How to Use a Multimeter to Measure Voltage, Current and Resistance
An essential tool in the DIYer’s kit for checking voltage, fuses, and general troubleshooting of appliances and auto repair
A digital multimeter or DMM is a useful test instrument for measuring voltage, current and resistance, and some meters have a facility for testing transistors and capacitors. You can also use it for checking continuity of wires and fuses. If you like to DIY, do car maintenance or troubleshoot electronic or electrical equipment, a multimeter is a handy accessory to have in your home toolkit.
Volts, Amps, Ohms — What Does it All Mean?
Before we learn how to use a multimeter, we need to become familiar with the quantities we are going to be measuring. The most basic circuit we will encounter is a voltage source, which could be connected to a load. The voltage source could be a battery or a mains power supply. The load might be an appliance such as a bulb or electronic component called a resistor. The circuit can be represented by a diagram called a schematic.
In the circuit below, the voltage source V creates an electrical pressure which forces a current I to flow around the circuit and through the load R. Ohm’s Law tells us that if we divide the voltage V by the resistance R, measured in ohms, it gives us a value for the current I in amps:
Current I = V/R
For more detailed information, take a detour to my other article:
Electricity 101: A Complete Beginner’s Guide
What Does a Multimeter Measure?
A basic multimeter allows you to measure the following:
- DC voltage
- DC current
- AC voltage
- AC current (not all basic meters have this function)
- Resistance
- Continuity — indicated by a buzzer or tone
In addition meters may have the following functions:
- Capacitance measurement
- Transistor HFE or DC current gain
- Temperature measurement with an additional probe
- Diode test
- Frequency measurement
The value measured by the instrument is indicated on an LCD display or scale.
Parts of a Meter
- The Display. This is usually a multidigit, 7 segment LCD display. Some laboratory instruments however have LED displays which are easier to read under certain lighting conditions.
- Rotary Range Selector Dial. This allows you to select the function which you will be using on the meter. On a non-autoranging meter, it also selects the range.
- Connection Ports. These are 4mm diameter female sockets into which 4 mm probe leads are plugged.
- Probes. These have a pointed tip on one end for touching against the point of measurement and a plug on the other end for insertion into a connection socket.
Choosing Test Probes
Most meters come as standard with pointed, needle tip probes. An alternative is crocodile (alligator) probes that have spring loaded clips. These are useful so that one or both probes can be connected to a circuit without holding the probes in place by hand.
Connection Sockets on a Meter
The arrangement is non-standard and depends on the brand/model of meter, so it’s important to understand the function of each socket to avoid damage to the meter:
- Com is the common socket into which the black probe lead is plugged. This is standard on all meters.
- VΩmA marked on a socket indicates that the red probe lead is plugged into it for measuring voltage, resistance or low current (“mA” means “milliamps”). If there is no mention of “mA” on this socket, there will be one or more separate sockets for connecting the probe lead to measure current. These sockets will be marked “A” or “mA” with the max current range (e.g. 10A for high current readings and 400 mA for lower current readings).
How Do I Setup a Multimeter to Measure Volts, Amps or Ohms?
Voltage, current and resistance ranges are usually set by turning a rotary range selection dial. This is set to the quantity being measured, e.g. AC volts, DC volts, Amps(current) or Ohms (resistance).
If the meter is non-autoranging, each function will have several ranges. So for example, the DC volts function range will have 1000V, 200V , 20V, 2V and 200mV ranges. Using the lowest range possible gives more significant figures in the reading.
How to Measure Voltage
- Power off the circuity/wiring under test if there is a danger of shorting out closely spaced adjacent wires, terminals or other points which have differing voltages.
- Plug the black ground probe lead into the COM socket on the meter (see photo below).
- Plug the red positive probe lead into the socket marked V (usually also marked with the Greek letter “omega” Ω and possibly a diode symbol).
- If the meter has has a manual range selection dial, turn this to select AC or DC volts and pick a range to give the required accuracy. So for instance measuring 12 volts on the 20 volt range will give more decimal places than on the 200 volt range.
If the meter is autoranging, turn the dial to the ‘V’ setting with the symbol for AC or DC (see “What Do the Symbols on the Range Dial Mean?” below). - A multimeter must be connected in parallel in a circuit (see diagram below) in order to measure voltage. So this means the two test probes should be connected in parallel with the voltage source, load or any other two points across which voltage needs to be measured.
