Measuring Current, Voltage and Power with a multimeter

As we’re targeting this tutorial at beginners, we’re going to show how to take measurements using simple basic and cheap budget multimeters that anyone can buy for very little cost. It should be said that there are far better multimeters on the market, with a range of pricing from tens to thousands of pounds. We think the budget multimeters are a good place to start, and as you get to know your electronics more. Only upgrade as you need to, otherwise you may end up wasting a lot of money on something you’ll hardly use.

IMPORTANT:

Caution!Ensure that meters are connected correctly, with the correct settings selected, and that the power is turned on or off in the circuit as directed. Not following this can destroy your meter, or even start a fire!

Multimeters are meters that have a multiple range of functions, giving more than one type of measurement, depending on the selection made on the meter. More old fashioned meters are analogue, with a dial indicating the reading, with more modern meters being digital, with a specific numeric value given in a readout. Analogue meters have an advantage in that it can be easier to see changes happening in the circuit, however, digital meters are easier to read. Examples on taking readings are given for both meter types.

Figure 3 A Budget Analogue Multimeter
Figure 3 A budget Analogue Multimeter with the needle indicating the value being measured
Fig 4 Budget Digital Multimeter
Figure 4 A digital multimeter gives a numeric reading of the value being measured

 


How to Measure Current with a Multimeter

Current is measured with an Ammeter. If you are using a multimeter, you will need to switch the measurement setting to Amps or A.

In order to measure current, your meter needs to be placed in line with the circuit (The circuit needs to be broken, and the meter bridges the gap).

IMPORTANT:

  • Caution!Be sure to note that the expected Amps in the circuit should not be higher than the range selected on the meter. If in doubt, use a high range to get a rough reading, then select a smaller range for more precision, once you have an idea of the result.
  • The circuit needs to have power to measure the current.
Schematic Symbol Description Circuit Schematic Reference
Ammeter Schematic Symbol Ammeter  

 

Ammeter Connection Schematic
Figure 5 Diagram showing circuit with ammeter connection
x

Measuring the bulb in the circuit gives a reading of approx. 0.26A.

Warning!Measuring the current of the battery: A word of warning:

We also decided to measure the current of the battery by connecting the multimeter directly to the battery terminals (no bulb). In contrast, the current was around 4A, about 15 times higher.

Warning!We don't recommend measuring current in batteries with a standard multimeter!

Most multimeters are rated up a maximum of 10A, so this single 1.5V AA battery is almost half the rated maximum before blowing the fuse in the meter, or blowing the meter itself (low budget meters are often not fused). The wires can get extremely hot when taking high current measurements, melting the insulation, or even catching fire.

Warning!Never measure the current of the mains! Instant meltdown and potential electrocution guaranteed!

 


How to Measure Voltage with a Multimeter

Voltage is measured with a Voltmeter. If you are using a multimeter, you will need to switch the measurement setting to Volts or V. Again, be sure to note that the expected voltage should not be higher than the range selected on the meter.

To measure Volts, connect the probes of the meter directly to the circuit at the points where you wish to measure. Unlike current, you do not need to break the circuit to take the measurement.

 

IMPORTANT:

  • Caution!Be sure to note that the expected Volts in the circuit should not be higher than the range selected on the meter. If in doubt, use a high range to get a rough reading, then select a smaller range for more precision, as you have an idea of what result to expect.
    • Your circuit needs to have power to measure the voltage.
Schematic Symbol Description Circuit Schematic Reference
Voltmeter Schematic Symbol Voltmeter  
Voltmeter Connection Schematic
Figure 7 Diagram showing circuit with Voltmeter connection
x
 
Measuring the volts across the bulb will give the same result as measuring the battery voltage, as the full voltage of the battery is available both to the bulb and to the meter probe leads.
 
 


How to Measure Power with a Multimeter

Power can either be measured with a specialist Wattmeter (a function that will not be available on most multimeters), or calculated from separate current and voltage measurements already taken with a multimeter, where power is the product (multiplied result) of current and voltage:

Power = Current x Voltage or Power is the product of Current and Voltage

As an example, if the measured current of our bulb is 0.26A, with a voltage across the bulb measuring 1.55V, the power is 0.26 X 1.55 = 0.4W - so, almost half a watt of power is used by the bulb.


A note about precision

It’s all about tolerance! Basic electronics circuits are quite imprecise, letting you get away with wild variations in what components state and what they actually are. This tolerance is normally expressed as a percentage of the value stated.

Batteries range in voltage as they are discharged, and simple electronics circuits won’t normally be affected by changes in voltage unless your battery is almost flat.

Your budget multimeter won’t give you a true and proper reading, but one that is close enough and will do the job. High precision meters are available, but come with a high cost, and they’re an expensive luxury that a beginner in electronics won’t need.

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