Measuring electrical resistance is a fundamental part of working with circuits and electronic devices. Resistance determines how much current can flow through a circuit, which is essential for making sure components don't get damaged or overloaded. One tool that engineers and technicians use to measure resistance is an ohmmeter. In this blog post, we'll dive into the world of ohmmeters, exploring the different types available, how they work, and some common applications.
What is an Ohmmeter?
An ohmmeter is a specific type of electronic equipment that is primarily used to determine the electrical resistance of a circuit. The ohm is the unit of resistance. Calculating an object's electrical resistance involves determining how much resistance it has to current flow. Micro, mega, and milli-ohmmeters are a few examples of the several types of meters that are available, each with a distinct sensitivity level. The micro-ohmmeter is utilized in bonding contact applications because it can calculate extremely low resistances with great precision at particular test currents.
Fig 1. Ohmmeter measures the electrical resistance of a circuit.
It is an electronic device that is used to measure electrical characteristics and parameters and it is of two general types:
Analog ohmmeters are the most basic type, consisting of a needle that moves across a scale to indicate resistance. Digital ohmmeters use electronic displays to show the resistance value numerically. Multimeter ohmmeters are a versatile type that can measure other electrical values besides resistance. Each type has its own set of pros and cons, so it's important to choose the right one for the job.
Working in the second type is easier and more accurate considering that most operations of measuring and changing the range are automatic. The quantities measured by the ohmmeter are generally as follows:
- Direct current DC and alternating AC amperes
- Voltage direct current DC and alternating AC
- Interruption and continuity of circuit
Ohmmeter Measuring Methods
An ohmmeter is an electronic device used to measure the electrical resistance of a circuit or component. To measure resistance, the ohmmeter sends a known current through the circuit or component and measures the resulting voltage. By using Ohm's Law (V = IR), the ohmmeter can calculate the resistance of the circuit or component.
There are two primary methods of measuring resistance: direct and indirect.
Direct Measurement Method
In the direct measurement method, the ohmmeter is directly connected to the circuit or component being tested. The ohmmeter sends a known current through the circuit or component and measures the resulting voltage. Based on Ohm's Law, the resistance of the circuit or component can then be calculated by dividing the measured voltage by the known current.
Direct measurement methods are generally fast and accurate, and they are useful for measuring low-resistance values. Some common examples of direct measurement methods are the DC direct current ampere measurement method and the AC ammeter-voltmeter measurement method.
DC Direct Current Ampere Measurement Method
The DC direct current ampere measurement method is a type of direct measurement method that uses a DC source to send a known amount of current through the circuit or component. The ohmmeter then measures the voltage drop across the circuit or component and uses Ohm's Law to calculate the resistance.
To perform a measurement using the DC direct current ampere method, the circuit or component being tested must be disconnected from the power source. The positive and negative leads of the ohmmeter are then connected to the ends of the circuit or component, and the ohmmeter is set to the resistance measurement range.
Once the ohmmeter is set up, a small DC current is sent through the circuit or component. As mentioned earlier, the ohmmeter measures the voltage drop across the circuit or component and uses Ohm's Law to calculate the resistance. The calculated resistance is then displayed on the ohmmeter's display.
One limitation of the DC direct current ampere measurement method is that it can only be used for circuits or components that can handle a DC current without damage. Additionally, the DC current source used in this method has a fixed value, which means that it may not be suitable for measuring very high or very low resistance values.
AC ammeter-voltmeter measurement method
The AC ammeter-voltmeter measurement method is another type of direct measurement method that can be used to measure the resistance of a circuit or component. In this method, an AC (alternating current) voltage source is used to send a known amount of AC current through the circuit or component being tested. The ohmmeter then measures both the AC voltage and AC current, and calculates the resistance using Ohm's Law.
To perform a measurement using the AC ammeter-voltmeter method, the circuit or component being tested must be disconnected from the power source. The positive and negative leads of the ohmmeter are then connected to the ends of the circuit or component, and the ohmmeter is set to the resistance measurement range.
Once the ohmmeter is set up, an AC voltage is applied to the circuit or component. As I mentioned earlier, the ohmmeter measures both the AC voltage and AC current flowing through the circuit or component, and calculates the resistance using Ohm's Law.
