What is a Multimeter?
What is a Multimeter?
A multimeter is a tool for measuring that can be used to measure a number of different electrical properties. A typical multimeter can measure voltage, resistance, and current. When it can do all three, it is called a volt-ohm-milliammeter (VOM), because it has a voltmeter, an ammeter, and an ohmmeter built in. Some can measure things like temperature and capacitance, which are two more properties. Its important to know each multimeter symbols when using it to test electrical appliances around the house.
The readings on an analog multimeter are shown by a microammeter with a moving pointer. Digital multimeters (DMM, DVOM) have displays with numbers and have almost made analog multimeters obsolete because they are cheaper, more accurate, and more durable.
Multimeters come in many different shapes, sizes, and prices. They can be small and easy to carry or very accurate bench instruments. A cheap multimeter can cost less than US$10, while a lab-grade one with a calibration certificate can cost more than US$5,000. Top multimeter brands are Fluke and UNI T.
History of Multimeter
In 1820, the galvanometer was the first device with a moving pointer that could measure current. The Wheatstone bridge was used to measure resistance and voltage by comparing the unknown voltage or resistance to a known voltage or resistance. Even though the devices worked well in the lab, they were too slow to be useful in the field. These galvanometers were hard to use and big.
The D'Arsonval–Weston meter movement has a moving coil that holds a pointer and turns on pivots or a tight band ligament. The coil spins in a permanent magnetic field and is held in place by small spiral springs that also carry electricity into the moving coil. It gives a proportional measurement instead of just a detection, and the deflection is the same no matter how the meter is turned. Instead of balancing a bridge, values could be read directly from the instrument's scale, which made measurement quick and easy.
The basic moving coil meter can only be used to measure direct current, which is usually between 10 A and 100 mA. It can be easily changed to measure higher currents by adding shunts, which are parallel resistances or to measure voltage by adding multipliers, which are series resistances. A rectifier is needed to measure voltages or currents that change direction. In 1927, the copper oxide rectifier made by the Union Switch & Signal Company in Swissvale, Pennsylvania, which later became part of the Westinghouse Brake and Signal Company, was one of the first rectifiers that worked well.
The Oxford English Dictionary records that the word "Multimeter" was first used in 1907.
The first multimeter was made by a British Post Office engineer named Donald Macadie. He got tired of having to carry around a lot of different tools to keep telecommunications circuits in good shape. Macadie made a meter that could measure amps, volts, and ohms. He called it the Avometer because it could do so many things. The meter comprised a moving coil meter, voltage and precision resistors, switches and sockets to choose the range, and a case to hold everything.
The Automatic Coil Winder and Electrical Equipment Company (ACWEECO) was set up in 1923 to make the Avometer and a coil winding machine that MacAdie had also designed and patented. Even though Mr. MacAdie had shares in ACWEECO, he kept working for the Post Office until he retired in 1933. Hugh S. MacAdie, his son, started working for ACWEECO in 1927 and became the Technical Director. The first AVO went on sale in 1923, and until the last Model 8, many of its features were almost the same.
General Properties of Multimeter
Any meter will put some load on the circuit being tested. For example, a multimeter with a moving coil movement and a full-scale deflection current of 50 microamps (A), which is the highest sensitivity usually available, must draw at least 50 A from the circuit being tested for the meter to reach the top of its scale. This could put so much load on a high-impedance circuit that it changes the circuit and gives a low reading. You can also talk about the full-scale deflection current in terms of "ohms per volt" (/V). The number of ohms per volt is often called the instrument's "sensitivity." So, a meter with a 50 A movement will have 20,000 /V of "sensitivity." "Per volt" means that the impedance of the meter in the circuit being tested is 20,000 times the full-scale voltage that the meter is set to. For example, if the meter is set to 300 V full scale, the meter's impedance will be 6 M. The best (highest) sensitivity that most analog multimeters with no internal amplifiers can offer is 20,000 /V. For meters (like VTVMs, FETVMs, etc.) that have amplifiers built in, the input impedance is set by the amplifier circuit.
Avometer
The first Avometer had a sensitivity of 60 /V, three direct current ranges (12 mA, 1.2 A, and 12 A), three direct voltage ranges (12, 120, 600 V, or optionally 1,200 V), and a 10,000 resistance range. In 1927, this was improved so that it had 13 ranges and 166.6 /V (6 mA) movement. From 1933 on, a "Universal" version with more alternating current and alternating voltage ranges was available, and in 1936, the Avometer Model 7 came with both 500 and 100 /V. From the middle of the 1930s until the 1950s, 1,000 /V became the standard for radio work sensitivity, and this number was often written on service sheets. But companies in the United States like Simpson, Triplett, and Weston made 20,000 /V VOMs before the Second World War, and some of these were sent to other countries. After 1945 and 1946, 20,000 /V became the standard for electronics, but some companies made instruments that were even more sensitive. Low-sensitivity multimeters were still made for industrial and other "heavy-current" uses. These were thought to be more durable than the more sensitive ones.