- Touch the black probe against the first point of the circuitry/wiring.
- Power up the equipment.
- Touch the other red probe against the second point of test. Ensure you don’t bridge the gap between the point being tested and adjacent wiring, terminals or tracks on a PCB.
- Take the reading on the LCD display.
Note: A lead with a 4mm banana plug on one end and a crocodile clip on the other end is very handy. The croc clip can be connected to ground in the circuit, freeing up one of your hands.
Connecting Probe Leads to Measure Voltage
Series and Parallel Connections
Measuring Voltage — Meter in Parallel With Load or Voltage Source
Safety First When Measuring Mains Voltages!
- Before using a meter to measure mains voltages, ensure test leads aren’t damaged and that there are no exposed conductors which could be touched inadvertently.
- Only use meters and test leads that have a CAT rating suitable for the measuring job. Choose leads with proper insulation, finger guards and shrouded plugs.
- Double check that test leads are plugged into the common and voltage sockets of the DMM (see photo below) and not the current sockets. This is essential to avoid blowing up the meter.
- Set the range dial on the meter to AC volts and the highest voltage range.
- Ideally use test leads with shrouded crocodile clips on the ends, power off the circuit, connect the leads and power up again so you don’t have to hold leads. TV repair technicians used to have a saying that the safest place to keep one of your hands is in your pocket when making measurements. When holding a probe in each hand, there’s always the danger of making contact with neutral or ground with one hand and hot (live) with the other resulting in a shock across the heart, a potentially lethal scenario. Alternatively the negative probe lead can be connected to neutral using a crocodile connector so that measurements can be taken with one hand using the positive needle probe.
Ideally buy and use a meter with a least CAT III or preferably CAT IV protection for testing mains voltages. This type of meter will incorporate high rupturing capacity (HRC) fuses and other internal safety components that offer the highest level of protection against overloads and transients on the line being tested. A meter with less protection can potentially blow up causing injury if it is connected incorrectly, or a transient voltage generates an internal arc.
If you are measuring voltage at a consumer unit/breaker box/fuse box, this video from Fluke Corporation outlines the precautions you should take:
Safe Practice When Taking Single Phase Measurement
Also these safety guidelines by Fluke explain the hazards of voltage spikes and the Overvoltage Installation Category:
Finally this guide lists what not to do when taking measurements:
10 dumb things smart people do when testing electricity
Autoranging Meters
Autoranging meters detect the magnitude of the voltage and select the range automatically to give the most amount of significant digits on the display. You must however set the mode to resistance, volts or current and also connect the probe leads to the proper sockets when measuring current.
What Do the Symbols on the Range Dial Mean?
How to Measure Current
- Turn off the power in the circuit being measured.
- Connect the probe leads as shown in the photo below. Plug the black ground probe lead into the COM socket.
- Plug the red positive probe lead either into the mA socket or the high current socket which is usually marked 10A (some meters have a 20 A socket instead of 10A). The mA socket is often marked with the maximum current and if you estimate that the current will be greater than this value, you must use the 10 A socket, otherwise you will end up blowing a fuse in the meter. On some meters, there is no additional socket for measuring current and the same socket is used as for measuring voltage (usually marked VΩmA).
- A multimeter must be inserted in series in a circuit in order to measure current. See the diagram below.
- Turn the dial on the meter to the highest current range (or the 10A range if the probe is in the 10A socket). If the meter is autoranging, set it to the “A” or mA setting. (See the photo above for an explanation of symbols used).
- Turn on the power.
- If the range is too high, you can switch to a lower range to get a more accurate reading.
- Remember to return the positive probe to the V socket when finished measuring current. The meter is practically a short circuit when the lead is in the mA or 10 A socket. If you forget and connect the meter to a voltage source when the lead is in this position, you may end up blowing a fuse at best or blowing up the meter at worst! (On some meters the 10A range is un-fused).
Connecting Probe Leads to Measure Current
Measuring Current — Meter in Series
What Multimeter Should I Buy? Recommended Products From Amazon
Fluke, a leading US manufacturer of digital instrumentation, promotes the Fluke 113 model for general purpose use in the home or for car maintenance. This is an excellent meter and can measure AC and DC volts, resistance, check continuity and diodes. The meter is auto-ranging, so ranges don’t have to be set. It is also a true-RMS meter. It doesn’t measure current, so If you need to measure AC and DC current, the Fluke 115 has this added facility.
