Digital Multimeters ( DMM )
The digital multmeter is an instrument capable of measuring dc voltage, ac voltage, dc current, ac current, resistance, conductance and decibels. Thus DMM offers increased versatility.
Some DMMs can measure the temperature, frequency etc. Now a days DMMs are popularly used and they have almosti entirely replaced the analog multimeters.
Fig. 4.1 1.2 shows the photograph of a digital multimeter. A DMM has a digital display and a function selector switch. The range selection takes place automatically. There are four input terminals, out of which two terminals
shown in Fig. 4.11.2 are used for measurement of all thei general purpose quantities such as ac/dc voltage, resistance, capacitance and diode, transistor testing.
These terminals are not used for current measurenent Por current measurement, two separate terminals (4 mA IOA) are provided as shown in Fig. 4.11.2. For current measurement upto 400 mA. we have to use oh. i erminals 400 mA and COM. whereas for cureni measurement between 400 mA and 10 A, we have to use th i terminals 10 A and COM
Important specifications of DMM:
Some important specifications of DMM are as follows: DC voltage t
Five voltage ranges are available. The voltage range in from ± 200 mV to ± 1000 V.
Accuracy is about ± 0.03%
Resolution is about 10 uV. AC voltage:
Five voltage ranges are avalable. Te voltage range in from 200 mV to 750 V.
Accuracy is dependent on frequency. Best accuracy obtained is about 0.5 %
Resolution is about 10 uV. Resistance:
Generally six resistance ranges are available. Te resistance range is from 200 9 to 20 M2.
Accuracy is about ± 0.1%of reading. DC current:
Five ranges are available. This range is from ± 200 u to ± 2000 mA.
Accuracy is about ± 0.3 %of reading.
Resolution is about ± 0.01 A. AC current:
Five current ranges are available. This range is from 200 uA to 2 Amp.
Accuracy is dependent on frequency. Best accura obtained is about ± 1%
In addition, it measures the parameters such as temperature, i frequency, power etc. and it can be used for testing of capaciton diodes and transistors.
Advantages of DMM:
As compared to analog multimeters; digital multimeters ae having following
Advantages :
They are having high input impedance. So there is no loading effect.They are having higher accuracy. An unambiguous reading is obtained.
The output can be interfaced with external equipment. They are available in smaller sizes.
Digital display so easy and accurate reading. Automatic range adjustment.
Easy to carry.
Disadvantages:
Needs battery for its operation. Internal circuit is complex.It is costlier than the analog multimeters.i
Applications of digital multimeter:
1. To measure DC voltage and2. To measure the resistance
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