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Tuesday, 5 March 2019

Multimeter Introduction


Elementary method of knowing current flow through a conductor was to Use a compass needle. When the compass needle was taken close to the current carrying conductor it deflected. It was identified that the magnetic field produced by the current carrying conductor that was responsible for that deflection. Instead of the straight conductor when the compass needle was used in the centre of a coil of wire carrying current the deflection was found to be more, significant. Based on this the magnetic effect of current refinements were made and suspension galvanometer came into use. This suspension galvanometer is the base for the popularly used moving coil instrument. However in spite of the refinements and the new measuring instruments the suspension galvanometer has still its use in high sensitivity laboratory measurements, provided the delicacy and portability are not very important.

Measurement of direct current, voltage, are done with moving coil instrument. The basic meter is a current meter. For the measurement of alternating current and alternating voltage an additional feature called the rectifier circuit is required. The reason is that the moving coil current meter cannot read alternating current.

In order to provide a sensitive instrument we prefer a basic meter that takes the minimum possible current for its full scale deflection. A list of commonly available moving coil current meters is given this topic. The best one is the 20 ยต A meter. In order to measure larger values of current, using this basic meter, we have to employ shunt resistance across the meter. Thus higher ranges of current for measurements are possible.

Voltage measurement is an indirect measurement. We only measure the current. A resistor will be used in series with the basic current meter. Then the dial or scale of the current meter can be calibrated in terms of volts. The series resistor is also called the multiplier resistor.

Resistance measurement also uses measurement of current. The scale of the ammeter can be calibrated in terms of the ohmic value.

A multimeter is a one piece combined unit that can be used for the measurement of direct current, voltage, alternating current, alternating voltage and resistance basically. It can also be used for measurement of audio power. The multimeter consists of shunts multiplier resistors in it with a switching arrangement for selection of the quantity to be measured and its range. Again the basic instrument used is a moving coil meter of the best sensitivity. This type of arrangement of a multimeter is called an analog multimeter. Such multimeters are economical, for use by a technician.

Electronic multimeters are also available that have better performance characteristics compared to the analog multimeters. Electronic voltmeters are different from conventional multimeters in that the electronic voltmeters use some electronic devices, in the measurement of the electrical quantities. Such electronic voltmeters are available currently using integrated circuits. Way back in seventies the electronic voltmeters used vacuum tubes and latter the transistors. They were mostly of the analog type that gave the reading over a scale using a pointer. The trend changed and now we have Digital Voltmeters and Digital Multimeters. The earlier versions have almost become obsolete. Though we call it as an electronic voltmeter, it can be used as multimeter in a laboratory or service station. The performance of such electronic test meters is better than the conventional type of multimeters.

However considering the cost aspect of the digital multimeter, the conventional analog meter still survives owing to its low cost; with technicians.

We can give a general name to the electronic instrument that performs the same functions of a conventional multimeter as an Electronic Test Meter (E.T.M.) or Electronic Multimeter.

There are several models of these Electronic Multimeters, also called as Digital Multi Metres (D.M.M.). There are hand held type, table tope type with 3.½, 4.½ and 6.½ digits of display, in different display types.

The world class test equipment presently offers two in one models. A cathode ray oscilloscope and can also have the functions of a Digital Multimeter, leading to a two in one model.
Further hand held oscilloscopes are now in market, which shows the Technology change. In this blog the following topics are covered:

Sensitivity of an ammeter and its expression
List of commercially available moving coil ammeters
Method of extension of the range of an ammeter
Procedure for measurement of internal resistance of an ammeter
Multi range ammeter
Volt meter
Multirange voltmeter
Measurement of alternating current
Measurement of alternating voltage
Measurement of resistance
Electronic Multimeter
Digital multimeter
Probes that are additional accessories to multimeters

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