An alternative is the Fluke 177 model which is a high accuracy instrument (the specification is 0.09% accuracy on DC volts). I use this model for more accurate testing and professional use and it can measure AC and DC voltage and current, resistance, frequency, capacitance, continuity and diode test. It can also indicate max and min values on each range.
Fluke 177 Multimeter with Auto-Ranging Facility
Measuring Large Currents with a Clamp Meter (Tong Tester)
On most multimeters, the highest current range is 10 or 20 amps. It would be impractical to feed very high currents through a meter because normal 4 mm sockets and test leads wouldn’t be capable of carrying high currents without overheating. Instead, clamp meters are used for these measurements.
Clamp meters (as the name suggests), also known as tong testers, have a spring loaded clamp like a giant clothes peg which clamps around a current carrying cable. The advantage of this is that a circuit doesn’t have to broken to insert a meter in series, and power needn’t be turned off as is the case when measuring current on a standard DMM. Clamp meters use either an integrated current transformer or hall effect sensor to measure the magnetic field produced by a flowing current. The meter can be a self contained instrument with an LCD which displays current, or alternatively the device can output a voltage signal via probe leads and 4mm “banana” plugs to a standard DMM. The voltage is proportional to the measured signal, typically 1mv represents 1 amp.
Clamp meters can measure hundreds or thousands of amps.
To use a current clamp, you simply clamp over a single cable. In the case of a power cord or multicore cable, you need to isolate one of the cores. If two cores carrying the same current but in opposite directions are enclosed within the jaws (which would be the situation if you clamp over a power cord), the magnetic fields due to the current flow would cancel out and the reading would be zero.
How to Measure Resistance
- If the component is on a circuit board or in an appliance, turn off the power
- Disconnect one end of the component if it’s in a circuit. This may involve pulling off spade leads or desoldering. This is important as there may be other resistors or other components having resistance, in parallel with the component being measured.
- Connect the probes as shown in the photo below.
- Turn the dial to the lowest Ohm or Ω range. This is likely to be the 200 ohm range or similar.
- Place a probe tip at each end of the component being measured.
- If the display indicates “1”, this means that resistance is greater than can be displayed on the range setting you have selected, so you must turn the dial to the next highest range. Repeat this until a value is displayed on the LCD.
Connecting Probe Leads to Measure Resistance
How to Check Continuity and Fuses
A multimeter is useful for checking breaks in flexes of appliances, blown filaments in bulbs and blown fuses, and tracing paths/tracks on PCBs
- Turn the selecting dial on the meter to the continuity range. This is often indicated by a symbol which looks like a series of arcs of a circle (See the photo showing symbols used on meters above).
- Connect the probe leads to the meter as shown in the photo below.
- If a conductor on a circuit board/ a wire in an appliance needs to be checked, make sure the device is powered down.
- Place the tip of a probe at each end of the conductor or fuse which needs to be checked.
- If resistance is less than about 30 ohms, the meter will indicate this by by a beep tone or buzzing sound. The resistance is usually indicated on the display also. If there is break in continuity in the device being tested, an overload indication, usually the digit “1”, will be displayed on the meter.
Connecting Probe Leads to Check Diodes or Continuity
How to Check Diodes
A multimeter can be used to check whether a diode is short circuited or open circuited. A diode is an electronic one way valve or check valve, which only conducts in one direction. A multimeter when connected to a working diode indicates the voltage across the component.
- Turn the dial of the meter to the diode test setting, which is indicated by a triangle with a bar at the end (see the photo showing symbols used on meters above).
- Connect the probes as shown above.
- Touch the tip of the negative probe to one end of the diode, and the tip of the positive probe to the other end.
- When the black probe is in contact with the cathode of the diode (usually indicated by a bar marked on the component) and the red probe makes contact with the anode, the diode conducts, and the meter indicates the voltage. This should be about 0.6 volts for a silicon diode and about 0.2 volts for a Schottky diode. When the probes are reversed, the meter should indicate a “1” because the diode is open circuit and non-conducting.