Several companies still make high-quality analog multimeters, such as Chauvin Arnoux in France, Gossen Metrawatt in Germany, and Simpson and Triplett in the United States (USA).
Pocket watch meters
In the 1920s, a lot of people used meters that looked like pocket watches. Most of the time, the metal case was connected to a negative connection, which caused a lot of electric shocks. Most of the time, the technical details of these devices were not very good. For example, the one shown has a resistance of only 25 /V, a scale that is not linear, and there is no way to set zero on both ranges.
Vacuum tube voltmeters
Vacuum tube voltmeters also called valve voltmeters (VTVM or VVM), were used to measure voltage in electronic circuits that needed a high input impedance. The VTVM usually used a cathode follower input circuit to give it a fixed input impedance of 1 M or more. This meant that it did not put much load on the circuit being tested. Before electronic high-impedance analog transistors and field effect transistor voltmeters were made, VTVMs were used (FETVOMs). Modern digital meters (DVMs) and some modern analog meters also use electronic input circuitry to achieve high input impedance—their voltage ranges are functionally equivalent to VTVMs. The input impedance of some poorly designed DVMs (especially some early designs) would vary over the course of a sample-and-hold internal measurement cycle, causing disturbances to some sensitive circuits under test.
Additional scales
Many multimeters have added features like decibel scales and measurement functions like capacitance, transistor gain, frequency, duty cycle, display hold, and continuity, which makes a buzzing sound when the resistance being measured is low. A technician's toolkit may have more specialized equipment besides multimeters, but some multimeters have extra features for more specialized uses (temperature with a thermocouple probe, inductance, connectivity to a computer, speaking measured value, etc.).
Operations of Multimeter
A multimeter has a DC voltmeter, an AC voltmeter, an ammeter, and an ohmmeter all in one. An analog multimeter that is not amplified is made up of a meter movement, range resistors, and switches. VTVMs are analog meters that are amplified and have active circuitry.
DC voltage is measured with an analog meter movement by connecting a series resistor between the meter movement and the circuit being tested. Higher voltages can be read by adding more resistance to the meter movement through a switch, which is usually a rotary knob. The full-scale voltage of the range is equal to the product of the range's basic full-scale deflection current and the sum of the series resistance and the range’s own resistance. As an example, a meter movement that needed 1 mA to move the full scale and had an internal resistance of 500 would have 9,500 series resistance on a 10 V range of a multimeter.
For analog current ranges, matched low-resistance shunts are connected in parallel with the meter movement to send most of the current around the coil. Again, the shunt resistance for a theoretical 1 mA, 500 movements on a 1 A range would be just over 0.5.
Instruments with moving coils can only measure the average value of the current going through them. To measure alternating current, which goes up and down repeatedly, a rectifier is added to the circuit so that each negative half cycle is inverted. This creates a DC voltage that is not zero and changes over time. If the waveform is symmetrical, the maximum value of the DC voltage will be half the AC voltage from peak to peak. Since the rectified average value and the root mean square (RMS) value of a waveform are only the same for a square wave, simple rectifier-type circuits can only be set up for sinusoidal waveforms. To find the relationship between RMS and the average value for other wave shapes, you need a different calibration factor. This kind of circuit usually only works with a small range of frequencies. Since real-world rectifiers have a voltage drop that isn't zero, accuracy and sensitivity are bad at low AC voltage values.
Switches set up a small battery inside the instrument to send an electric current through the thing being tested and the meter coil. Since the amount of current available depends on how charged the battery is, which changes over time, the ohm scale on a multimeter usually has a way to set it to zero. Most analog multimeters have circuits where the meter deflection is inversely proportional to the resistance. This means that full scale is 0 and that a higher resistance means a smaller deflection. Since the ohms scale is compressed, the lower resistance values have better resolution.
Amplified instruments make it easier to set up networks of series and shunt resistors. The internal resistance of the coil is separated from how the series and shunt range resistors are chosen. This turns the series network into a voltage divider. When AC measurements are needed, the rectifier can be put after the amplifier stage, which makes the low-range accuracy better.
