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A Multi meter is an electronic instrument, every electronic technician and engineers
widely used piece of test equipment. Multi meter is mainly used to measure the three
basic electrical characteristics of voltage, current and resistance. It can also be used to test
continuity between two points in a electrical circuit. This post mainly introduces the basic
information of multi meter, applications and types of multi meter are in. Let’s see all of
Multi meter has multi functionalities 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. Multi meters can be used for testing batteries, household wiring, electric
motors and power supplies.
Types of Multi meters:
There are different types of multi meter like Analog, Digital and Fluke multi meters.
Digital Multi meter:
We mostly used multi meter is digital multi meter (DMM). The DMM performs all
functions from AC to DC other than analog. It has two probes positive and
negative indicated with black and red color is shown in figure. The black probe
connected to COM JACK and red probe connected by user requirement to measure
ohm, volt or amperes. The jack marked VΩ and the COM jack on the right of the
picture are used for measuring voltages, resistance and for testing a diode. The two
jacks are utilized when LCD display that shows what is being measured .
Overload protection that prevents damage to the meter and the circuit, and protects
2. The Digital Multi meter basically consists of a LCD display, a knob 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 performthe corresponding measurement. To measurethe
resistance, currentflows froma constant currentsourcethrough the unknown
resistor and the voltage across the resistor is amplified and fed to a Analog to
Digital Converter and the resultant output in formof resistanceis displayed on
the digital display. To measure an unknown ACvoltage, the voltage is first
attenuated to get the suitable rangeand then rectified to DC signal and the
analog DC signalis fed to A/D converter to get the display, which indicates the
RMS value of the AC signal. Similarly to measurean AC or DCcurrent, the
unknown inputis firstconverted to voltage signal and then fed to analog to digital
converter to get the desired output(with rectification in case of AC signal).
Analog Multi meter:
The Analog Multi meter or VOM (Volt-Ohm- Milli ammeter) is constructed using
a moving coil meter and a pointer to indicate the reading on the scale. The moving
coil meter consists of a coil wound around a drum placed between two permanent
magnet. As current passes through the coil, magnetic field is induced in the coil
which reacts with the magnetic field of the permanent magnets and the resultant
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.
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For measurement of DC current, the D Arsonval movement described above can
be directly used. However the current to be measured should be lesser than the full
scale deflection current of the meter. For higher currents the current divider rule is
applied. Using different values of shunt resistors, the meter can also be used for
multi range current measurement. For current measurement the instrument is to be
connected in series with the unknown current source.
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 measurement, 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 measurement of resistance, the unknown resistance is connected in series with
the meter and across a battery, such that the current passing through the meter is
directly proportional 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.
The two mainfactors affectingthe measurementare the sensitivityandthe accuracy. Sensitivityrefers
to the reciprocal of the full scale deflectioncurrentandismeasuredinohmspervolt.
Fluke Multi meter:
The fluke multi meters are protected against the transient voltage. It is a small portable device
used to measure voltage, current and test diodes. The multi meter has multi selectors to select the
desired function. The fluke MM 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 moved to the appropriate port for the desired measurement. The fuse is protected to
prevent the damage, if connected to wrong port.
Current measurements are easy to make, but they are done in a slightly
different way to the way in which voltage and other measurements are
made. However current measurements often need to be made to find
out whether a circuit is operating correctly, or to discover other facts
associated with its current consumption.
... both digital and analogue multi meters are able to measure current....
Basics of current measurement
Current measurements are made in a different way to voltage and other measurements. Current
consists of a flow of electrons around a circuit, and it is necessary to be able to monitor the
overall flow of electrons. In very simple circuit is shown below. In this there is a battery, a bulb
which can be used as an indicator and a resistor. To change the level of current flowing in the
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circuit it is possible to change the resistance, and the amount of current flowing can be gauged by
the brightness of the bulb.
( A simple circuitinwhichtomeasure current )
When using a multi meter to measurecurrent, the only way that can
be used to detect the level of current flowing is to break into the
circuit so that the currentpasses through the meter. Although this can
be difficult at times, it is the best option. A typical current
measurement can be made as shown below. Fromthis it can be seen
that the circuit in which the currentis flowing has to be broken and
the multi meter inserted into the circuit. In somecircuits where
currentmay often need to be measured, terminals with a shorting link
may be added to facilitate the currentmeasurement
In order that the multi meter does not alter the operation of the circuit when it is used
to measure current, the resistance of the meter must be as low as possible. For measurements
of around an amp, the resistance of a meter should be much less than an ohm. For example if a
meter had a resistance of one ohm, and a current of one amp was flowing, then it would
develop a voltage of one volt across it. For most measurements this would be unacceptably
high. Therefore resistances of meters used to measure current are normally very low.
How to measure current with an analogue multi meter
It is quite easy to use an analogue meter to measure electrical current. There are a few minor
differences in way that current measurements are made, but the same basic principles are used.
6. ... analogue multi meters are also able to measure current easily and accurately....
When using the meter it is possible to follow a number of simple steps:
1. Insert the probes into the correct connections - this is required because there may be a
number of different connections that can be used. Be sure to get the right connections as
there may be separate connections for very low or very high current ranges.
2. Set switch to the correct measurement type (i.e. to measure current) and range for the
measurement to be made. When selecting the range, ensure that the maximum for the
particular range chosen is above that anticipated. The range on the multimeter can be
reduced later if necessary. However by selecting a range that is too high, it prevents the
meter being overloaded and any possible damage to the movement of the meter itself.
3. When taking the reading , optimise the range for the best reading. If possible adjust it so
that the maximum deflection of the meter can be gained. In this way the most accurate
reading will be gained.
4. Once the reading is complete, it is a wise precaution to place the probes into the voltage
measurement sockets and turn the range to maximum voltage position. In this way if the
meter is accidentally connected without thought for the range to be used, there is little
chance of damage to the meter. This may not be true if it left set for a current reading,
and the meter is accidentally connected across a high voltage point!
How to measure current with a digital multi meter
To measure current with a digital multi meter it is possible to follow a few simple steps:
1. Turn the meteron
2. Insertthe probesintothe correct connections - inmanymetersthere are a numberof different
connectionsforthe probes.Oftenone labeled commonintowhichthe blackprobe isnormally
placed.The otherprobe shouldbe enteredintothe correctsocket forthe currentmeasurement
to be made.Sometimesthere isaspecial connectionforcurrentmeasurements,andsometimes
a separate one for eitherloworhighcurrentmeasurements.Selectthe correctone forthe
3. Setmainselectorswitchonthe meterswitchto the correct measurementtype,(i.e.current)
and range for the measurementtobe made.Whenselectingthe range,ensure thatthe
maximumrange isabove the expectedreadinganticipated.The range onthe DMM can thenbe
reducedasnecessary.Howeverbyselectingarange that istoo high,itpreventsthe meterbeing
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4. Whenthe measuringthe current, optimise the range forthe bestreading.If possibleenable all
the leadingdigitstonotreadzero,and inthis way the greatestnumberof significantdigitscan
5. Once the readingiscomplete,itisa wise precautiontoplace the probesintothe voltage
measurementsocketsandturnthe range to maximumvoltage.Inthiswayif the meteris
accidentallyconnected withoutthoughtforthe range used,there islittle chance of damage to
the meter.Thismay notbe true if it leftsetfora current reading,andthe meterisaccidentally
connectedacrossa highvoltage point!
Although it is not as common to measure electrical current as it is to
measure voltage, it is nevertheless and important ability to be able to
measure current. Also knowing how to measure current to gain the best
of the multi meter is also important.