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.
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
Follow the steps below to measure resistance:
Ω
. Note that the jacks used may be the same ones used to measure volts.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 applies its own voltage to the unknown resistance and then measures the current it produces to calculate a resistance value readout.
Even though it reads out resistance, the ohmmeter is still a current-measuring device at heart. The ohmmeter is created from a DC meter by adding 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 dead 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 to display 0 Ω when the test leads are clipped together (zero external resistance). When the leads are left open, the meter reads infinite (I) resistance or over-limit (OL) resistance. When a resistance is placed between the leads, the readout increases according to how much current that resistance allows to flow.
An ohmmeter should never be left on the ohms function when not in use to conserve its battery. Since the current available from the meter depends on the battery's state of charge, the DMM should be zero adjusted to start. This may require no more than a test of touching the two probes together.
Figure \(\PageIndex{1}\) shows how resistance measurements are taken.
Note
1 000 Ω = 1 kΩ
1 000 000 Ω = 1 MΩ
Figure \(\PageIndex{1}\): Using a DMM to measure resistance (CC BY-NC-SA; BC Industry Training Authority)