# What is an Ohmmeter? Circuit Diagram, Types and Applications

There are different types of meters available for electronic devices testing, etc. The electronic devices testing equipment like ammeter, ohmmeter, voltmeter, and multimeter are used to test the circuit resistance, voltage and current to check the wiring connection whether the connection is correct or not. So the circuit testing can be done using a device called ‘Ohmmeter’. But without identifying the working concept, it is unfeasible for connecting this device to any circuit for testing the soldering components. However, to be a skilled technician, one should require being an expert in this to do many things than simply reading a test device. This article discusses an overview of ohmmeters, circuit working, types, and applications.

## What is an Ohmmeter?

An ohmmeter can be defined as, it is one kind of electronic device mainly used for calculating electrical resistance of a circuit, and the unit of resistance is ohm. Electrical resistance is a calculation of how much an object resists allowing the flow of current through it. There are different types of meters available with different sensitivity levels such as micro, mega and milli-ohmmeters. The micro-ohmmeter is used for calculating very low resistances with high precision at specific test currents, and this ohmmeter is used in bonding contact applications.

Micro-Ohmmeter is a portable device, mainly used for calculating current, voltage, as well as for diodes testing. This type of meter comprises several selectors for choosing the preferred function, and it automatically ranges to select most measurements. Mega-ohmmeter is mainly used for calculating the large resistance values. Milli-Ohmmeter is useful for calculating low-resistance at high precision to verify the value of the electrical circuit.

### Ohmmeter Working Principle

The working principle of ohmmeter is, it comprises of a needle and two test leads. The needle deflection can be controlled with the battery current. Initially, the two test leads of the meter can be shorted together to calculate the resistance of an electrical circuit. Once the two leads of the meter are shorted, then the meter can be changed for appropriate action in a fixed range. The needle comes back to the highest point on the meter scale, and the current in the meter will be highest. An ohmmeter circuit diagram is shown below.

Once the testing of the circuit is done then the test leads of the meter must be detached. Once the two test leads of the meter are connected to the circuit then the battery gets discharged. When the test leads get shorted then the rheostat will be adjusted. The meter needle can be reached to the lowest position that is zero, and then there will be zero resistance among the two test leads.

## Types of Ohmmeter

The classification of this meter can be done based on the application in three types namely series type ohmmeter, shunt type ohmmeter, and multi-range type ohmmeter. The brief discussion of meters is given below.

### 1) Series Type Ohmmeter

In series type ohmmeter, the component which we want to measure can be connected with the meter in series. The resistance value can be calculated through the shunt resistor R2 using D’Arsonval movement which is connected parallel. The R2 resistance can be connected in series with the battery as well as R1 resistance. The measuring component is connected in series by the two terminals A as well as B.

Whenever the measuring component value is zero then there will be a huge flow of current through the meter. In this situation, the shunt resistance can be corrected until the meter specifies the full-load current. For this current, the needle turns aside in the direction of 0 ohms.

Whenever the measuring component is detached from the circuit then the circuit resistance turns into the unlimited & flow of current in the circuit. The needle of the meter deflects towards the infinity. The meter illustrates the infinite resistance when there is no flow of current & the zero resistance once the huge flow of current through it.

Whenever the measuring component is connected in series with the circuit, and the resistance of that circuit is higher, the meter needle will deflect in the direction of the left. And if the resistance is little, then needle turn aside in the direction of right.

### 2) Shunt Type Ohmmeter

The connection of shunt type ohmmeter can be done whenever the calculating component is connected in parallel with the battery. This type of circuit is used to calculate the low-value resistance. The following circuit can be built with the meter, the battery, and the measuring component. The measuring component can be connected across the terminals A & B.

When the resistance value of the component is zero then the current in the meter will become zero. Similarly, when the resistance of the component becomes vast then the flow of current through the battery & the needle illustrates the full-scale deflection in the direction of the left. This type of meter has no current on the scale in the direction of left as well as the infinity spot in their right direction.

### 3) Multi-Range Ohmmeter

The multi-range ohmmeter range is very high, and this meter includes an adjuster, and the range of a meter can be selected by an adjuster based on the requirement.