How digital multimeter works? Explain with diagram

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• How digital multimeter works? Explain with diagram

Before going into the details of how digital multimeter works, let us see its applications. It is widely used laboratory instrument for different types of measurements. Its important features are given below –

1. It can measure AC/DC voltage, AC/DC current and resistance very accurately.
2. Accuracy is maximum due to minimum loading effects.
3. Audio and visual indication for continuity testing in electrical wiring.
4. Automatic sign indication i.e. (+) or (–) indication.
5. Automatic null setting.

Working

To understand how digital multimeter works, let us see the block diagram of DMM is given below. The working of each block to measure different types of electrical quantities is as follows.

How to measure resistance?

To measure unknown resistance using digital multimeter, connect an unknown resistor across its input probes. Keep rotary switch in position-1. The proportional current flows through the resistor, from constant current source. According to Ohm’s law voltage is produced across it. This voltage is directly proportional to its resistance. This voltage is buffered and fed to A-D converter, to get digital display in Ohms.

How to measure AC voltage?

Connect an unknown AC voltage across input probes. Keep rotary switch in position-2. The voltage is attenuated, if it is above the selected range and then rectified to convert it into proportional DC voltage. It is then fed to A-D converter to get the digital display in Volts.

How to measure AC current?

Current is indirectly measured by converting it into proportional voltage. Connect an unknown AC current across input probes. Keep the switch in position-3. The current is converted into voltage proportionally with the help of I-V converter and then rectified. Now the voltage in terms of AC current is fed to A-D converter to get digital display in Amperes.

How to measure DC current?

The DC current is also measured indirectly. Connect an unknown DC current across input probes. Keep the switch in position-4. The current is converted into voltage proportionally with the help of I-V converter. Now the voltage in terms of DC current is fed to A-D converter to get the digital display in Amperes.

How to measure DC voltage?

Connect an unknown DC voltage across input probes. Keep the switch in position-5. The voltage is attenuated, if it is above the selected range and then directly fed to A-D converter to get the digital display in Volts.