 10KΩ)

Here are photos of a 50KΩ audio potentiometer

Minimum is 0Ω as expected

Middle is 8KΩ

Maximum is 54.2KΩ, close to the ideal 50KΩ

If, when centered, the resistance is more like 85% or 15% of the total resistance, then its a log pot. This is a 50KΩ analog potentiometer. When centered, the resistance is about 8KΩ.

Example 3. Testing a sensor

Potentiometers are resistors that change value when they are moved. A Light Dependent Resistor (LDR) is a resistor that changes value with the amount of light it receives. This one has a range of about 20K max.

First, set the range, in this case 20KΩ seems pretty good. In bright light, it measures about 610 Ω

Slightly shaded it's 5.84KΩ (remember this is still a well-lit photo)

Voltage

Voltage is "potential energy".

A water pump is like a voltage supply (also known as a battery).
The pump pushes water through a hydraulic system, and the voltage supply pushes electrons through an electronic system.

The higher the rated pressure of the pump, the more 'work' the water can do.
Likewise, the higher the voltage the more 'work' (Watts) the electrons can do.

Voltage is used to provide power (via a battery or wall plug) and its also used as a way of transmitting data. For example, music is recorded from a microphone as an analog voltage signal, if that voltage waveform is applied to a speaker the voltage performs the work of making air move and produces sound.

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Voltage comes in two flavors (yum): Alternating Current (AC) and Direct Current (DC). Here is another quick tour of the differences.

Direct current voltage is what comes out of batteries. The battery is at 9V, and it pretty much keeps that voltage constant, until it dies. The chemical reactions inside the battery creates DC voltage.
Electronic circuits really like DC voltage.

Alternating current voltage is what comes out of the wall. The generator at the US power plant creates a voltage that oscillates, going from -60V to 0 to +60V to 0 again, 60 times a second. At the European power plant its -120V to +120V at 50 times a second.
AC voltage is great for power plants because its easy to transform AC voltages (using a transformer) up to 50KV for long distance travel and then down to 240V or 120V to safely power your home. Those big honking grey things that you see next to buildings that hum are the huge transformers.
Motors (like your washing machine and refrigerator compressor pump) like running off of AC voltage.

You can turn AC voltage into DC voltage very easily by using a very small transformer to bring the 120V down to a reasonable level like say 16VAC. This is basically what's inside a wall wart plug or your laptop power supply.
Its much harder to turn DC into AC, you will need an inverter which are more expensive than transformers/rectifiers.

Batteries only supply DC voltage and wall plugs only supply AC voltage. However, it is possible to have both AC and DC voltage at a certain point:
If an AC voltage is oscillating between -60V and +60V it has 120V AC and 0V DC because the average voltage of -60V and +60V is 0V.
If an AC voltage is oscilating between 0V and 120V then it has 120V AC and 60V DC because the average voltage of 0V and 120V is 60V.

In the above oscilloscope image, the dashed horizontal line in the center is ground (0V) and each dashed division is 5V. The scope is displaying a signal that has both AC and DC components. There is an alternating voltage (a square wave) that is about 4V high at about 100Hz and a DC (mean average) voltage that is around 7V. Use the dashed divisions to verify for yourself that this is so.

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Voltage testing is very common, you'll use it a lot. Use it for:

• Testing if your power supply is working, are you getting 5V out of that 7805 regulator?
• Verifying that your circuit is getting enough power: when all of the blinky lights are on, is the power supply dropping too low?
• Verifying signals to and from chips to make sure they are what you expect once the circuit is up and running
• Testing batteries, solar cells, wall plugs, and power outlets (carefully!)

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You can only test voltage when the ciruit is powered

If there is no voltage coming in (power supply) then there will be no voltage in the circuit to test! It must be plugged in (even if it doesn't seem to be working)

Voltage is always measured between two points
There is no way to measure voltage with only one probe, it is like trying to check continuity with only one probe. You must have two probes in the circuit. If you are told to test at a point or read the voltage at this or that location what it really means is that you should put the negative (reference, ground, black) probe at ground (which you must determine by a schematic or somewhere else in the instructions) and the positive (red) probe at the point you would like to measure.

If you're getting odd readings, use a reference voltage (even a 9V battery is a reasonable one) to check your voltage readings. Old meter batteries and wonky meters are the bane of your existence but they will eventually strike! Good places to take reference voltages are regulated wall plugs such as those for cell phones.

Voltage is directional If you measure a battery with the red/positive probe on the black/negative contact and the black probe on the positive contact you will read a negative voltage. If you are reading a negative voltage in your ciruit and you're nearly positive that this cannot be, then make sure you are putting the black probe on the reference voltage (usually ground)

DC voltage and AC voltage are very different Make sure you are testing the right kind of voltage. This may require pressing a mode button or changing the dial.

There are often two seperate modes for AC and DC voltage. Both will have a V but one will have two lines, one dashed and one solid (DC) and one with have a wave next to it (AC).

This meter has the double line for DC voltage, and 5 ranges, from 200mV to 600V. The lightning bolt symbol is a gentle reminder that this voltage is extremely dangerous.

There is also the V-wave symbol for AC, and two ranges since most AC voltages that are measured are power voltages and are pretty big. (For small AC waveforms, a scope is best since you will be able to see the waveform itself)

This autoranging meter makes it pretty clear which mode you want to be in

This ranged meter has 5 ranges, the top range is 750 VAC or 1000 VDC, to switch between DC and AC you need to press the DC/AC button on the upper right.

When the probes are not connected to anything, they should display 0V. They might flicker a bit if they pick up ambient voltage (your home is a big radiator of 60Hz voltage which can couple into your meter probes).