How to Use a Multimeter (DMM) 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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How to Use a Multimeter - Basic Definitions
Volts - This is the pressure in an electrical circuit
Amps - This is a measure of the current flowing in an electrical circuit
Ohms - A measure of the resistance to flow in a circuit
Voltage Source - This produces a current flow in a circuit. It could be a battery, generator, mains supply or bench power supply
Load - 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
Ground - This is usually the point in a circuit to which the negative terminal of a battery or power supply is connected
DC - Direct current. Current flows only one way from a DC source, an example of which is a battery
AC - 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
See diagrams and photos below
- 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)
- Next you need to decide whether the voltage being measured is AC or DC. If you are measuring the voltage from a mains socket outlet or the output voltage of a transformer, you need to select AC. Voltages of batteries, or the output of a power supply circuit or adapter is likely to be DC
- Multimeters may have several ranges for each function. For example the DC measuring mode may have ranges of 200mv, 2v, 20v, 200v and 1000 V in order to facilitate the measurement of a large range of voltages. Turn the dial of the meter to a range which is just above the voltage being measured, and ensure that you pick the AC voltage or DC voltage range. So for instance if you are measuring the voltage of a car battery which is approximately 12 volts, you can set the range to 20v. This gives the most number of decimal places in the reading. Setting the range to 200 volts gives less decimal places. If the meter is autoranging, set it to the "V" setting. (See the photo near the bottom of the article for an explanation of symbols used).
- 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
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.
- 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.
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
Connecting Probe Leads to Measure Voltage
Series and Parallel Connections
Measuring Voltage - Meter in Parallel With Load
When measuring mains voltages, always turn off power before connecting measuring probes. If this isn't possible, connect to the neutral first.
How to Measure Current
See diagram and photos below
- 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 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 at the bottom of the article 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.
Clamp Meter From Amazon
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 symbols used on meters below)
- The probes 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 symbols used on meters below)
- 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/meter/monitor. 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 duration of power up (useful for fridges, freezers and air conditioners which cut in and out).
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!
True RMS Multimeters
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 . 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.
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
Buying a DMM
Fluke, who are a leading manufacturer of digital instrumentation, recommend the 113 model below for general purpose use in the home or for car maintenance.
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.
© 2012 Eugene Brennan
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