Ohmmeter: How Does it Work? (Series, Multi-Range & Shunt Type Ohm Meters)

What is an Ohmmeter?

An ohmmeter, also written as ohm meter, measures electrical resistance. The instrument applies a known test stimulus and measures the response produced by electric current. Micro-ohmmeters and milliohmmeters are designed for very low values, while Megohmmeters, including instruments made by Megger, use higher test voltages to measure insulation resistance.

A measured value depends on the device, its temperature and the test method. The resistance of metallic conductors usually rises with temperature. Semiconductor temperature behaviour depends on material, doping and operating region, so it should not be reduced to one rule for every device.

Three common classic analogue ohmmeter arrangements are:

1. Series ohmmeter.
2. Shunt ohmmeter.
3. Multi-range ohmmeter.

Working Principle of Ohmmeter

A basic analogue ohmmeter uses a battery, a limiting resistor, a zero-adjust resistor and a current-sensitive movement. The unknown resistance completes the test circuit. Current then sets the pointer position on a scale calibrated in ohms. A digital meter instead measures voltage produced by a known test current and calculates resistance from Ohm’s law.

In a series analogue circuit, a very high unknown resistance produces almost no current, so the pointer rests near the infinite-ohms end of the scale. Shorted test leads produce maximum current, and the zero-adjust control sets that position to zero ohms.
Measure resistance only on a de-energised circuit using a meter and procedure rated for the task. Discharge capacitors first and isolate the component when parallel circuit paths could alter the reading.

D’Arsonval Movement

A D’Arsonval movement is a permanent-magnet moving-coil mechanism used in analogue DC measuring instruments. Current in the movable coil interacts with a permanent magnetic field and produces torque that moves the pointer.

The movement places a light rectangular or circular coil in the air gap of a permanent magnet. A soft-iron core provides a low-reluctance path and shapes the flux so the field in the coil gap is strong and nearly radial.

The field and coil geometry provide useful torque from a small current. Spiral control springs carry current to and from the moving coil and provide restoring torque. The pointer settles where electromagnetic and spring torques balance.

Reversing the current reverses the torque. A bare movement therefore responds to polarity and is intended for DC unless a rectifier or another conversion circuit is added. Coil current is proportional to pointer deflection, but an analogue ohmmeter’s resistance scale is nonlinear because current changes inversely with total circuit resistance.

Eddy-current damping opposes motion and lets the pointer settle quickly without repeated oscillation. The control springs, rather than damping, set the final steady position.
A mirror strip on the scale helps the user align the pointer with its reflection and reduce parallax error. It does not create the indication.

A D’Arsonval movement offers these practical characteristics:

1. Pointer deflection is linear with movement current.
2. Eddy-current damping provides a fast, stable indication.
3. The movement can operate with low power.
4. The air-cored moving coil avoids the hysteresis associated with a moving iron element.
5. A strong internal field and magnetic shielding can reduce, but not eliminate, stray-field error.

The movement also has limitations:

1. The bare movement does not indicate alternating current correctly; it needs a conversion circuit and otherwise responds to DC current only.
2. Its magnets, bearings, coil and springs make it more delicate than many moving-iron mechanisms.
3. Spring ageing, magnet ageing, friction, temperature and mechanical shock can change calibration.

Analogue ohmmeters use a DC source because the movement in PMMC instruments responds directly to DC. This gives a useful indication of DC resistance, but there is no universal factor for converting it to AC resistance. Skin effect, proximity effect, inductance, capacitance and material losses make AC impedance depend on frequency and construction.

Series type Ohmmeter

A series-type ohmmeter places the unknown resistance in series with limiting resistor R₁, zero-adjust resistor R₂, source E and movement resistance R_m.
With the test terminals shorted, total resistance is at its minimum. The current and pointer deflection are therefore at their maximum.

The user shorts the leads and adjusts R₂ until the pointer reaches the zero-ohms mark at full-scale current. With terminals AB open, current approaches zero and the pointer rests at the infinite-resistance mark. The ohms scale therefore runs in the opposite direction to the movement-current scale.

An unknown resistance produces a current between those two limits. The calibrated nonlinear scale converts the resulting pointer position into ohms. Best resolution occurs near the middle of the selected range.

As the battery voltage changes with age and use, the shorted-lead current changes. The zero-adjust control compensates within a limited range, so the user should repeat the adjustment after changing range and before taking readings. A meter that cannot be zeroed needs a battery or service check.

Shunt Type Ohmmeter

A shunt-type circuit places the unknown resistance in parallel with the meter movement. A battery and adjustable series resistor establish the test current. A switch disconnects the battery when the instrument is not being used.

With zero unknown resistance, terminals A and F are shorted and most current bypasses the movement. The pointer remains at its zero-deflection position, which is labelled zero ohms for this circuit.

When the unknown resistance is very high, little current uses the shunt path and the movement approaches full-scale deflection. The series adjustment sets the required full-scale current under this open-terminal condition.

Full-scale deflection therefore represents a very high or open-circuit resistance. Intermediate values divide current between the movement and the unknown shunt, producing a calibrated pointer position.

Battery voltage, contact resistance, movement friction and calibration drift still affect the result. The adjustment corrects the reference point but cannot remove every source of error.

Multi-Range Ohmmeter

A multi-range analogue ohmmeter switches different resistors or sources into the test circuit so one movement can cover several resistance decades. The user selects a range and performs the specified zero adjustment before reading the matching scale multiplier.

The connection of the unknown resistance depends on whether the selected circuit is series or shunt type. The range switch changes the circuit constants so the most useful part of the scale covers a different band of values.

Short-circuit behaviour also depends on the circuit topology: a series ohmmeter has maximum movement current, while a shunt ohmmeter has minimum movement current. For an unknown value, start on a safe high range and step down until the indication falls in a readable part of the scale.

A range label such as ×1 or ×100 is a scale multiplier, not a statement that one particular resistance must cause full-scale deflection. Apply the selected multiplier to the indicated scale value. For precision below a few ohms, a four-wire Kelvin instrument is preferable because separate force and sense leads remove most lead and contact resistance from the result.

Multi-range ohmmeters extend coverage, but range count alone does not determine accuracy. Resolution, test current, lead configuration, calibration, temperature and the device under test all matter. Modern digital multimeters often add autoranging, relative mode and four-wire resistance functions.