How do you test for high resistance?

The main drawback associated with shunt ammeters is their fundamentally high input impedance design. This drawback becomes more significant with decreasing current, because a larger shunt resistor must be used in order to develop a measurable voltage. However, as long as the shunt resistor is significantly smaller than the resistance of the DUT and the currents to be measured are not very small (not much lower than microamp level [10-6A]), shunt ammeters work fine.

Voltage Burden

The terminal voltage of an ammeter is called the voltage burden. This voltage burden developed across the meter could result in significantly lower current through the load than before the meter was inserted, therefore, the ammeter can't read the current it was intended to measure.

An ideal ammeter would not alter the current flowing in the circuit path, so it would have zero resistance and zero voltage burden. A real ammeter will always introduce a non-zero voltage burden. In general, the error term caused by an ammeter is stated as the ammeter's voltage burden divided by the resistance of the DUT. A shunt ammeter's voltage burden is typically on the order of hundreds of millivolts

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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