Digital Multi Meter (DMM)

Digital Multi Meter (DMM)

Introduction

It is a common & important laboratory instrument. It is used to measure AC/ DC voltage, AC/DC current and resistance with digit al display. It gives digital display, which is ver y accurate. It has an advantage of very high input resi stance. It also provides over ranging indicator.

How digital multimeterworks ?

The block diagram of DMM is given below. The working of each block to mea sure different types of electrical quantities is as follows.

How to measure resistance?

Connect an unknown resistor a cross its input probes. Keep rotary switch in t he position-1 (refer block diagram below). The pro portional current flows through the resistor, from constant currentsource. According to Ohm’s law voltage is produced across it. This voltage is d irectly proportional to its resistance. This voltage is b uffered and fed to A-D converter, to get digital display in Ohms.

Block diagram of DMM

How to measure AC voltage?

Connect an unknown AC voltage across the 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 the n 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.