Ohmmeter for Resistance Measurement

Ohmmeters

• 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.

Ohmmeters Construction

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

D’Arsonval Movement

• 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.

Ohmmeter Types

• 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.

Applications of an Ohmmeter