Description of Digital Multimeter:
In this getting started tutorial, you will learn what is a digital multimeter and how it can be used for electrical and electronics works. Apart from the basic measurements and testing we will also do Cable insulation test for 1Kv.
Digital Multimeter is one of the most basic instruments for electrical engineers, technicians, and electricians. Digital Multimeter was first introduced in the late 1970s, and is more reliable and accurate now. For decades the needle-based analog meters were used for measuring the Voltage, Current, and resistance only. Digital Multimeters somehow replaced the Analog meters as the digital Multimeters are provided with multiple options. All over the world engineers and technicians are working in different fields of engineering “industries, Power Plants, Construction, Electronics, etc”. The engineers and technicians frequently check the power supply and different electrical parameters. From my own personal experience, while making a new PCB, I continuously check the short circuits, wires continuity, the regulated voltages, resistors values, etc.
In this article, you will learn how to use a digital Multimeter and how this will help you in electrical and electronics works. I will try to explain everything through images explaining how to check AC and DC voltages, AC and DC currents, how to check the continuity of a cable, resistance, temperature, frequency, and I will also explain how to check electronics components like capacitors, diodes, resistors and LEDs etc.
It really doesn’t matter if you are using a digital Multimeter for the first time, and you are just a beginner, trust me you will find this very interesting and useful.
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What are AC and DC currents?
In the electrical and electronics world we have two kinds of currents one is AC current “Alternating Current” and other one is DC current “Direct current”. Electric charge in Direct Current only flows in one direction, while the electric charge in Alternating current changes direction periodically.
Direct current has many uses, from the charging of batteries to large power supplies for electronic systems, motors, and more. Very large quantities of electrical energy provided via direct-current are used in smelting of aluminum and other electrochemical processes. While on the other hand the AC current “Alternating current” is generally used for power distribution, which is why the mains sockets in our homes and at work provide an alternating current to power whatever is needed, but direct current, DC is more widely used for the electronics boards themselves and for many other applications.
How to Measure AC Current with Digital Clamp Multimeter:
As we know that AC current can be measured by connecting a multimeter in series but some Multimeter don’t have option for AC current so we can use clamp Multimeter to measure the AC current.
In the picture give below, as you can see a Clamp Multimeter is connected with a single core live cable that shows the AC current of the running electrical load. So, before you measure the AC current first of all, set the Clamp meter knob on the Ampere, and turn ON the clamp Multimeter. This is ready for measuring the AC current, go ahead and start measuring the AC current. You can measure the AC current flowing through all the 3 phase, RED, YELLOW, and BLUE. You can also use this to check how much current is drawn by your Water Pump, your outdoor A/C unit, etc.
How to Measure DC Current with Digital Clamp Multimeter:
For measuring the DC current in an electronic circuit, the digital Multimeter should be connected in series with the load. For this the multimeter should be connected carefully. The Red test lead of the digital multimeter is connected with the Ampere Slot. The COM test lead of the digital Multimeter should be connected with the ground of the DC power supply. The positive wire from the DC power supply is connected with the +Ve contact of the load while the ground of the load is connected with the RED test lead of the digital Multimeter. This makes the Series connection.
How to Measure AC and DC Voltage with Digital Multimeter?
Measuring AC or DC voltage using a digital Multimeter is very simple. We only need to select the AC or DC voltage on the Multimeter using the Multimeter knob. While measuring the voltage try to set the knob on a higher voltage usually greater than the voltage under test.
AC voltage can be lethal, make sure you wear the protective gloves. Never touch the wires with bare hands.
How to Measure AC Voltage with Digital Multimeter:
For reading the single phase AC voltage (180Vac-230Vac), set the selection knob of the digital Multimeter on the AC voltage. While the Digital Multimeter test leads are connected, use these test leads to measure the AC voltage. While measuring the voltage make sure you set the selection knob on the next higher voltage. In the picture below you can see I am reading the AC voltage delivered to the extension board which is 228.2Vac.
In the same way we can measure voltage of any outlet socket, we can check the output of the transformers, we can check voltage available at the input contacts fo the breakers, contactors, etc.
