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How to Use a Multimeter to Measure Voltage, Current and Resistance

Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.

What is a Multimeter?

Multimeters are widely used by professionals in several fields including industrial maintenance and testing, research, appliance repair and electrical installation. However a digital multimeter or DMM is also an invaluable test instrument for home and DIY use. The instrument can used for measuring voltage, current and resistance and can check:

  • Battery voltages
  • Vehicle electrics and electronics
  • Continuity of cables and power cords
  • Home appliances and electronic devices
  • Fuses

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'll 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 a device 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 in a loop 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

Schematic of a simple circuit.

Schematic of a simple circuit.

Example:
A 12 volt car battery is connected to a load with a resistance of 4 ohms. What is the current?

I = V/R

V = 12 volts
R = 4 ohms

So current I = 12/4 = 3 amps

A simple circuit consisting of an AA cell and a bulb. The AA cell is the voltage source that causes current to flow in a loop around the circuit and through the bulb.

A simple circuit consisting of an AA cell and a bulb. The AA cell is the voltage source that causes current to flow in a loop around the circuit and through the bulb.

For more detailed information on current, voltage and resistance, AC and DC, take a detour to my other article:

Watts, Amps, Volts, Kilowatt Hours (kWh) and Electrical Appliances - Basic Electricity Explained

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.

A professional model 177 Fluke multimeter with an accuracy of 0.09 % on DC volts.

A professional model 177 Fluke multimeter with an accuracy of 0.09 % on DC volts.

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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.
A range selector dial is used to select the function (volts, amps, resistance) and range. Note the symbols used for AC and DC. See graphic further down this article for an explanation.

A range selector dial is used to select the function (volts, amps, resistance) and range. Note the symbols used for AC and DC. See graphic further down this article for an explanation.

Multimeter probes.

Multimeter probes.

4mm plug on the end of a multimeter probe.

4mm plug on the end of a multimeter probe.

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

  1. 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.
  2. Plug the black ground probe lead into the COM socket on the meter (see photo below).
  3. 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).
  4. 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).
  5. 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.
  6. Touch the black probe against the first point of the circuitry/wiring.
  7. Power up the equipment.
  8. 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.
  9. 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.

Exposed conductor of test lead

Exposed conductor of test lead

Connecting Probe Leads to Measure Voltage

Test leads and 4mm sockets on a DMM, setup to measure voltage

Test leads and 4mm sockets on a DMM, setup to measure voltage

Series and Parallel Connections

Explaining series and parallel connections (R1, R2 and R3 are resistors)

Explaining series and parallel connections (R1, R2 and R3 are resistors)

Measuring Voltage - Meter in Parallel With Load or Voltage Source

DMM connected in parallel with load to measure voltage across it

DMM connected in parallel with load to measure voltage across it

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.

Safety First When Measuring Mains Voltages!

  1. Before using a meter to measure mains voltages, visually inspect it first. Check the meter, probes and accessories are free from damage. Never use test leads with exposed conductors which could be touched inadvertently or a meter with cracks in the casing or exposed metal. Make sure probes are pushed securely into sockets.
  2. 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.
    Fluke has a safety guide here giving more information on safe multimeter use.