Ohmmeter: Measure Resistance Accurately

An ohmmeter is an electrical instrument used to measure the resistance in a circuit or a component.
Works based on when an ohmmeter applies current to the circuit or component, it measures the resulting voltage and calculates the resistance value using ohms law formula \(V=IR\).
To measure resistance, we can also use an analog multimeter and digital multimeter.
We cannot find out the resistance using ohmmeter in a working or test circuit.
To check the resistance, we need to disconnect the power and measure the resistance.
The micro-ohmmeter measures low resistance and the mega ohmmeter measures the high resistance of the circuit.
The ohmmeter is very convenient to use but less accurate.

Analog multimeter

mix of milliammeter (microammeter) with a set of resistances in series and a constant battery source.
when current flow through the circuit or component, the pointer deflects in the meter.
Display | Pointer | Range selector knob | mA or \(\mu\)A | Multimeter Dial | Jack/Ports | Probes/leads

This type of movement is used in DC measuring instruments.
The main principle is that when a current-carrying coil is placed in a magnetic field, it feels a force that can deflect the pointer of a meter and we get the reading in the instrument.

Series Ohmmeter
Measures high resistance values for DUT.
It uses two resistors (series and zero adjust) to find out unknown resistance across the resistor.
The zero adjust resistor is in parallel with D’ Arsonval (meter movement).
The device has an internal source voltage to produce current and shows resistance through meter deflection.

Shunt Ohmmeter
Shunt meter measures low resistance values in a circuit.
The infinity reading is adjusted instead of zero resistor.
This type of ohmmeters is not used as their measuring range is low (5 to 400 \(\Omega\)).
Unlike series type, this meter movement is in parallel with the resistance to be found.

Multi-Range Ohmmeter
To measure a wide range of resistance values, this meter provides a selection switch.
The initial reading is set to zero using adjuster.
To know unknown resistance, connect it in parallel to the instrument.
The adjustment is done such that, the meter shows full-scale value.