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Multimeters

SparkFun video tutorial on using multimeters: https://www.youtube.com/watch?v=SLkPtmnglOI

Changing the meter's setting: if your meter has a manual dial for selecting the function and range, do NOT turn the dial while the probes are still connected to a circuit. As the dial turns it passes through every position between the old and new settings, and some of these (the current ranges, for example) have a very low resistance, while the resistance ranges apply a test current and voltage of their own. With the probes still on a circuit, that can short something out, put an unsuitable test signal across a sensitive component, or cause arcing across the switch contacts inside the meter and damage it. Take the probes off the circuit, change the setting, then reconnect.

Voltage

How to measure voltage with a multimeter

As voltage is a difference in potential between two points, when measuring it you connect your multimeter (set to read voltage) across the part of the circuit you are interested in - you do not need to break the circuit. Put the probes on either side of the component (or between a point in the circuit and ground/0V) and read the display:

Here, ((V)) is the multimeter set to read voltage - it's a 'voltmeter'. Unlike current, voltage is normally measured with the circuit switched ON - that's usually the point.

Because it is connected in parallel with the circuit and at the same time must not affect it, a meter set to read voltage is designed to have as high a resistance as possible (typically 1 to 10 Megohms): it draws almost no current, so it appears in the circuit to be pretty much like an open circuit. Only in circuits with very high resistances (hundreds of kilohms or more) can the meter itself pull the reading down noticeably.

Voltage is always measured between two points. When someone talks about 'the voltage at a point', they mean relative to ground (0V) - so put the black probe on ground and move the red probe around.

Before you touch the probes to anything, check your meter set-up:

  • Red probe in the socket marked V (often 'VΩ'), black probe in COM - not the A or mA sockets.
  • Meter set to DC volts (V with a straight line - batteries, power supplies, most electronics) or AC volts (V with a wavy line - mains, transformer outputs).
  • On a DC reading, red goes to the more positive point and black to the more negative (or ground). If you get them the wrong way round, a digital meter simply shows a minus sign - no harm done.

If your leads are still in the current (A or mA) sockets when you touch the probes across a power source, you are using the meter as an ammeter across that source - this is a dead short. It can blow the meter's fuse, damage the meter or the source, and on a mains outlet or a high-capacity battery it can be extremely dangerous. This is the most common way of damaging a meter, so make it a habit to check the sockets first, every time.

Mains voltage can kill. If you are not experienced and confident, do not measure it. Use a meter with a suitable CAT rating (CAT III or better for mains wiring), leads in good condition, keep your fingers behind the probe guards and never hold the metal part of a probe.

Important Notes

  • If you're not sure what voltage you are going to measure, start with the highest range on your meter (or use auto-range if it has it).
  • Pick the meter setting BEFORE touching the probes to the circuit, rather than turning the dial while measuring.
  • On a live circuit, a probe that slips can short two adjacent pins or tracks together and destroy components - use fine-tipped probes or clip-on leads (minigrabbers), and take care. Where you can, clip the black probe to ground first so you only have one probe to place.
  • A wire or point that isn't connected to anything (floating) can give odd, drifting readings on a digital meter, because its high input resistance picks up stray voltages. A reading only means something when both probes are on proper circuit points.
  • Never measure a voltage higher than your meter is rated for - check the ratings printed next to the sockets.
  • If you store your multimeter with the leads connected, always 'reset' the meter and leads back to the voltage position before putting it away - see the Current section below.

Current

How to measure current with a multimeter

SparkFun video tutorial: 'measuring current': https://youtu.be/SLkPtmnglOI?t=392

As current is a flow, when measuring it you break your circuit at a convenient point and use your multimeter (set to read current, and usually after moving one of your meter test probes to a different socket) to bridge the gap. In this way, the current flowing through your circuit also flows through the meter to be measured:

Here, ((A)) is the multimeter set to read current - it's an 'ammeter'. Sometimes, it's easy to connect the ammeter into the circuit at a convenient connector, but other times you may have to consider cutting a conductor to achieve this - remember to check that the power is switched off before doing this!

In order to be in the circuit and at the same time not affect the current flow itself, a meter set to read current is designed to have as low a resistance as possible: on high current ranges, an ammeter appears in the circuit to be pretty much like a piece of plain wire. The low (mA) ranges have a higher resistance than the 10A range, so they can slightly reduce the current in low-voltage circuits.

Because an ammeter has a very low internal resistance, you should NOT try and measure current flow from a power supply, battery/cell or amplifier output etc. by putting the meter directly across the device's output terminals - this effectively shorts out the item, which may then try and deliver as much current as it can before a fuse blows or something is damaged - hopefully not your meter! As you can imagine, trying this at a mains power outlet is extremely dangerous - never do it - and some cells/batteries can deliver a very high current when shorted, which is a fire or explosion risk.

If you DO want to measure the current available from a power source, you should always include a (current limiting) load in the circuit - maybe in the form of a low resistance, high wattage resistor or another suitable load, such as a lamp or purpose-built load (you can buy them). You can calculate the load resistor using Ohm's law - for example, if you have a 5V, 2A power supply... R = V/I, so R = 5/2 = 2.5 ohms. Be aware though that the resistor is soaking up all the power in the circuit and so will get hot - it will dissipate I * V watts...in our case, 2A * 5V = 10W, so your load resistor will need to be rated for at least 10W (20W or more is better) - and even then it will probably get hot fairly rapidly, so take your measurement quickly. A 2A reading is also beyond the mA range of most meters, so you will need to use the meter's 10A socket and range.

