Apr 062013

Мultimeters are among the most common measuring instruments after clocks and thermometers. The device was invented in 1820, in the form of a galvanometer. It originally measured the resistance and voltage using the Wheatstone bridge. It was used in laboratory conditions, it was very slow and not easily portable. A portable multimeter was invented in 1920 and it was used for measuring voltage and current. The inventor of the multimeter is considered the English engineer Donald Macadie, who had to carry a lot of heavy measuring tools in maintaining postal telecommunications. Снимка  Macadie invented a combined device that could measure volts, amps and ohms, that is why he called it Avometer. The device contained a movable coil, various precise resistors and switches of ranges. The first Avometer was released in 1923 and did not change much in comparison to its latest model 8. Снимка

At the same time pocket multimeters were sold as well. They looked very much like pocket watches and cost less than Avometres. The technical characteristics of these devices were very rough, the scales were nonlinear and very often the measurement wasn’t reliable. The measuring device was constantly improved, new functions were added, such as measurement of decibels, capacitance, frequency, duty factor, etc. Some multimeters (German multizet) have special additional features for measuring inductance, temperature, and some have the function to connect to a computer. There are multimeters that „pronounce“ the given value, etc. Снимка

Modern multimeters (multitesters, Volt-Ohm meters, Volt-Ohm-milliammeters) are electronic measuring instruments, measuring several parameters. Some devices have an electronic display or an analog (pointer) scale, calibrated for measuring all types of variables. The device is portable and extremely useful for outdoor measurements, at home and in mobile teams of electricians doing industrial fieldwork. Multimeters not only have a variety of functions but can look in different ways. There are devices which are pen-shaped, handle-shaped, some have the shape of pliers, etc.

Multimers can be found in a huge range of prices. There are models which cost around 10 lv as well as certified precise laboratory equipment which costs thousands of lv.
The typical multimeter is a combination of a multirange voltmeter (AC and DC), a multirange ammeter and a multirange ohmmeter, often measuring frequency, temperature, parameters of transistors and magnetic quantities.
How to measure with a multimeter? The standard multimeter consists of four parts:
1. Display 2. Switch with graduated scale for measuring values ​​3. Two, three or four built-in labeled jacks (connectors), one of which is necessarily COM 4. Probes, usually red and black. Снимка

Some devices have buttons on their upper side, one of which is usually ON/OF, and the rest may be HOLD – for saving the measured value, LIGHT – for illuminating the display, etc. The multimeter is powered by a battery which is mounted in the device under a hood at its rear side and is usually included in the sales package. Very often the multimeter is in a rubber housing  to prevent damages if dropped. The display has 4 categories and can also show a negative sign. Снимка After inserting the battery and placing the black probe into the jack labeled COM, and the red probe, depending on what we measure, in one of the other jacks, e.g. the one labeled mAVΩ, it is time to check the condition of the device. By setting the switch at a sound signal position, usually marked by propagating sound waves, you can hold the probes’ handles so that their metal terminals come into contact. As a result, you should hear a sound, which indicates the presence of a circuit. The third jack has a value shown in amperes and is used for measuring large currents.

A multimeter’s scale has separate sections for measuring DC and AC voltage. The DC voltage is indicated by a V- sign and the AC voltage, respectively by V ~, and the values ​​of the ranges of measuring are selected by the rotating switch. There are simple devices without a display, which only detect the presence of voltage and its magnitude out of several predefined values, each of which referring to a LED. Снимка
If you don’t know the value of the measured voltage, it is recommended to set the switch at the highest value. If you measure DC voltage but you swap the places of the probes, the value of the measured voltage will appear on the screen with a negative sign without causing any problems to the multimeter.
To measure the voltage of a battery for example, the switch must be set at a constant voltage and a value, which is approximate but less than that, displayed on the battery.

When measuring resistances you need to set the switch on the scale, indicated by the sign Ω. If you don’t know the resistance value – set the switch at a higher value. The resistances have colored lines, called the resistors’ color code. If you are not familiar with the meaning of the lines, you can refer to a number of online calculators that are easy to use, but if you have no internet access but you have a multimeter, you can approach the probes’ terminals to the two legs of the resistor . Снимка

