What Is Ohmmeter?

by Matthew Mills | Last Updated: February 10, 2021
What Is Ohmmeter?

In order to test your electronic devices, you need different types of instruments like voltmeter, ohmmeter, ammeter, etc. They can be used to test circuit properties like the voltage, resistance, and current to see if all the connections are correct.

For expert technicians, it is not enough to just read a testing instrument. You cannot connect an ohmmeter to a circuit for testing purposes if you have no idea how it operates. So What Is Ohmmeter?

An ohmmeter is a device used for measuring a material’s electrical resistance, which represents the opposition it presents to electric current flow. This instrument consists of a variable resistor, a meter movement that is calibrated in ohms, and a battery.

There are three main types of ohmmeters, and these are micrometers, teraohmmeters, and megohmmeters, and they are different from each other in regards to the resistance range they are able to measure.

The device indicates the resistance reading through a mechanical meter movement operating on an electric current. To produce the required current to operate the meter movement, there must be an in-built voltage source inside the ohmmeter. It must also possess ranging resistors that allow the optimum amount of current to pass through, regardless of the resistance.

What Is Ohmmeter?

Generally, ohmmeters have low accuracy. You can use it for a wide range of applications to find out the approximate resistance value. It can be used to determine the resistance of different parts of a circuit like a machine field coils, the heater element, circuit continuity, inspecting the semiconductor diodes, sorting and measuring of a resistor, etc.

Additionally, it has applications in academics as well. You can use it in a laboratory along with a bridge to know the approximate resistance value. This gives you abundant time to balance the bridge.


How does an ohmmeter work?

How does an ohmmeter work?

An Ohmmeter works according to Ohm’s Law which says that the steady current flowing through the resistor is directly proportional to the voltage difference across its ends. The mathematical representation of Ohm’s Law is V=IR.

If you wish to determine a conductor’s resistance, you need to connect the black and red wires to the negative and positive terminals. The device has a needle that shows the resistance of the circuit or the wire.

The deflection of the needle depends on the battery current. At first, you must short together the leads to measure the circuit’s resistance. After doing this, you can alter the meter for the required operation in a specific range and the needle goes to the maximum value on the scale and the meter current is at maximum.

After you are done testing the circuit, you must remove the test leads. The battery loses charge when the leads are joined with the circuit. When the test leads are shorted and the necessary adjustments are performed on the rheostat, the needle returns to zero. This indicates that there is no resistance between the leads.


How to use an ohmmeter?

Here are the steps you need to follow in order to measure resistance using an ohmmeter:

1. Setting up your instrument

Step 1

First, you need to make sure the Ohmmeter is plugged in or has a battery inside. Most Ohmmeters come with a battery that is in-built or separately packed along with instructions for installation. Regardless of whether your device is analog or digital, you’ll need to complete this step.

Step 2

The next step requires you to connect the test leads to the right sockets. The black lead should be connected to the negative slot while the red leads go into the positive plug. You must plug the short ends into the sockets because the longer ends are used to test the resistance of a circuit.

Step 3

The third, you need to do the zero adjustments by touching the two lead ends together. As you bring them together, the ohmmeter’s needle should point rightward. If this doesn’t happen, you need to shift the adjustment knob until when the needle reaches 0 while the leads are still in contact with each other.

Unlike most of the normal measuring scales, the ohmmeter scale displays the reading in the reverse direction. This means that the resistance value decreases as the needle moves towards the right and increases as it moves towards the left. If you are using a digital ohmmeter for measuring the circuit or wire, then the reading on the screen should be zero when you connect the two leads.

Step 4

If it’s possible, you should test out your ohmmeter on a regular resistor brought from the store. You should get yourself a resistor where the value of resistance is already mentioned. Next, calculate this resistance using the ohmmeter and check if the two readings match.

To test the ohmmeter, you can use plenty of other things apart from the resistor, like a mark made with a graphite pencil on paper, or a strip of aluminum. But of course, you have to know the resistance value beforehand. You can do this by placing the leads of the ohmmeter on the conductor’s either end and check if the screen shows the proper resistance value.

How to use an ohmmeter
How to use an ohmmeter
How to use an ohmmeter

2. Measuring the resistance

Step 1

Make sure there is no power flowing into the circuit. Disconnect it completely from the power source or turn it off. The circuit or wire you are measuring must be completely dead because otherwise the measurement may be inaccurate and you can suffer from an electric shock as well.

Since the Ohmmeter acts as a source of power supply current and voltage to your circuit, you don’t need any other source of power. If you conduct the test on a powered circuit, it can damage the circuit, the device, as well as yourself.

