Electrical resistance describes a material’s opposition to the flow of electric current, measured in ohms. To determine this value, a specialized tool called an ohmmeter, often integrated into a multimeter, is used. A common point of confusion for those new to electrical troubleshooting is whether resistance should be checked by connecting the meter in series or in parallel with a component. Measuring resistance requires a specific preparatory step to ensure accuracy.
Measuring Resistance Requires Isolation
The correct procedure for measuring the resistance of a single component is to completely isolate it from the rest of the electrical circuit. This means the component must be removed or at least disconnected from any parallel pathways. Furthermore, the circuit must be completely powered down before the measurement is attempted.
An ohmmeter determines resistance by internally applying a small, known voltage and then measuring the resulting current that flows through the component being tested. The meter then uses Ohm’s Law (resistance equals voltage divided by current) to calculate the final resistance value.
If the component remains connected to an energized circuit, the existing voltage and current will interfere with the ohmmeter’s test current. This interference results in a highly inaccurate reading. For safety, always confirm the circuit’s power source is disconnected and any stored energy, such as in large capacitors, has been discharged before connecting the ohmmeter’s probes.
Understanding In-Circuit Measurement Errors
The question of measuring resistance in series or parallel is irrelevant because the component must be isolated for an accurate reading. Attempting to measure resistance while a component is still wired into the system leads to misleading data. The ohmmeter measures the total resistance between its two probes, which can include unintended pathways.
If a component is left in the circuit with other components wired in parallel, the ohmmeter’s test current will travel through all available paths. The meter will then measure the equivalent resistance of that entire parallel network, not the resistance of the single component. This reading will always be lower than the component’s actual value.
If the ohmmeter is inserted “in series” into a break in the circuit path, it measures the resistance of the component plus the resistance of all other connected components in that loop. In both the parallel and series scenarios, the resulting measurement is insufficient for determining the specific value of a single component.
How Ohmmeter Use Differs From Voltage and Current Checks
The procedure for checking resistance stands alone because of how the ohmmeter operates, which is different from how voltage and current are checked. Voltage is measured with a voltmeter, and it must be done while the circuit is actively powered. The voltmeter’s probes are connected in parallel, or across, the component to measure the electrical pressure difference between two points.
Current is measured with an ammeter, which also requires the circuit to be powered on. To measure current, the circuit must be physically interrupted, and the ammeter must be inserted into the path so that the entire flow of electricity passes through the meter, a technique known as a series connection. The ammeter’s low internal resistance ensures it does not significantly impede the current it is measuring.
The ohmmeter is the only meter function that must supply its own power to perform the test. This need for the meter to be the sole source of power is the primary reason why circuit power must be off and the component isolated. Measuring resistance in isolation prevents interference and ensures the reading is specific to the component itself, contrasting sharply with the procedures for voltage and current.