How to Use a Multimeter to Measure Voltage, Current and Resistance
What is a Multimeter?
A digital multimeter or DMM is a useful 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.
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Volts, Amps, Ohms - What Does it All Mean?
This is the pressure in an electrical circuit
This is a measure of the current flowing in an electrical circuit
A measure of the resistance to flow in a circuit
This produces a current flow in a circuit. It could be a battery, portable generator, mains supply to a home, alternator on your car or bench power supply in a lab or workshop
A device or component which draws power from a voltage source. This could be an electronic resistor, bulb, electric heater, motor or any electrical appliance
This is usually the point in a circuit to which the negative terminal of a battery or power supply is connected
Direct current. Current flows only one way from a DC source, an example of which is a battery
Alternating Current. Current flows one way from a source, reverses, and then flows the other way. This happens many times a second at a rate determined by the frequency which is typically 50 or 60 hertz. The mains supply in a home is AC
If you would like some more detailed information about these quantities, take a detour to this hub:
Using a Multimeter - Measuring Functions on the Instrument
A basic multimeter facilitates the measurement of the following quantities:
- DC voltage
- DC current
- AC voltage
- Continuity - indicated by a buzzer or tone
Some basic meters don't have an AC current range.
In addition meters may have the following functions:
- Capacitance measurement
- Transistor HFE or DC current gain
- Temperature with an additional probe
- Diode test
Voltage, current and resistance ranges are usually selected by turning a rotary dial, and the measurement is indicated on an LCD display or scale. Laboratory bench DMMs sometimes have seven segment LED displays.
Damaged probes and test leads are a hazard. Never use a damaged probe to measure mains voltages!
Voltage, Current and Resistance Ranges
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 setting 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
When measuring mains voltages, always turn off power before connecting measuring probes. Always connect to the neutral first.
WARNING !!! Safety First When Measuring Mains Voltages!
- Before using a meter to measure mains voltages, ensure the test leads aren't damaged and that there are no exposed conductors which could be touched inadvertently
- Double check that the 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
- Always turn off the power (if possible) before inserting the probes into a socket outlet, using the switch on the socket. Insert a probe into the neutral pin first before inserting a probe into the hot (live) pin of the socket. If you insert the probe into the hot (live) pin first and the meter is faulty, current could flow through the meter to the neutral probe. If you then inadvertently touch the probe, there is a possibility of shock.
- Finally turn on the power switch and measure the voltage
Ideally buy and use a meter with a least CAT III or preferrably 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 surges on the line being tested or overloads. A meter with less protection can potentially blow up causing injury if a surge occurs or it is connected incorrectly.
If you are measuring voltage at a consumer unit/breaker box/fuse box, this video from Fluke Corporation outlines Safe practices when taking single phase measurements
Identifying Live or Hot Wires
A "VoltStick" is a standard tool in any electricians tool kit, but useful for homeowners also. I use one of these for identifying which conductor is live whenever I'm doing any home maintenance. Unlike a neon screwdriver tester (phase tester), you can use one of these in situations when live parts/wires are shrouded or covered with insulation and you can't make contact with wires. It also comes in useful for checking whether there's a break in a power flex and where the break occurs. Fluke non-contact detector
Note: It's always a good idea to use a neon tester to double check that power is definitely off when doing any electrical maintenance.
Connecting Probe Leads to Measure Voltage
Series and Parallel Connections
Measuring Voltage - Meter in Parallel With Load
What Do the Symbols on the Range Dial Mean?
Autoranging meters detect 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 lead to the proper socket when measuring current.
Fluke Multimeter with Auto-Ranging Facility
What Multimeter Do I Need? - Buying a DMM
Fluke, who are a leading manufacturer of digital instrumentation, recommend the for general purpose use in the home or for car maintenance. This is an excellent meter and can measure AC and DC volts, current, 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 Fluke 113 model
An alternative is the Fluke 177 model which is a high accuracy instrument (the specification is 0.09% accuracy on DC volts). A model like this is ideal for more accurate testing and professional use.
How to Measure Current
- Turn off the power in the circuit being measured
- A multimeter must be inserted in series with the load in a circuit in order to measure current.
Plug the ground probe lead into the COM socket and 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
- Connect the meter in series as in 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
Measuring Large Currents with a Clamp Meter
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) 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
- Turn off all power to the circuit being measured
- Disconnect one end of the resistance from the circuit. This may involve pulling off spade leads or desoldering a component. This is important as there may be other resistances in parallel with the resistance being measured
- The probes are connected to the meter in the same way as for measuring voltage
- 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 resistance being measured
- If the display indicates "I", 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)
- The probe leads are connected to the meter in the same way as for measuring voltage
- 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)
- The probes are connected to the meter in the same way as for measuring voltage
- 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.
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:
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!) . Another way is to cut the neutral core, add 4mm banana plugs to each of the cut ends and plug these into the meter. 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 in direction is either 50 or 60 times per second, depending on the country in which you live. The change in direction is known as the frequency and measured in Hertz. 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
Questions & Answers
To be clear, am I correct in my interpretation that if I want to check that there is 230v in my electrical connections in a light fitting that is glowing dimly, I need the lamp in first to complete the circuit, then I check either end of the fitting placing the meter in parallel? Conversely, if I were to use the meter in lieu of the lamp, then this would be in series and the reading would be false or the meter would simply not work?
© 2012 Eugene Brennan