There are different types of ohmmeters with different levels. Like micro, mega, and milli ohmmeter, etc. Low resistance is measured by a micro-ohmmeter and high resistance by a mega-ohmmeter. Using an ohmmeter is easy and convenient, but the result obtained is less accurate.
The instrument which is used to measure the electrical resistance of a circuit and the resistance of the components of an electric circuit is called an ohmmeter. An ohmmeter can also be used to find the value of an unknown resistor. Resistance is measured in ohms.
Symbol of Ohmmeter
Working Principle of Ohmmeter
When current is passed through a circuit or component, the meter pointer begins to deflect. When the pointer moves to the left of the meter, it will represent high resistance and will respond to low current. When the pointer is deflected to the right of the meter, it will represent low resistance and will respond to a high current.
In an ohmmeter and an analog multimeter, the resistance measurement scale is non-linear. The meter indicator measuring resistance is zero on the absolute scale (on the right) and is the maximum read value at rest. The pointer has to be brought to the zero position before it can be used. After coming to zero, the test can be done. Resistance meters typically range from 1 ohm (1Ω) to 1 megohm (1MΩ). When the two probes are connected on each side of the resistor, the indicator meter starts to deflect.
To read the ohmmeter, the selector knob is turned to an estimated ohm range or, when set to maximum, gives an estimated reading. If the value is too high, the pointer remains at zero. You can try adjusting the resistance range dial to a lower multiplier range or keep adjusting the knob to get accurate results.
When the knob adjustment is complete, calculations need to be done with the results read on the scale. If the multiplier range is marked ‘x10’, the reading has to be multiplied by 10 ohms. If the multiplier range notation is written as ‘X1K’, then the reading needs to be multiplied by 1000 ohms.
Types of Ohmmeter
The following are the three main types of ohmmeters.
- Series Ohmmeter.
- Shunt Ohmmeter.
- Multi-Range Ohmmeter.
When an electrical circuit or component is to be measured, it is connected in series with the meter. So this type of combination is a series ohmmeter. The value of resistance is calculated using D’Arsonval movement through shunt resistor R2 which is connected in parallel. The R2 resistance is connected in series with the battery and the R1 resistance. The electrical circuit or component to be measured is connected in series by two terminals A and B.
Whenever the value of the electrical circuit or component to be measured is zero, a large current flows through the meter. In this case, the shunt resistance cannot be corrected until the meter specifies the full-load current. For this current, the needle is rotated in the direction of 0 ohm.
Whenever the electrical circuit or component to be measured is disconnected from the circuit, the circuit resistance becomes unlimited and current flows in the circuit. The meter needle is deflected towards infinity. The meter displays infinite resistance when no current flows through it and zero resistance once a large amount of current flows through it.
Whenever the electrical circuit or component to be measured is connected in series with the circuit and the resistance of that circuit is high, the meter needle is deflected in the left direction and when the resistance is low, the needle deflects in the right direction.
Series Ohmmeter used to High resistance measurements and machine field coils, etc.
In this type of ohmmeter the electrical circuit or component to be measured is connected in parallel with the battery. It is mainly used to measure low-value resistance. The following circuit is made with the meter, battery and the circuit or component to be measured. The circuit or component to be measured is connected with terminals A and B.
When the resistance value of the electrical circuit or component is zero, then zero current flows in the meter. Similarly, when the resistance of the electrical circuit or component is high, the current flow through the battery and the needle is represented by a full-scale deflection to the left. In this type of meter, there is no current on the scale in the left direction and infinity in the right direction.
Shunt Ohmmeter used to Detect low resistance value, precise bridge circuit and heater element etc.
Multi-range ohmmeters have a very long range. For measuring a wide range of resistance values, this meter provides a selectable switch. Using the adjuster set the initial reading to zero. To find the unknown resistance, a series of resistance values are connected in parallel with the meter. Adjustments are made in such a way that the meter can display the full scale value.
Other Types of Ohmmeters
The Following are other types of Ohmmeters.
This ohmmeter measures relatively low resistance in the range of 1μΩ to 2500Ω. The meter consists of a set of resistors with different current ranges.
It uses the 4-wire Kelvin method to measure the resistance of an inductive load. It uses filters to eliminate AC waves. Some of them are 10A-5m, 10A-25m, 10A-250m, 1A-2500m, 100mA-25Ω, 10mA-250, 1mA-2500.
It is used to Measures resistance for motors, transformers, components, circuit breakers and switches and RTD measurements etc.
The digital milli-ohmmeter calculates resistance in the range of 100 μΩ to 2000Ω with high accuracy. It uses 4-wire resistance technology to measure resistance.
Applications are winding resistance measurements for electric motors, and bond testing for generators, railways, ships, Voltage and current measurement, Testing diodes and PCB tracks, etc.
Mega ohmmeter (Megger)
Megger instrument measures the megaohms and giga-ohms resistance in a circuit. It is suitable for insulation resistance measurement. The meter has a measurement range of 0.5Ω to 2,000,000 MΩ.
It is used to Insulation cables, Testing of capacitors, Grounding, and short circuit testing etc.
It is also known as a digital multimeter for resistance measurement. It also measures current and voltage in electronic circuits. This meter is easy to read when compared with analog. You can measure resistance in ohms, kilo-ohms and mega-ohms on a digital display.
It is used to measures voltage, resistance (Ω, kΩ, MΩ), currents, and so on.
Applications of Ohmmeters
- An ohmmeter can be used to ensure the continuity of the circuit which means that if there is enough current through the circuit or there is a very large flow of current then the circuit will fall apart.
- They are widely used in electronic laboratories in engineering to test electronic components.
- These are used for debugging small ICs such as PCBs and other stuff that needs to be executed in sensitive equipment.