Digital instruments, which have to have amplifiers, measure resistance in the same way that analog instruments do. For measuring resistance, a small, constant current is usually run through the thing being tested, and the digital multimeter reads the voltage drop that happens as a result. This gets rid of the scale compression that happens with analog meters, but it does require a source of precise current. A digital multimeter with auto-ranging can change the scaling network so that the measuring circuits can use the full accuracy of the A/D converter.
In all types of multimeters, stable and accurate measurements depend on how well the switching elements work. The best DMMs have switches with gold-plated contacts. Less expensive meters use nickel plating or none at all and instead use solder traces on the printed circuit board to make the contacts. The long-term accuracy and precision of an instrument are limited by how accurate and stable its internal resistors and other parts are (e.g., due to changes in temperature, aging, or voltage/current history).
Multimeter Functions
Depending on the model, each of these tools can give different readings. So, basic types of multimeters are mostly used to measure amperage, resistance, voltage, and continuity. A complete circuit can be tested by doing the following.
Some types of multimeters can be used to measure acidity, light level, alkalinity, wind speed, and relative humidity with the help of special sensors or add-ons.
Digital Multimeter (DMM or DVOM)
Digital multimeters are often used today because they are accurate, last longer, and have more features. In a digital multimeter, the signal being tested is turned into a voltage, and the signal is then prepped by an amplifier whose gain can be controlled electronically. The amount being measured is shown as a number on a digital multimeter, which eliminates parallax errors.
Modern digital multimeters may have an embedded computer, which provides a wealth of convenience features. Measurement enhancements available include:
Analog Multimeter
A multimeter may have a galvanometer meter movement or, less often, a bar graph or simulated pointers like a liquid-crystal display (LCD) or vacuum fluorescent display. There were a lot of analog multimeters and a good analog instrument cost about the same as a DMM. Analog multimeters had the same problems with precision and reading accuracy that were talked about above. Because of this, they were not made to be as accurate as digital instruments.
Analog meters were easy to use in situations where the overall trend of a measurement was more important than a single exact value. Changes in angle or ratio were easier to understand than changes in a digital readout's value. Because of this, some digital multimeters have a second display which is a bar graph. This display usually has a faster sampling rate than the main display. The response of these fast sampling rate bar graphs is better than that of a physical pointer on an analog meter. This makes the older technology obsolete. Advanced digital meters could track and show changes in DC, AC, or a combination of the two better than analog meters could. They could also separate and show both DC and AC components at the same time.
Physically and electrically, analogue meter movements are more fragile than digital ones. Many analog multimeters have a "off" position on the range switch to protect the meter movement while it's being moved. This puts a low resistance across the meter movement, which slows it down. When not in use, a shorting or jumper wire can be used to protect meter movements that are made up of separate parts in the same way. Because the shunt has a low resistance, meters with a shunt across the winding, such as an ammeter, may not need any more resistance to stop the meter needle from moving on its own.
The way a moving pointer's meter moves an analog multimeter is almost always a moving-coil galvanometer of the d'Arsonval type. The moving coil is held in place by jeweled pivots or taut bands. In a basic analog multimeter, the current that moves the coil and pointer comes from the circuit being measured. It is usually better to draw as little current as possible from the circuit being measured, which requires delicate mechanisms. In ohms per volt, you can measure how sensitive an analog multimeter is. For example, at full-scale deflection, a very cheap multimeter with a sensitivity of 1,000 /V would draw 1 mA from a circuit. [28] More expensive (and mechanically more delicate) multimeters usually have sensitivities of 20,000 ohms per volt or higher, with 50,000 ohms per volt (drawing 20 microamperes at full scale) being about the upper limit for a portable, general-purpose, non-amplified analog multimeter.
Difference between Analog and Digital Multimeter
| Analog Multimeter | Digital Multimeter |
|---|---|
| Analog Multimeter is used to measure limited electrical quantities like voltage, current, and resistance. | With a digital multimeter, you can figure out things like voltage, current, capacitance, resistance, diode and impedance values, etc. |
| The analog multimeter is bigger in size. | The digital multimeter is smaller in size. |
| This meter has a scale next to the pointer that shows the reading. | On an LCD screen, a multimeter shows the reading as a number. |
| Analog multimeters are calibrated manually. | Digital multimeters are calibrated automatically. |
| Its construction is simple. | Its construction is complicated because of the involvement of components like electronics and logic. |
| Because of parallax errors and wrong pointer readings, analog multimeters are less accurate. | Digital multimeters are very accurate. |
| It can show reading without ADC. | It needs ADC to exhibit the reading. |
| There is not a stable input resistance. | Input resistance is stable |
| These are cheaper. | These cost a lot. |
| It has less electric noise. | More electronic noise comes from it. |
