Before setting up the multimeter, you need to make sure you have all the equipment in order for you to be able to test anything. The things you will need for this test are:
Two meter probes (one black and one red)
Batteries (If your meter doesn’t come with any)
A fuse ( if your meter doesn’t come with one)
It would be even better if your meter came with a calibration certificate – this would ensure that your meter is in good working order and test results would be more accurate. This is not mandatory, as you can work out if the meter works accurately by comparing the results with another meter.
It is preferable to get an ohmmeter with auto ranging functionalities because then you don’t have to waste time fiddling with manual ranging.
Setting up the Ohmmeter
The first thing we must do is to check if the meter turns on. When you turn the dial, the device should turn on. If it doesn’t, then you need to make sure that a new battery is placed in.
If the device you are using has multiple functionalities such as measuring voltage, current etc and you want to measure resistance, you need to make sure that it is set to measure Ohm.
After you have done this, insert the red probe in the positive (red) terminal and the black probe in the COM (black) terminal. When testing for resistance, you must not use the meter to test a live circuit. Doing this will result in damaging the meter or blowing the fuse. If your meter has a fuse and it is damaged, you can replace it by opening the back of your meter. To check if your meter is working correctly, place the red and the black probes of the leads together. If you meter is broken, there will be an ‘OL’ on the digital display but if the meter is not broken, a small resistance should be displayed.
Removing the test piece
The first thing we must do before testing the resistance of a component is removing it from the circuit it is in. For example, if you wanted to find the resistance of a copper wire, you would have to disconnect all electrical components and voltage sensitive equipment from the circuit first. If you’re measuring resistance of a resister, you first must remove the resister from the circuit and then test the resister.
The way the multimeter measures resistance is by sending a small amount of voltage (typically less than 1 volt) to one end of the item you are testing. Current created by voltage flows through the item or component and goes back to the multimeter through the other probe to give us the measurement of resistance.
It is not possible for you to find the resistance of a component whilst it is still in a circuit. The reason for this is that the multimeter will pick up other resistances found in that circuit as well as the resistance of the component you are testing, which means that the measurement you will be getting will not be correct.
Once you have followed these steps, we can move on to testing.
Using an ohmmeter to test
If you are testing a resister, you need to get one of the meter probes and place it on the connecting lead on one of the sides of the resister, then you need to place the second probe on the other connecting lead on the opposite side like demonstrated in the image below:
Once you have done this a reading should be displayed on your meter.
When testing an insulated copper wire, you must make sure that a bit of copper is exposed on both sides of the wire. With the meter probe, touch one side of the wire ensuring that you have touched the copper part of the wire and not the plastic insulation. Then, with the second probe, touch the other side of the copper wire, again ensuring that you have made contact with the copper and not the plastic insulation, if you touch the plastic insulation you will get an incorrect reading.
When you want to test the resistance of a bulb, you must remove it from the circuit, then place one probe on the base of the bulb (the metal part under the glass) and the other probe on the electrical foot contact (the bottom of the bulb). You should then get a reading for your bulb.
If you are going to measure the resistance of an audio speaker, you need to make sure you have access to the two wires at the back of the speaker. Next, you need to place the red probe on one of the wires of the speaker and the black probe on the other side of the speaker, making sure that you placed the probe on the metal part of the wire and not the plastic/rubber insulation.
Internal damage testing
Earlier, I mentioned that if your multimeter displays ‘OL’ it means your meter is damaged. However, the ‘OL’ which means ‘open’ can also mean the component you are measuring is damaged. When a circuit is opened it means that the circuit has been switched off because you have prevented electricity from flowing through the complete path of the circuit. A good circuit will display ‘OL’ when it is turned off. If a circuit is broken, it is technically, switched off permanently (meaning, there is a break in the continuity of the conductor).
When the ohmmeter tests for resistance, it sends a certain amount of voltage through the circuit (on some instruments, it gives you a choice of test voltage). If the circuit is open the voltage sent by the meter is never returned to it which is why it displays ‘OL’.
A switch which is working properly should display a low resistance when it is on. If the switch displays a high resistance, the switch will be worn out and will have too much resistance within it.
Upon learning how to master the Ohmmeter, you will be able to solve many problems relating to electrical appliances that you previously were not able to solve. In addition to this, if you are an electrician, then knowing how to correctly use an ohmmeter is vital and it will help you perform your jobs far better. However, whether you are an electrician or not, an ohmmeter is a tool that every person should have for electrical work.