Why do you measure resistance? To determine the status of a circuit or component. The higher the resistance, the lower the current flow, and vice versa.

In general, the resistance of components used to control circuits (such as switches and relay contacts) starts out very low and increases over time due to factors such as wear and dirt. Loads such as motors and solenoids decrease in resistance over time due to insulation breakdown and moisture

How to measure resistance with a digital multimeter

To measure resistance:
1. Turn off the electrical circuit.

• If the circuit includes a capacitor, discharge the capacitor before taking any resistance reading.

2. Turn the digital multimeter’s dial to resistance, or ohms, which often shares a spot on the dial with one or more of the other test/measurement modes (continuity, capacitance or diode; see figure below).

• The display should display OLΩ because, in resistance mode, even before the test leads are connected to a component, a digital multimeter automatically starts taking a resistance measurement.
• The MΩ symbol may appear on the display because the resistance of the open (not supplied) test leads is too high.
• When leads are connected to a component, a digital multimeter automatically uses Autorange mode to adjust to the best range.
• Pressing the range button allows the technician to manually set the range.
• The best results will be achieved if the component to be tested is removed from the circuit. If the component is left in the circuit, the readings may be affected by other components in parallel with the component to be tested.

3. First insert the black test wire into the COM socket.
4. Then insert the red lead into the VΩ socket.

• When finished, remove the leads   in reverse order  : red first, then black.

5. Connect the test leads across the component being tested.

• Ensure that the connection between the test leads and the circuit is good.

Tip:   For very low resistance measurements, use the relative mode (REL; see point 11). It may also be referred to as zero or delta (Δ) mode. It automatically subtracts the test lead resistance – typically 0.2 Ω to 0.5 Ω. Ideally, if the test leads touch (short together), the display should show 0 Ω.

Other factors that can affect resistance readings: foreign matter (dirt, solder flux, oil), body contact with the metal terminals of the test wires, or parallel circuit paths. The human body becomes a parallel resistance path, which reduces the overall circuit resistance. Thus, avoiding touching the metal parts of the test leads to avoiding errors.

6. Read the measurement on the screen.
7. When finished, turn off the multimeter to prevent battery drain.

8. Press the RANGE button to select a specific fixed measurement range.

• Be sure to note the beep (such as K or M) after the measurement in the display.

9. Press the HOLD button to capture a fixed measurement – viewable later.
10. Press the MIN/MAX button to capture the lowest and highest measurement.

• The multimeter beeps each time a new reading is recorded.

11. Press the relative button (REL) to set the multimeter to a specific reference value.

• Measurements are shown above and below the reference value.

The significance of the resistance reading depends on the component being tested. In general, the resistance of any component varies over time and from component to component. Small changes in resistance are usually inconclusive but may indicate a pattern to note. For example, as the resistance of a heating element goes up, the current passing through the element decreases, and vice versa. See the chart below.

When working on a circuit board, it may be necessary to lift one of the resistor leads off the board to measure the correct resistance of the resistor. The resistance measurement displayed by a digital multimeter is the total resistance through all possible paths between the test lead probes. Care must be taken when measuring resistance across a component that is part of the circuit.

The resistance of all components connected in parallel to one being tested affects the resistance reading, and will usually lower it. Always check the circuit diagram for parallel tracks.