# Digital Multimeter: Working and Application

- Link: https://studyelectrical.com/2022/07/digital-multimeter-working-and-application.html
- Published: 2022-07-08T05:22:36+00:00
- Author: Tesla

A digital multimeter is an electronic instrument which can measure very precisely
the dc and ac voltage, current (dc and ac), and resistance. All quantities other
than dc voltage is first converted into an equivalent dc voltage by some device 
and then measured with the help of digital voltmeter.

The block diagram of a digital multimeter is shown below. The procedures of measurement
of different quantities are described below.

![digital multimeter-block diagram](https://studyelectrical.com/wp-content/uploads/
2022/07/digital-multimeter-block-diagram-1024x509.png)

Block diagram of a digital multimeter

## Voltage Measurement

For measurement of ac voltage, the input voltage, is fed through a calibrated, compensated
attenuator, to a precision full-wave rectifier circuit followed by a ripple reduction
filter. The resulting dc is fed to an [Analog Digital Converter](https://en.wikipedia.org/wiki/Analog-to-digital_converter)(
ADC) and the subsequent display system. Many manufacturers provide the same attenuator
for both ac and dc measurements.

## Current Measurement

For current measurement, the drop across an internal calibrated shunt is measured
directly by the ADC in the ‘dc current mode’, and after ac to dc conversion in the‘
ac current mode’. This drop is often in the range of 200 mV (corresponding to full
scale).

Due to the lack of precision in the ac–dc conversions, the accuracy in the ac range
is generally of the order of 0.2 to 0.5%. In addition, the measurement range is 
often limited to about 50 Hz at the lower frequency end due to the ripple in the
rectified signal becoming a non-negligible percentage of the display and hence results
in fluctuation of the displayed number. At the higher frequency end, deterioration
of the performance of the ADC converter limits the accuracy.

In ac measurement the reading is often [average or rms values](https://studyelectrical.com/?post_type=lp_course&p=13672/lessons/power-rms-average-values-of-standard-signals)
of the unknown current. Sometimes for measurement of current, a current to voltage
converter may also be used.

![Current-to-voltage-converter](https://studyelectrical.com/wp-content/uploads/2022/
07/Current-to-voltage-converter.png)

Current to voltage converter

The current under measurement is applied to the summing junction at the input of
the op-amp. The current in the feedback resistor I_(R) is equal to the input current
I_(in) because of very high input impedance of the op-amp. The current I_(R) causes
a voltage drop across one of the resistors, which is proportional to the input current
I_(in). Different resistors are employed for different ranges.

## Resistance Measurement

For resistance measurement the digital multimeter operates by measuring the voltage
across the externally connected resistance, resulting from a current forced through
it from a calibrated internal current source.

The accuracy of the resistance measurement is of the order of 0.1 to 0.5% depending
on the accuracy and stability of the internal current sources. The accuracy may 
be proper in the highest range which is often about 10 to 20 MΩ. In the lowest range,
the full scale may be nearly equal to>200 Ω with a resolution of about 0.01 Ω for
a 4½ digit digital multimeter. In this range of resistance measurement, the effect
of the load resistance will have to be carefully considered.

## How to Measure Voltage Using Digital Multimeter?

**DC Voltage Measurements**

To measure DC voltage, we place the Red lead into the V Ω COM port. Turn the dial
or switch to VDC or V. If it is a manual ranging meter set it for the proper range.

As in the example below, we want to measure a 9V battery so the best range would
be the 20 V range. If you have an auto-ranging meter you only need to set the function
on the dial to VDC or V.

![measure-voltage-using-multimeter](https://studyelectrical.com/wp-content/uploads/
2022/07/measure-voltage-using-multimeter.png)

measure-voltage-using-multimeter

**AC Voltage Measurement**

To measure AC voltage, we place the Red lead into the V Ω port and black lead into
the COM port. Turn the dial or switch to VAC or V. If it is a manual ranging meter
set it for the proper range.

As an example the meter would be set to the 200 V range to measure a 120V outlet.
If you have an auto-ranging meter you only need to set function to VAC or V .

Remember that it is always a good practice to connect the black lead first then 
the red.

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