How to Measure DC Voltage with Digital Multimeter:
As you know while measuring the AC voltage it really doesn’t matter which test lead of the digital multimeter is connected with the phase and which one is connected with the neutral of the motor or any other electrical device under test. But in DC voltage it matters. If you connect the Positive test lead with the GND and the negative or COM test lead with the Positive supply, it will show a negative sign with the reading. This will not damage the digital Multimeter. This thing can be really handy while testing wires of the same color. You can easily find which one is positive and which one is the GND.
While measuring the DC voltage, set the digital multimeter selection knob on the DC voltage. Now you can use the test leads to measure the voltage. I am using it on regular basis for checking the voltage of DC adaptors, regulated power supplies, Batteries voltage, etc.
In the picture below, you can see I am reading the voltage of a 1.5V battery cell. These batteries are provided with the + and – signs. + is the positive and – is the GND. So I Connected the Red test lead with the + and Black test lead with the – of the battery. I was able to read the voltage. This way you can read the DC voltage of an DC power supply, or the voltage across the resistors, etc.
Insulation Test of a cable using a Digital Multimeter:
Cables insulation test is one of the most frequent tests performed in Building constructions, Industries, Power Plants, etc. The insulation testing is the oldest and most widely used test for checking the quality of the cable insulation. The insulation test must meet the electrical safety testing standards. We can determine the insulation quality of different electrical equipment’s by using a digital Insulation tester. I will be using the Fluke Insulation Tester.
The insulation testing Meter should show higher resistance (range 50Mega ohm and so) for the Cable insulation test or Motor Winding insulation etc. If the insulation resistance (IR) of the cable is high, it means the cable is in good condition and assures more reliability.
So now I am going to explain how to perform the cable insulation test for an armored cable and rubber cable.
In the above picture we are doing insulation test for armored cable and simple rubber insulation. So first connect the red probe to the insulation point as shown in the below picture and black probe connect to the COM point. After connection set the rotating slot to your mention voltage point (mention voltage means the voltage is mentioned on the cable like 1kv or 0.5kv etc from the manufacturing company). After that if you are doing insulation test for armored cable then connect the red probe to the phase conductor and black probe to the armored, but make sure both side of the cable should be open and nobody touch the cable during the testing, and then push the test button in Multimeter once you push the test button it will start reading in Kohm or Mohm.
So once the test is performed, remove the probes and make the cable short to drop the voltage to avoid an electric shock as shown in the above picture. Safety comes first, make sure you wear the protective gloves. Below, picture gives you a simple overview of how the cable insulation tester is used.
Now lets perform the insulation test for the rubber cable. During this test we will connect the probes of the Insulation tester in the same way as we did before, we connect one probe of the insulation tester with one cable and the other probe of the insulation tester with the other cable. It really doesn’t matter which probe is connected with which cable.
In the picture above, I am simply trying to explain how to perform the insulation test, because in practical you will need to disconnect the cables from both the ends. Because during the insulation test, we apply high voltage which may damage your motor, PLC, transmitters, and electronic circuits etc.
The procedure is same, first we connect the test leads with the first two cables and perform the insulation test, and then we repeat this for the other cables.
One point to be noted if your Insulation tester drops voltage it means your cables are short with second conductor.
Some Safety Advice for Using Digital Multimeter:
Following are the safety Tips,
- During insulation testing once you apply high DC voltage then do not touch the cable because it is fully energize and may result in a heavy electric shock.
- Do not connect Multimeter with high voltage If your Multimeter Voltage range is less than the source voltage, because it will damage your Multimeter voltage point.
- Disconnect all kind of electrical and electronics sources(PLC, Motors driver transmitters or any electronics circuits) from the cable while performing insulation test, because it may damage everything permanently.
- The temperature should also be taken into consideration, perform the insulation tests at a standard conductor temperature (20 Degree Centigrade).
- Supply DC voltage from insulation tester should be according to cable specification, because it can stress the cable and may damage your insulation.
- After performing the tests, short the cables to de-energize the cables.
- While performing the insulation test look around and make sure the cable under test is not touching with other cables, because it will drop the voltage and will not perform good insulation test.
- Don’t perform continuity test for live cable. It may damage your multimeter.
- Switch off and remove the Leads from the Multimeter once you finish the testing.
- Switch to AC while measuring AC Voltage or AC current and same for DC.