The AC ammeter-voltmeter method is useful for measuring the resistance of circuits or components that cannot be tested with a DC current source, such as AC circuits. However, it has some limitations. One limitation is that it can only be used for circuits or components that can handle an AC current without damage. Additionally, the calculations involved in this method can be more complex than those in the DC direct current ampere measurement method.
Indirect Measurement Method
In the indirect measurement method, the ohmmeter measures other electrical values besides resistance and then calculates the resistance from those values. This method is often used for measuring high-resistance values, and it's useful when the circuit or component being tested cannot be directly connected to the ohmmeter.
One common example of indirect measurement is the bridge method, which involves using a Wheatstone bridge circuit to measure resistance. In this method, the circuit or component being tested is connected to the Wheatstone bridge circuit, and the bridge is balanced to measure the resistance. The balanced resistance value can then be used to calculate the resistance of the circuit or component.
Overall, both direct and indirect measurement methods have their advantages and limitations, and it's important to choose the right one for the job. The DC direct current ampere measurement method is a simple and straightforward way to measure resistance in certain situations, and it's useful for measuring low-resistance values. Indirect measurement methods are often used for measuring high-resistance values and are useful when the circuit or component being tested cannot be directly connected to the ohmmeter.
Working Principle of Ohmmeters
Ohmmeters operate on the principle that they have a needle and two test leads. The battery current can be used to control the needle deflection. To begin with, the meter's two test leads can be connected in series to determine an electrical circuit's resistance. Once the meter's two leads are shorted, it can be adjusted for the proper response within a predetermined range. The current in the meter will be at its highest when the needle reaches the highest point on the scale. Below is a circuit diagram for an ohmmeter.
Fig 2. Ohm meter working principle.
The test leads of the meter must be disconnected after the circuit has been tested. The battery is depleted as soon as the two test leads of the meter are connected to the circuit. The rheostat will be changed when the test leads short. The resistance between the two test leads will be zero once the meter needle has been moved to its lowest, 0 positions.
Exploring the Different Types of Ohmmeters
Based on its intended use, this meter can be divided into three categories:
- Series-type ohmmeters
- Shunt-type ohmmeters
- Multi-range type ohmmeters
Below is a succinct discussion of meters.
The component we want to measure can be connected to the ohmmeter in series when using a series-type ohmmeter. Through the parallel-connected shunt resistor R2, which uses D'Arsonval movement, the resistance value can be calculated. Both the battery and the R1 resistance can be connected in series with the R2 resistance. The two terminals A and B are used to connect the measuring component in series.
Fig 3. Series-type ohmmeter circuit drawing
A shunt ohmmeter can be connected when the computational component is connected in parallel with the battery. This type of circuit is used to calculate low-value resistors. The following circuit can be assembled with the measuring instrument, battery, and measurement parts. Measurement components can be connected via terminals A and B.
Fig 4. Shunt-type ohmmeter circuit drawing
The range of the multi-range ohmmeter is very high, this meter includes an adjuster that allows you to select the range of the meter according to your needs.
Fig 5. Multi-range type ohmmeter circuit drawing
The Uses of an Ohmmeter
Ohmmeters have a wide range of applications, from testing circuit boards to diagnosing automotive problems to troubleshooting telecommunications equipment. Engineers and technicians use ohmmeters to check the resistance of components like resistors, capacitors, and inductors, as well as to identify wiring problems and faulty connections.
Common Ohmmeter Issues and Troubleshooting
Like any piece of equipment, ohmmeters can sometimes experience issues. One common problem is incorrect readings, which can be caused by a variety of factors, such as incorrect calibration or a damaged component. Battery failure is another issue that can occur. Fortunately, many of these issues can be fixed with some simple troubleshooting steps.
Ohmmeters are an essential tool for anyone working with electrical circuits and devices. By understanding the different types of ohmmeters, the various measuring methods, and how they work, engineers and technicians can use these tools to diagnose and fix a wide range of problems. Whether you're working in electrical engineering, automotive repair, or telecommunications, an ohmmeter is a valuable addition to your toolkit!