On the screen you will see 0.00, 1 or the actual value of the resistance. If the reading is 1 or OL (overload), this means that the device is overloaded and you need to increase the range of measurement, for example from 2kΩ to 20kΩ. If the reading is 0 or close to 0, you have to reduce the scope, for example from 200kΩ to 20kΩ. However, if you have 20kΩ range and the reading is 5.5, this means that the resistor is 5,5kΩ. The process of measuring the resistance has a number of features with which you must comply:
don’t forget that resistances are influenced by the ambient temperature and as a rule their value is always given at 20°C; if the resistance is mounted on a board or a device, it is extremely difficult to be measured because the surrounding electronic components will significantly influence the measured value; you need to bear in mind that each resistor has a tolerance (deviation from the nominal value), which is about 5%, so if you measure a resistance of 20 kΩ, it is better to set the switch at a higher range. Снимка
Current measuring is one of the most frequent processes and at the same time it is quite difficult nowadays when most electronics is embedded and encapsulated. The difficulty stems from the fact that current must be measured through successive connection of the measuring device. While the voltage is measured in parallel – between phase and neutral, in current measuring, the circuit must be disconnected physically and the device must be connected successively. The red probe must be connected to the side of the power supply and the black one – to the side of the consumer. If you connect the probes the other way round – the displayed value will be negative. It is recommended that you use probes with alligator clips which allow the current to flow steadily through the device, without keeping your hands busy. It is essential to determine the exact range of the flowing current. If you want to measure the current over time, you should also take into account the starting current of certain consumers that can transcend the specified range for a short period of time.
Here we need to mention that the most common reason for burning the multimeter is if we try to measure voltage, but we have forgotten the switch in the area of ​​current measurements. The result is full short circuit and even the fuse won’t be able to save the device from irretrievable damage. Moreover, there is no fuse on the jack labeled 10A, as we will further discuss. If you need to measure current larger than 100mA, you must place the red probe in the connector, which is marked with 10A. By doing this you will be able to measure much larger currents. Of course, you will lose some of the sensitivity of the device. When you are not sure exactly what electricity to expect in a circuit, it is always good to start with a probe inserted into the 10A jack. Near this jack there is a sign unfused, which means that when measuring with a probe, connected to this jack, there is no fuse.
One of the most important functions of a multimeter is measuring the continuity (connectivity) of a circuit. This test is important because it gives information whether the circuits are properly connected. Hardware specialists often use this particular function of the device. In order to measure very low resistance between two given points (of the order of several ohms) you must position the switch at the sign that looks like waves spreading out of a speaker. Thus we use the multimeter as a continuity tester. Some of the simplest models of multimeters may not have this function. If you hear a sound, when connecting the terminals of the circuit, it means that the resistance it is very, very low. As a rule, the system must be deactivated when measuring connectivity. If you measure connectivity between two points of a circuit, in which there is a connected capacitor, you can hear a short sound, which will then stop. This only means that the capacitor is discharged and has disconnected the circuit. Снимка

We often use the multimeter to measure the polarity of diodes and LEDs. For this purpose, the switch must be set at the diode indication. Once this is done, you have to inspect the diode carefully and seek identification of the cathode. This is usually a darker band in the area of ​​the cathode. Then the device indicates the voltage drop on the diode in forward direction. The number that appears may be 0,2V for a germanium diode or 0,5V – for a silicon one. If you have inserted the probes in switched positions, then an indication OL will appear which means overloading. If the OL indication appears in both positions of the probes, this means that the diode is disconnected. If in both positions some indication of value appears, this means that the diode is broken, i.e there is short-circuit. In LEDs the longer leg corresponds to „+“ and the shorter one – to „-„. Once you determine the polarity, you have to carefully touch the positive terminal (respectively the anode) to the red probe and the negative terminal – to the black probe. If the polarity is respected, the LED will emit light. If the voltage drop in the LED is greater than 2 V, the device will give incorrect indication that the LED is disconnected!
Some devices have a separate coupling for measuring transistors. In order to measure the amplification of a transistor, we need to position the switch on a scale with a single partition marked with hFE (if available). There is an indication PNP for a direct transistor and NPN – for a reverse transistor. The arrangement of the openings is E for emitter, C for collector, and B for base – in this order, and then E again. This is done because transistors can have ESP and CBE outlets. When we place the transistor according to the indications on the display, we will see the amplification of the transistor by current. The displayed value must be considered approximate because it depends on the applied test current.

Since the multimeter provides small DC, the Darlington type of transistors that have relatively strong currents will produce erroneous readings, as well as transistors with in-built resistances. The transistor must not be warmed up by the heat of your hands, because the reading can be affected by the temperature. For this purpose, the transistor must be left in the device as long as the reading gets stabilized. Снимка
Some devices have separate inputs (sockets) for thermocouples, capacitors, temperature scale, option for analog indication – presentation of the measured value as a continuous line. Снимка

When measuring a capacitor, it must be first discharged by short-circuiting its outlets, then it must be placed in the appropriate socket, observing its polarity, if it is polarized. If the capacitor scale has divisions, it is good to start with the biggest one. It may happen that you leave the switch in the area of measuring the current and leave the probes between phase and neutral. This will lead to a short circuit, large current will run through the multimeter and its fuse will burn. This can happen without any external signs, but if you attempt to measure current with a burnt fuse, the only thing is that the indication on the display will be zero because the fuse has disconnected the circuit, which would otherwise close through the device. If you to change the fuse of the multimeter, you need to open it. The various devices can be opened in a different way – some have screws, others have clips. It is important that the new fuse, which you put in the place of the old one, has the same burn-up value. Usually its value is 200mA. Multimeters in the price range of 10 to 60-70lv can be used for practical purposes by electricians and hardware specialists. However, if the goal is to achieve higher accuracy and to obtain experimental results for scientific purposes, the device must be of a higher class, such as Agilent multimers. The functions of such a multimeter are significantly more, it is much more efficient and the results are much more accurate.
Thus for example, high-class devices can have automatic selection of range (autoranging), which means that the multimeter can adjusts its internal circuitry to measure correctly the fed voltage, current or resistance. On the other hand, the backlight of the display allows measurements in poorly lit places, e.g. outdoors, during the night. The automatic shut-off function preserves the battery and is very useful for forgetful users. The devices of this class usually have an outlet to connect to a computer, where the results of the measurements can be saved, presented graphically, etc. by using special software.




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