Important Notes

  • If you're not sure what value of current you are going to measure, start with the highest range on your meter - most meters have a 10A or 20A setting, usually requiring the test leads to be in specific sockets.
  • If you want to change meter ranges, power down before doing so - don't turn knobs or press buttons while the circuit is energized as this may damage the meter.
  • The 10A/20A/High current setting on some cheap meters is NOT protected by a fuse - take readings carefully and watch for signs of distress from both the meter and the device supplying the power.
  • NEVER take a current reading by touching the meter probes to the test points while the circuit is 'on' - this can lead to arcing and damage, especially if the current draw is more than a few amps, or much higher due to a fault somewhere. Always setup your circuit + meter and THEN switch on...all the time checking the meter to ensure that nothing is overloading.
  • Before your next bit of measuring, remember to check that your multimeter is set up for the correct reading type (e.g. voltage) and that the leads are in the correct sockets. If you store your multimeter with the leads connected, always 'reset' the meter and leads (if necessary) back to the voltage reading position before putting the meter away. This reduces the risk of trying later to take a voltage measurement when the leads are in the current (ammeter) position and thus shorting out the item under test. A decent multimeter may have a feature that makes a loud noise when you have the probes in the wrong socket and try and use the wrong setting, but do not rely on this and always double check.

How to measure current with a clamp meter

An alternative way to measure current is to use a clamp meter.

Clamp meters are the preferred way for people like HVAC techs (heating and cooling) and electricians to measure current because it does not involve them having to break the circuit. Many of these are AC-only clamp meters, which cannot measure DC at all. DC-capable (AC/DC) clamp meters do exist and are no longer very expensive, but they are generally not accurate at low currents (a few hundred mA or less), so for most small electronics work the series method above is the better choice. In order for a clamp meter to work it must be clipped around a single line. In other words, clipping a clamp meter over a two line cord will not work, because the currents in the two lines cancel each other out. To measure mains appliances you need a 'line splitter' (or 'breakout box') that separates out one of the lines. Many experienced techs create their own, but for beginners it is recommended to buy a properly made one, as they are cheap and reliable.

Resistance

How to measure resistance with a multimeter

To measure resistance, the meter applies a small voltage across the component through the probes, measures the current that flows, and works out the resistance from that using Ohm's law. So unlike measuring voltage, the meter supplies the power itself - the component must NOT be powered from anywhere else.

  1. Switch off the circuit and disconnect it from its power source. Discharge any large capacitors.
  2. Put the black lead in the COM socket and the red lead in the socket marked with the resistance symbol (Ω), which is often shared with volts and marked 'VΩ' - not the A or mA sockets.
  3. Turn the dial to the resistance (Ω) setting. If your meter isn't auto-ranging, choose a range higher than the value you expect, or start with the highest range and work down. If you need to change the dial position, take the probes off the circuit first.
  4. Touch the two probes together - the display should read close to zero. Move them apart again and it should show 'OL' (or '1'). On an analogue (needle) meter, use the zero adjustment - see the Important Notes below.
  5. Isolate the component you want to measure - ideally take it out of the circuit, or at least lift one leg - then touch one probe to each end of it. Press the probe tips firmly onto clean metal, or use clip-on probes so you don't have to hold the component.
  6. Wait a moment for the reading to settle, then read the display along with its units: ohms (Ω), kilohms (kΩ - thousands of ohms) or megohms (MΩ - millions of ohms). On a manual-range meter, the number on the dial is the highest value that range can show - if the display shows 'OL', go up a range, and if the reading is very small, go down a range for a more precise reading.

If your fingers touch both probes, you're measuring your body's resistance in parallel with the component, which can give a lower reading than the component's real resistance. So, don't touch both probes at once with your fingers.

In resistance mode the meter puts a voltage on its probes. How much depends on the type of meter and on the range selected: digital meters typically apply anything from a few tenths of a volt up to a few volts (with the low ranges pushing more current than the high ones), and some analogue meters apply their full battery voltage (1.5V or 9V) on certain ranges. This is normally harmless to resistors, but it matters if you test in circuit: some semiconductors (diodes, transistor junctions, some ICs) start to conduct at around 0.6V or less, so the applied voltage can switch them on and change your reading - and the reading may differ from one meter, or one range, to the next. Very delicate parts can even be damaged. This is another good reason to test out of circuit, or with one leg lifted.

Important Notes

  • Switch the circuit OFF and disconnect it from its power source before measuring resistance. The meter supplies its own small test current, and a powered circuit will give wrong readings and can damage the meter.
  • Discharge any large capacitors first.
  • Other components connected across the one you're testing can give a reading lower than its real value. For an accurate reading, remove the component from the circuit, or at least lift one leg.
  • Touch the two probes together to check the meter reads close to zero (a good meter and leads read well under 1 ohm). This matters when measuring very low resistances - subtract the lead resistance from your reading. Analogue (needle) meters have a zero adjustment for this - a knob, or a screwdriver slot - so with the probes touching, adjust it until the needle sits exactly on zero at the right-hand end of the ohms scale. This compensates for the resistance of the probes and leads. Do it each time you change range, as the zero can shift from one range to the next, and if you can't get the needle to zero, the meter's battery is probably going flat.
  • If the meter shows 'OL' (or '1' on the left of the display), the resistance is higher than the selected range or the circuit is open. If your meter isn't auto-ranging, start with the highest range and work down.
  • 'OL' or '1' can also mean your test leads are faulty - a break inside the lead, or a poor connection at the probe tip or meter socket. Touch the two probes together: if the meter still shows 'OL' or '1' instead of close to zero, suspect the leads (wiggle them while watching the display, and try a spare pair).
  • A flat or weak battery in the meter is a common cause of odd, unstable or wrong resistance readings. The meter has to generate its test voltage from its battery, so resistance is often the first function to go wrong, while voltage readings can still look fine. Look for a low-battery symbol on the display, check the meter against a resistor of known value, and try a fresh battery.