You should always make sure you are not testing on a live circuit. In some cases, it is not enough to just cut off the source of power. If the circuit has capacitors in it, it will take some time to discharge. If the electronic device is based on cathode ray tubes, like in the case of a television or if it is a microwave oven, you need to exercise caution and wait for a while before you can start testing the circuit.

Step 2

Next, you need to adjust the device to the proper scale. You can use the dial to make this adjustment, turning it in the center to adjust the scale, starting from 10s, to 100s, to 1000s, and so and so. As a best practice, you can adjust the meter to a scale that is just above the resistance of the circuit being measured.

For instance, if the circuit to be measured has an expected resistance of a few thousand, you need to adjust the ohmmeter scale to the 100s. As such, if the reading is “12”, you can say that the resistance of the circuit is 1200 ohms. Of course, you need to have a rough idea of the resistance value in order to adjust the scale.

Step 3

After this, you should place the two probes at either end of the circuit you are measuring. You can connect a probe to either end and it wouldn’t make any difference as far as the resistance reading is concerned.

This is because you will be measuring the resistance, the value of which remains the same regardless of the direction of flow of current. You can place the positive and the negative probe on any end to get the proper resistance value.

Step 4

This step involves determining the resistance of the circuit by reading the number on the device. Then you have to multiply this number by the scale that you’ve set your meter to, and you will get a value that equals the resistance of the circuit.

With decreasing resistance, current can flow more easily through the electronic circuit. So a circuit having a resistance of 0 ohms would allow current to flow through it unobstructed.

Step 5

If you wish to measure the resistance values for the different small components of a circuit, then individual testing needs to be performed for them. If you wish to test a particular resistor attached to the board, you can extract it using a soldering iron.

This is to make sure that there is no accuracy in the reading owing to interference from another component in the circuit. To put it another way, if you use an ohmmeter to test a resistor that is still attached to the circuit board, there is no way for you to make sure that the resistance value shown in the reading is only from that specific resistor.

Step 6

After you’re done measuring the resistance, you should turn off the ohmmeter. If you store the device without turning it off, the test probes are prone to get shorted, which would drain the battery. You can avoid this by disconnecting all the wires and turning the device off before keeping it aside.


How to read an ohmmeter?

In order to take a reading using an ohmmeter, first, you need to set its range by turning it on and turning the dial located on the front. If you have no idea regarding the resistance range of the resistor to be measured, you should begin from the highest setting in the megaohm range and reduce it gradually till you get a reading on your ohmmeter.

If you are working with an ohmmeter having a needle display, the needle will move towards the middle. In case the needle lies close to either end, you should change the range until the point you can see it lying near the full range.

How to read an ohmmeter

If the device is using a D’Arsonval display, there is likely to be a reflective arc lying beneath the numbers. You should adjust the meter or your head till the scale is right in front of you and all you can see is the needle. If instead, you see its reflection that would mean you are viewing it at a slight angle and cannot spot the needle’s exact position.

While you are looking straight at the scale, find out where along the scale the needle lies. There will be many different scales and you need to ensure that you are reading the needle’s position correctly. The needle will show the resistance value of the resistance being tested.


How to test a transformer with an ohmmeter?

How to test a transformer with an ohmmeter

If you are using an ohmmeter to test an AC transformer, you need to measure the resistance of the coiled wires lying near the core. Then you touch the leads to either end of the wiring and read the resistance value shown in the display.

Check if this value is the same as that mentioned on the datasheet of the transformer. If the difference between the readings is quite large, it is quite probable that there is something wrong with the transformer and so you should replace or remove it immediately. Just to be sure, you can conduct the test 3-4 times since the readings are not always accurate.


How to test a capacitor with an ohmmeter

For this purpose, you can use a digital or analog device. Take it to the highest ohm range, at least in the thousands. At this point, a small amount of current is generated by the meter when you connect the leads to the opposite ends of the capacitor. After you connect the leads, you should keep them there for a few seconds.

How to test a capacitor with an ohmmeter

In the case of an analog meter, the needle will start rising from a low value, climbing steadily as the charge accumulates in the capacitor. This will happen provided the capacitor is working properly. Otherwise, the needle will remain in the same position.

In the case of a digital ohmmeter, a good capacitor will cause the reading on the display to rise slowly until the capacitor gets discharged. After that, the value will go back to 0 and start rising again. In case of a bad capacitor, the resistance value will be extremely low and it will remain unchanged.

What Is Ohmmeter – Final Word

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