A Multimeter is an electronic instrument, every electronic technician and engineers widely use. A multimeter is mainly used to measure the three basic electrical characteristics voltage, current, and resistance. It can also be used to test stability between two points in an electrical circuit. The multimeter has multiple uses like, it acts like ammeter, voltmeter, and ohmmeter. It is a handheld device with positive and negative indicator needle over a numeric LCD digital display. Multimeters are used for testing batteries, household wiring, electric motors, and power supplies.
Types of Multimeters:
There are different types of multimeter like Digital, Analog and Fluke multimeters.
The mostly used multimeter is a digital multimeter. It performs all functions from AC to DC other than analog. It has two points positive and negative indicated with black and red color. The black point connected to COM JACK and redpoint connected by user necessity to measure ohm, volt or amperes. The jack marked VΩ and the COM jack is used for measuring voltages, resistance and for testing a diode. The two points are operated when LCD display that shows what is being measured (volts, ohms, amps, etc.).
The Digital Multimeter basically consists of an LCD display, a dial to select various ranges of the three electrical characteristics, an internal circuitry consisting of a signal conditioning circuitry, an analog to digital converter. The PCB consists of concentric rings which are connected or disconnected based on the position of the knob. Thus as the required parameter and the range is selected, the section of the PCB is activated to perform the corresponding measurement. To measure the resistance, current flows from a constant current source through the unknown resistor and the voltage across the resistor is amplified and served to an Analog to Digital Converter and the resulting output in form of resistance is displayed on the digital display. To measure an unknown AC voltage, the voltage is first reduced to get the suitable range and then rectified to DC signal and the analog DC signal is fed to A/D converter to get the display, which indicates the RMS value of the AC signal. Similarly to measure an AC or DC current, the unknown input is first converted to a voltage signal and then fed to an analog to digital converter to get the desired output.
Advantages of a Digital Multimeter are its output display which directly shows the measured value, high accuracy, and ability to read both positive and negative values.
The Analog Multimeter is made using a moving coil meter and a pointer to indicate the reading on the scale. The moving coil meter involves a coil wound around a drum placed between two permanent magnets. As current passes through the coil, a magnetic field is induced in the coil which reacts with the magnetic field of the permanent magnets and the reproduced force causes the pointer attached to the drum to deflect on the scale, indicating the current reading. It also consists of springs attached to the drum which provides an opposing force to the motion of the drum to control the deflection of the pointer.
For measurement of DC voltage, a resistor is connected in series with the meter and the meter resistance is taken into account such that the current passing through the resistor is same as the current passing through the meter and the whole reading indicates the voltage reading. For voltage measurements, the instrument is to be connected in parallel with the unknown voltage source. For multi-range measurement, different resistors of different values can be used, which are connected in series with the meter.
For the measurement of resistance, the unknown resistance is connected in series with the meter and across a battery, in such a way that the current passing through the meter is directly related to the unknown resistance.
For AC voltage or current measurement, the same principle is applied, except for the fact that the AC parameter to be measured is first rectified and filtered to get the DC parameter and the meter indicates the RMS value of the AC signal.
Advantages of an Analog Multimeter are that it is very cheap, doesn’t require a battery, can measure fluctuations in the readings.
The fluke multimeters are protected against the passing voltage. It is a small portable device used to measure voltage, current, and test diodes. The multimeter has multiple selectors to select the desired function. The fluke multimeter automatically ranges to select most measurements. This means the magnitude of the signal does not have to be known or determined to take an accurate reading, it directly moves to the appropriate port for the desired measurement. The fuse is protected to prevent the damage if connected to the wrong port.