How to measure electrical units

Part ofDesign and TechnologyElectronic and microelectronic control systems

Key points

  • Current is electron flow, measured in amps (A) or milliamps (mA). Use an ammeter in series.
  • Voltage is the pushing force, measured in volts (V) or millivolts (mV). Use a voltmeter in parallel.
  • Resistance restricts current, measured in ohms (Ω), kilohms (kΩ), or megaohms (MΩ). Use an ohmmeter/multimeter.
  • Ohm's law is V=ꞮR (Voltage = Current × Resistance).
  • Multimeters measure V, Ɪ, R. Connect for voltage (parallel), current (series), resistance (circuit off). Start high, work down.
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Understanding units

  1. Current (flow of electrons)
    Amp (A) = big current (like in a kettle)
    Milliamp (mA) = tiny current (like in LED)
    1000mA = 1A
    Measurement Tool: Ammeter, connected in series with the circuit.

  2. Voltage (pushing force)
    Volt (V) = normal battery voltage (like 9V battery)
    Millivolt (mV) = tiny voltage
    1000mV = 1V
    Measurement Tool: Voltmeter, connected in parallel with the component.

  3. Resistance (restricts flow)
    Ohm (Ω) = small resistance
    Kilohm (kΩ) = thousand ohms
    Megaohm (MΩ) = million ohms
    Measurement Tool: Ohmmeter or multimeter.

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How to use a multimeter.

Multimeter graphic with test leads, LCD display, Common (COM) port, A port, VΩ and selection knob labelled
Diagram of a digital multimeter
  1. Measuring voltage:
    • DC voltage: set the multimeter to the DC voltage (DCV) setting.
    • connection: connect the black probe to the COM port and the red probe to the VΩ port, place the probes across the component or section of the circuit where you want to measure the voltage
    • reading: read the voltage value displayed on the multimeter
  2. Measuring current:
    • DC current: set the multimeter to the DC current (DCA) setting
    • connection: connect the black probe to the COM port and the red probe to the A port, break the circuit and connect the multimeter in series so that the current flows through the multimeter
    • reading: read the current value displayed on the multimeter
  3. Measuring resistance:
    • resistance: set the multimeter to the resistance (Ω) setting
    • connection: connect the black probe to the COM port and the red probe to the VΩ port, place the probes across the component whose resistance you want to measure, ensure the circuit is powered off
    • reading: read the resistance value displayed on the multimeter

SAFETY TIP

Always start on highest setting range and work down to protect the multimeter!

Yellow multimeter on white background with black and red testing leads
Yellow multimeter set to measure current in amps

Calculating Expected Values

Using Ohm’s Law (V=ꞮR), you can calculate the expected values for voltage, current, and resistance in a circuit.

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What is Ohm’s law?

Illustration of triangle showing Ohm's law components - voltage v, current i and resistance r
Ohm's law. V is equal to I multiplied by R, I equals V divided by R and R equals V divided by I.

Ohm’s Law describes the relationship between voltage (V), current (Ɪ), and resistance (R) in an electrical circuit.

The law states that:
Voltage = Current × Resistance
or
V=ꞮR

Illustration of triangle showing Ohm's law components - voltage v, current i and resistance r
Ohm's law. V is equal to I multiplied by R, I equals V divided by R and R equals V divided by I.

If you know any two of these quantities, you can calculate the third.

The formula can be rearranged as follows:

To find voltage:

Voltage = ?Resistance = 4ΩCurrent = 3A

V=ꞮR

To find current:

Current = ?Voltage = 9VResistance = 180Ω

Ɪ=V/R

To find resistance:

Resistance = ?Voltage = 6VCurrent = 4mA

R=V/Ɪ

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Test yourself

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