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
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How to measure resistance with a digital multimeter[/caption]
To measure resistance:
1. Turn off the electrical circuit.
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).
3. First insert the black test wire into the COM socket.
4. Then insert the red lead into the VΩ socket.
5. Connect the test leads across the component being tested.
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.
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.
11. Press the relative button (REL) to set the multimeter to a specific 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.
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