A multimeter can be useful not only to a specialist electrician, but also to almost every owner in everyday life. This is a multifunctional and compact device that measures current, voltage, and many other parameters. It only takes a couple of minutes to test the resistance with a multimeter. On sale today you can find both electronic models and analog ones, but for the most part the difference between them lies only in the way the information is displayed and the details.
Varieties
First, a few words about the types of devices. Previously, an analog multimeter was most often used, in which ordinary arrows were installed to display readings. Today, electronic models are more in demand, but analog models are not in a hurry to become a thing of the past, they are used mainly by professionals.
The reasons for this lie in the following. Turnouts work more stably in areas of electromagnetic fields. In addition, electronic models require power (usually batteries), and battery wear can directly affect measurement errors. It is also worth noting the possibility of failure due to strong electrostatic discharges. An analog multimeter shows a more accurate result.
There are advantages to digital models as well. They display information in a more accessible way, and ways to display the difference between the measured indicators and the reference ones.
Instrument control basics
Many models of multimeters have their own characteristics, but there are also rules common to all varieties. For example, to start measurements, touch the ends of the metal probes (they are equipped with handles made of insulating material) to the conductor.
The value of the parameter that is currently being measured by the multimeter must be within the range that is set by a special switch on the case.
That is why it is recommended to take measurements by setting the maximum measurement mode, and then adjusting the accuracy or vice versa. However, the most technically advanced devices are able to determine the measurement limits automatically.
You should also remember the rules:
The connection diagram of the probes is as follows. The one with the black wire is inserted into the COM jack (negative pole), the red one into the VΩma jack. Remember that there is a wide range of models on the market today, so the nuances of use may vary. To avoid an unfortunate failure, it is recommended to additionally read the user manual.
Device
In the vast majority of models, the basis of the device is completely identical. The only difference may be the designations, a number of additional features and measurement limits. In any case, all the controls for the device are located on the front panel. Among them: sockets for connecting probes, a screen, as well as a resistance measurement mode switch.
The 1CL7106 chip is responsible for the hardware component. When measuring voltage, the signal passes through resistor R17 and is transmitted to input 31. The current is perceived by the resistors, depending on which range has been set by the user. The resulting voltage drop is fed to input 32.
probes
In budget models of testers, probes most often do not differ in special quality. In this case, you should not judge by their appearance, since they are specially made as beautiful and glossy as possible. Attention should be paid, first of all, to the wire - it should be as elastic as possible and hold well.
In order to pierce the insulation of a wire or to find the pins of a microcircuit with a small pitch, the ends of the probe are made in the form of needles. Bronze is used as a material for their manufacture, which does not hold sharpening very well. In some cases, low-quality probes can break off at the embedding points. Finally, low-quality probes can make unreliable contact in the multimeter sockets.
As a solution, specialists most often “bring them to mind” on their own. To do this, they solder the wires to the tips and fit the connectors into the sockets. In this case, the tips must be tinned, otherwise the indicators will be different depending on the pressure. To reduce the resistance, the wires can be replaced with a cable of a thicker section, the complete ones have a resistance of up to 0.5 ohms and more.
Check before work
The current-carrying conductors in the multimeter probes wear out over time, which has an extremely negative effect on the measurement accuracy. That is why it is important to check them before starting work. This is done simply. The switch is set to the lowest range, after which the wires are closed with each other. Next, the insulation on the handles is checked in the same way. If the contact is bad, the readings on the screen will start to go astray. Separately, it should be noted the option of checking in dialing mode. In the event of an unstable audio signal, the contacts must be replaced.
Instruction
So, how to measure resistance with a multimeter? This requires only three steps, but first you must make sure that the network under test is completely de-energized.
The black test lead is inserted into the COM jack, after which the red test lead is inserted into VΩmA. Then you need to turn on the device. This is most often done by turning the measurement switch. To work with the smallest resistances, you will need to put the switch on the letter "omega" and set the range to 200, that is, within 0.1-200 Ohms (measurement of low resistances). Next, a check is made for the short circuit of the measuring circuit, for which the probes are closed to each other. If the multimeter is working, an indicator of the order of 0.3-0.7 will appear on the screen and, as already mentioned, it should be constant. This indicator displays the resistance of the test leads themselves. If this figure is higher or changes frequently, the wires should be updated. If the wires are open, the screen should read one, indicating very high (infinite) resistance.
In order to make a measurement, it is required to simultaneously touch the contacts of the circuit. If the system is working properly, the multimeter will take readings. If a power failure test is performed, the tester will display new readings. The resistance in this case should be quite low, up to 1.5 Ohm. If you want to measure the resistance of a current consumer, for example, a light bulb or transformer windings, the indicator can jump up to 150-200 ohms. There is a rather characteristic feature: with an increase in the power of the current consumer, checking the resistance of the device with a multimeter shows a lower result.
If the numbers on the tester screen do not change during measurements, you should switch to a higher range.
If the multimeter displays all the same values, go to a new range and keep trying. There is an important point here. If you put the switch on 2000k and take the probe contacts with your bare hands, it turns out that we are measuring the resistance of the body, which, of course, will affect the results.
Features and nuances
The operation of the multimeter has several important features at once that can affect the result of its work. Let's look at some important examples.
Quite often, a situation arises when it is required to measure the resistance of a part already soldered into the board. In this case, you can not even try to measure the assembly - the result is guaranteed to be incorrect. The reason is simple: any element on the board is connected to others, so the multimeter during the test will only show a general indicator. If you only want to test one element, you will have to retrieve it from the schema.
In the case of multi-terminal elements, dismantling is also an absolute must. You can check their resistance only after that. Otherwise, the result cannot be relied upon.
The insulation resistance of cables should only be measured in warm and dry conditions, as icing and moisture will give an incorrect result.
Do not forget about the condition of the multimeter probes. The most accurate result can only be obtained with serviceable parts. You can check their condition as follows: attach the bare ends to each other and move them. If the multimeter readings jump strongly, then the probes must be urgently replaced. With faulty parts, accurate data cannot be counted on.
Finally, the health of the battery should be noted. Each specialist will say that as soon as the battery starts to discharge, the tester's readings go further and further from the truth. Most often, a discharge indicator icon appears on the screen. In this case, either replace the battery or recharge the device.
Measurement example
Let's take an example of how to check the impedance of headphones. Most often they are connected to a PC or player using a miniJack connector. It consists of three parts. The closest to the holder is the common channel, then there are separate ones for the right and left channels.
To check, just touch one multimeter probe to the common channel, and the second to the right and left in turn. The exact resistance is indicated in the data sheet of the headphones, but most often it is about 40 ohms. If the readings are very different, then there is a short circuit in the wire. To check, we additionally measure like this. We touch one probe to the right channel, and the second - to the left. Ideally, the resistance should be exactly twice as much.
As you can see, resistance measurements are quite simple to carry out. You must be sure that the multimeter is working and understand the value of the measured value.