Measuring Current and Voltage in a circuit

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: 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 damage your meter.

Multimeters are meters that have a range of functions, giving more than one type of measurement, depending on the selection made on the meter. Meters are either analogue, with a dial indicating the reading, or digital, with a specific numeric value given in a readout. Examples on taking readings are given for both meter types.

Fig4-Budget-Analogue-Multimeter  Fig5-Budget-Digital-Multimeter
A budget Analogue Multimeter with the needle indicating the value being measured A Digital Multimeter gives a numeric reading of the value being measured


Measuring Current:

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:

  • 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, as you have an idea of what result to expect.

  • The circuit needs to have power to measure the current.

Schematic Symbol  Description
 Symbol-Ammeter Ammeter

Fig6-Ammeter-Connection-Schematic

Diagram showing circuit with ammeter

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

WarningMeasuring 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.

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

Most multimeters are rated up a maximum of 10A, so this single 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.

WarningNever measure the current of the mains! Instant meltdown and potential electrocution guaranteed!

Measuring Voltage:

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:

  • 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
 Symbol-Voltmeter

Voltmeter

 Fig7-Voltmeter-Connection-Schematic

Diagram showing circuit with ammeter

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.

Measuring Power:

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 using the following formula:

P = I V or Power = Current x 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.