What Is an Ohmmeter?
An ohmmeter is a measuring instrument used to determine the resistance of an electrical circuit.
It is one of the indispensable measuring instruments in electronics and electrical work, and is available in a wide range of accuracy and range. They are called ohmmeters because they use Ohm’s law to measure resistance.
There are several types of ohmmeters: milliohmmeters are specialized for resistance values of mΩ or less, insulation ohmmeters are used to measure the resistance of insulators, and ground ohmmeters measure the resistance between a grounded conductor and the earth. Although simple testers are sometimes used to check the resistance of circuits, ohmmeters with high accuracy are required in development and manufacturing.
Uses of Ohmmeters
Ohmmeters are used to measure the resistance of electric circuits. The following are examples of applications of ohmmeters:
1. Fault Locating
Resistance testers are useful for locating faults in electric circuits. By measuring the resistance of each part of an electric circuit, it is possible to identify the part that is out of the normal range. This type of measuring instrument is indispensable for maintenance and security of electric circuits.
2. Circuit Design and Development of Electrical Equipment
It is also used in circuit design. When designing circuits, accurate resistance measurement of elements is necessary. Some ohmmeters have a basic accuracy of 0.02% and a resolution of 1µΩ. Such ohmmeters are used in electronic device design, manufacturing, and quality control.
They also play an important role in the development of electrical devices. Electrical equipment as a product also requires accurate resistance values, which are obtained with ohmmeters.
3. Research and Development and Education
Resistance meters are also used in the field of research and development of basic materials. Ohmmeters are used to investigate the properties of materials and circuits. They are also used in educational settings, particularly in the study of electrical and electronic engineering, to measure circuits. This enables students to understand the characteristics of circuits and how to measure resistance.
Principle of Ohmmeters
Resistance meters use Ohm’s law to measure resistance. Ohm’s law is a law that states that resistance, the ratio of current to voltage, is constant. This law can be used to determine the resistance in a circuit.
Ohmmeters contain a power supply and a measurement circuit. The measurement power supply is generally supplied by a battery or AC adapter. The measurement circuit also incorporates an ampere meter to detect the circuit current to be measured and a volt meter to measure the circuit voltage.
Most precision ohmmeters use a contact-type four-point method of measurement: two of the four wires pass a constant current through the object, while the other two act as voltmeters to measure the voltage. By measuring the voltage produced when a constant current flows, resistance can be determined from Ohm’s law.
Simple testers use the 2-terminal method, which is inexpensive and easy to construct, while the 4-terminal method can measure resistance more accurately than the 2-terminal method because it is not affected by the resistance of the measuring leads or contact resistance.
Types of Ohmmeters
Various types of ohmmeters are available depending on the application. The following are examples of ohmmeter types:
1. Analog Type Ohmmeter
Analog ohmmeters use an analog pointer to indicate the resistance value. The needle vibrates according to the current flowing in the circuit, indicating the resistance value. Although they are relatively inexpensive, it is necessary to select the appropriate range according to the resistance range of the circuit to be measured.
2. Digital Type Ohmmeter
Digital ohmmeters are resistance meters that show resistance values on a 7-segment display. They are capable of high-precision measurement and are often equipped with a function that automatically switches the range. Testers with voltage and current measurement functions are also part of digital ohmmeters.
3. Clamp Type Ohmmeter
This ohmmeter measures resistance by generating a magnetic field around the circuit with a clamp between the clamps. The advantage of this type of tester is that it can make measurements without coming into contact with the circuit to be measured. It is often used as a ground ohmmeter.
4. Ohmmeter
This ohmmeter is used to diagnose the insulation condition. Insulation resistance is measured by grounding one terminal, applying high voltage, and measuring the leakage current. Ohmmeters capable of applying a high voltage of about 1,000 VDC according to the system voltage are also available. Ohmmeters are widely used for maintenance at production sites.