How to Use a DMM
By Rageb Ahsan – Electrical Engineer
A multimeter is a very useful instrument for an electrician. Every now and then electrical personnel need it for profession. A trained electrician or an educated Electrical Engineer isn’t the lone user of a multimeter. For household purposes one can use it; an electronics hobbyist may also use it for his purposes and so on.
Let’s talk about the analog multimeter first. An analog multimeter has a graduated dial on which a pointer moves to indicate the measured value of the variable. Well, the variables are voltage, current and resistance which are measured with this meter. You have to select the variable on the meter before making any measurement.
A rotary selector switch is there below the dial to make the selection. You can see different positions of the selector switch with markings of Voltage (AC/DC), Ampere (AC/DC) and Ohms. Sorry man, I haven’t mentioned yet what is ampere and ohm. These are the measuring units for electrical current and resistance respectively.
Voltage (AC/DC) may have several ranges of measurement. Similar the case is for ampere. So before going to measure any of these parameters, it would be better if you have some idea about the value of the parameter which you are going to measure. If you have the idea, you should select a range well above the estimate. We can have an example here. Our domestic electricity voltage is 110V or 240V. If you know it to be 110V, for measuring it you can select a range of 200V on the meter. But if you have any confusion, it’s wise to select a range higher than 240V.
The very first reading will help you to be confirmed of it. In case you find it to be close to 110V, you can now select the 200V range and you may get a more accurate measurement. Any higher range selection is not harmful for the instrument. But having selected a range lower than the measured value, may damage the instrument. If you select a much higher range, the instrument is safe but the measurement is less accurate.
Volt is the unit of potential difference between two points. So when you like to find out a voltage, you actually desire to find the potential difference between two points. Hence you have to make contact of the test leads with the two points of interest. Well, you have to push the other ends of the test leads into the correct sockets on the meter first. Take the two test leads on two hands and make contact with two target points with the tips of the leads. Take care not to touch the open tips with yourfingers. The two targets should be clean of dirt and foreign material. Otherwise you may not get a correct result.
For getting an accurate result in measurement you must view the pointer avoiding any parallax. You know what parallax is? Let me explain. Your eye must be perfect vertical with the dial when you look at the pointer. In any other angle you’ll add some error to the reading. Some manufacturers add a strip of mirror along the side of the scale that helps to get a parallax free view.
Measuring unit of electrical current is ampere. So ampere is a flow (of electrons). For measuring the flow the instrument must be placed in the path of flow. So you have to make a room for the meter to place in the flow path. The following diagrams would make it clear to you.
Fig.1 shows the circuit of which you like to measure the current. Fig.2 shows how to connect the meter in the circuit to get the same current passing through it. You have to keep in mind that before measuring a current you must select ampere (AC/DC) and the proper range on the multimeter. The meter is shown here to be placed after the switch and before the lamp. But it’s immaterial to place here or anywhere in the current path. This is termed as a series connection.
Resistance measurement is another vital function of a multimeter. If you want to measure the resistance value of a lamp you just take it out of the socket, set your meter at Ohm and touch the two terminals of it with the test leads of the meter. You get a result. If the value is very low in comparison with the range you may choose a lower range to get a more correct measurement. If you have to measure the resistance of a tiny resistive element on a printed circuit, you have to disconnect its at least one terminal from the circuit. Otherwise you won’t get the correct value.
The difference in making measurement with an analog meter and a digital one is very a little. It’s rather simple to measure with a digital meter. There is no chance of any parallax. Otherwise it’s all the same.