- If the meter reads “1” when the probes are placed either way, the diode is likely to be faulty and open circuit. If the meter indicates a value close to zero, the diode is shorted circuited.
- If a component is in circuit, resistances in parallel will affect the reading and the meter may not indicate “1” but a value somewhat less.
How to Measure Wattage and the Power Consumption of an Appliance With a Multimeter
Watts = Volts x Current
So to measure the power in watts of a load/appliance, both the voltage across the load and the current passing through it must be measured. If you have two DMMs, you can measure the voltage and current simultaneously. Alternatively measure the voltage first, and then disconnect the load so that the DMM can be inserted in series to measure current. When any quantity is measured, the measuring device has an influence on the measurement. So the resistance of the meter will reduce current slightly, and give a lower reading than the actual value with the meter not connected.
Two ways to measure current drawn by an electrical appliance:
- The safest way to measure the power consumption of an appliance powered from the mains is to use a power adapter. These devices plug into a socket and the appliance is then plugged into the adapter which displays information on an LCD. Typical parameters displayed are voltage, current, power, kwh, cost and how long the appliance was turned on (useful for fridges, freezers and air conditioners which cut in and out). You can read more about these gadget in my article here:
Checking Power Consumption of Appliances With an Energy Monitoring Adapter - An alternative way of safely measuring current drawn by an electrical appliance is to make up a test lead using a short piece of power cord with a trailing socket on one end and a mains plug on the other. The inner neutral core of the power cord could be freed and separated from the outer sheath, and current measured with a clamp meter or probe. (Don’t remove the insulation!) Alternatively, cut the neutral core, fit shrouded 4 mm plugs to the two cut ends and plug these ends into the meter to measure current. Unplug the test lead from the mains before making connections to the meter and adjusting the range.
Only make connections and adjust range on the meter with the power off!
How to Check Peak Voltages — Using a DVA Adapter
Some meters have a button which sets the meter to read max and min RMS voltages and/or peak voltages (of the waveform). An alternative is to use a DVA or Direct Voltage Adapter. Some components such as CDI (Capacitor Discharge Ignition) modules on vehicles, boats and small engines produce pulses which vary in frequency and can be short duration. A DVA adapter will sample and hold the peak value of the waveform and output it as a DC voltage so the component can be checked to see whether it’s producing the correct voltage level. A DVA adapter typically has two probe leads as input for measuring voltage and either two output leads with banana plugs or a connector with fixed plugs attached for plugging into a meter with standard spaced sockets. The meter is set to a high DC voltage range (e.g. 1000 volts DC) and the adapter typically outputs 1 volt DC per 1 volt AC input.
Important information for anyone using a DVA to check ignition circuits!
In this application, the adapter is used for measuring the primary voltage of a stator/ignition coil, not the secondary voltage, which could be about 10,000 volts or more.
Fluke also manufacture meters that can capture the peak level of short transients e.g. — The Fluke-87–5, Fluke-287 and Fluke-289 models.
True RMS Multimeters
The voltage supply to your home is AC, and voltage and current vary in polarity over time. The waveform is sinusoidal as in the diagram below and the change of direction of current is known as the frequency and measured in Hertz (Hz). This frequency can be 50 or 60 Hz, depending on which country you live in. The RMS voltage of an AC waveform is the effective voltage and similar to the average voltage. If the peak voltage is Vpeak, then the RMS voltage for a sinusoidal voltage is Vpeak / √2 (approx 0.707 times the peak voltage). The power in a circuit is the RMS voltage multiplied by the RMS current flowing in a load. The voltage normally printed on appliances is the RMS voltage even though this is not usually stated.
A basic multimeter will indicate RMS voltages for sinusoidal voltage waveforms. The supply to our homes is sinusoidal so this isn’t a problem. However if a voltage is non sinusoidal, e.g. a square or triangular wave, then the meter will not indicate the true RMS voltage. True RMS meters however are designed to correctly indicate RMS values for all shaped waveforms.
The AC Supply Feeding Our Homes is a Sine Wave
Disclosure
This story includes affiliate links for which the author may receive commission on sales. The author is an Amazon Associate.
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Disclaimer
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters. Using a DMM to measure voltages can be dangerous. Read and understand all manuals before use and take reasonable precautions including wearing personal protection equipment (PPE) to prevent against injury.
This guide was first published on Hubpages on 7th June, 2012
