Why measure resistance? To determine the condition 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.

To measure resistance:
1. Turn power to circuit OFF.

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

2. Turn 

• The display should show OLΩ because, in Resistance mode, even before test leads are connected to a component, a digital multimeter automatically begins taking a resistance measurement.
• The MΩ symbol may appear in the display because the resistance of open (unattached) test leads is very high.
• When the leads are connected to a component, a digital multimeter automatically uses the Autorange mode to adjust to the best range.
• Pressing the Range button allows a technician to manually set the range.
• 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 could be affected by other components in parallel with the component to be tested.

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

 

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

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

5. Connect test leads across the component being tested.

• Make sure that contact between the test leads and circuit is good.

5. Connect 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 test lead resistance—typically 0.2 Ω to 0.5 Ω. Ideally, if test leads touch (are shorted together), the display should show 0 Ω.

Other factors that can affect resistance readings: Foreign substances (dirt, solder flux, oil), body contact with the metal ends of the test leads, or parallel circuit paths. The human body becomes a parallel resistance path, lowering total circuit resistance. Thus, avoid touching metal parts of test leads to avoid errors.

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

Advanced digital multimeter options

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

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

9. Press the HOLD button to capture a stable measurement—it can be viewed 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 (REL) button to set the multimeter to a specific reference value.

• Measurements above and below the reference value are displayed.

9. Press the HOLD button to capture a stable measurement—it can be viewed later.10. Press the MIN/MAX button to capture the lowest and highest measurement.11. Press the relative (REL) button to set the multimeter to a specific reference value.

Resistance Measurement Analysis

The significance of a resistance reading depends on the component being tested. In general, resistance of any one component varies over time and from component to component. Slight resistance changes are usually not critical but may indicate a pattern that should be noted. For example, as the resistance of a heating element rises, the current passing through the element decreases, and vice versa. See diagram below.

When working on a circuit board, it may be necessary to lift one of the leads of the resistor from 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. Caution is required when measuring resistance across a component that is part of a circuit.

The resistance of all components connected in parallel with a component being tested affects the resistance reading, usually lowering it. Always check the circuit schematic for parallel paths.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

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Related resources

Introduction to Resistance Measurements

You have studied voltage and current measurements, but you will find resistance measurements different in several ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own current through the unknown resistance and then measures that current to provide a resistance value readout.

Role of the Battery

Even though it reads out resistance, the ohmmeter is still a current-measuring device at heart. The ohmmeter is created from a DC current meter by the addition of a group of resistors (called multiplier resistors) and an internal battery. The battery supplies the current flow that is eventually measured by the meter. For this reason, use an ohmmeter only on de-energized circuits.

In the process of measuring resistance, the test leads are inserted in the meter jacks. The leads are then attached to the ends of whatever resistance is to be measured. Since current can flow either way through a pure resistance, there is no polarity requirement for attaching the meter leads. The meter’s battery sends a current flow through the unknown resistance, the meter’s internal resistors, and the current meter.

The ohmmeter is designed so that it will display 0 Ω when the test leads are clipped together (zero external resistance). The meter reads infinite (I) resistance or over limit (OL) resistance when the leads are left open. When a resistance is placed between the leads, the readout increases according to how much current that resistance allows to flow.

To conserve its battery, an ohmmeter should never be left on the ohms function when not in use. Since the current available from the meter depends on the state of charge of the battery, the DMM should be zero adjusted to start. This may require no more than a test of touching the two probes together.

Figure 8 shows how resistance measurements are taken.

Note:

1000 Ω = 1 kΩ

1 000 000 Ω = 1 MΩ

 

Figure 8: Using a DMM to Measure Resistance

1. Turn off power to circuit.
2. Plug the black test lead into the common input jack. Plug the red or yellow lead into the resistance input jack.
3. Select the resistance setting.
4. Touch the probe tips across the component or portion of the circuit.
5. View the reading and note the unit of measurement, ohms, kilohms, or megohms.

Resistance Measurement Procedures

Follow the steps below to measure resistance:

1. Before testing begins the technician should always know what reading to expect based on the manufactures specifications, name plate rating, Ohms’s law and Kirchhoffs law. Testing blindly is dangerous and counterproductive.
2. Turn off the power and prove the circuit to be measured is “dead” using the T3 testing method and the voltage measuring procedures. Be sure to wear your PPE as we always assume a circuit is “live” until proven otherwise
3. Remove or isolate the component to be tested.
4. Plug the test probes into the appropriate probe jacks, Common and

   Ω

   . Note that the jacks used may be the same ones used to measure volts.
5. Select the ohms function by turning the function switch to ohms. Start with the lowest setting.
6. Touch the probes together to check the leads, connections and battery life. The meter should display zero or a very small amount of resistance for the test leads. With the leads apart, the meter should display OL or I, depending on the manufacturer.
7. Connect the tips of the probes across the break in the component or portion of the circuit for which you want to determine resistance. If you get an OL (over limit), switch to the next highest setting.
8. View the reading on the display unit. Be sure to note the unit of measurement.
9. Turn the meter off when testing is complete to prolong battery life.

Video: Measuring Resistance