The main aim of this section is to design sensing circuits for measuring both AC and DC voltages ranging from 0-100 VAC and VDC, and display the measured values using a basic ADC for Part A and for Part B using an internal ADC of a microcontroller.
NOTE// IF YOU NEED SIMULATION FILE EMAIL ME AT:MAHLOBONKANYISO@GMAIL.COM
Part A
1. Introduction to voltmeters
A voltmeter is an instrument used to measure voltage or electrical potential difference between two
points in basic electric circuits. Voltmeters may have an accuracy of a few percent of full scale, and
are used with voltages from a fraction of a volt to several thousand volts. Two common voltage
measurements are direct current (DC) and alternating current (AC). Although voltage measurements
are the simplest of the different types of analog measurements, they present unique challenges due to
noise considerations. Voltmeters are classified in to two types namely, analog and digital voltmeter.
Analog voltmeter consists of pointer that moves across a scale and the movement is proportional to
the voltage measured. Analog voltmeters are further classified based on their principle of
construction. Some of the commonly known analog voltmeters are Permanent Magnet Moving Coil
Voltmeter, Rectifier Type Voltmeter, Electrostatic Type Voltmeter, and Moving Iron Type Voltmeter
[1].
Analog voltmeters generally have an error percentage of 5% and the parallax error is often an issue.
However, analog voltmeters can be used to measure ranging from few volts to several thousand volts.
To overcome the defects of analog voltmeters, digital voltmeters were introduced. A digital voltmeter
(DVM) measures an unknown input voltage by converting the voltage to a digital value and then
displays the voltage in numeric form. DVMs are usually designed around a special type of analog-to-
digital converter (ADC) called an integrating converter. The percentage of error in digital voltmeters
is usually less than 1% and the accuracy can be increased in precision digital voltmeters with high
speed measurement and option of storing the values in a memory. The first digital voltmeter was
invented and produced by Andrew Kay of Non-Linear Systems and later founder of Kaypro in 1954
[2]. DVM measurement accuracy is affected by many factors, including temperature, input
impedance, and DVM power supply voltage variations. However this implementation is quite rugged
and very much reliable.
2. Aims and objectives
The main aim of this section is to design sensing circuits for measuring both AC and DC voltages
ranging from 0-100 VAC and VDC, and display the measured values using a basic ADC for Part A
and for Part B using an internal ADC of a microcontroller.
, 3. System design
Figure 1: Digital voltmeter system block diagram
Figure 2: Digital voltmeter functional block diagram
3.1. DC and AC voltage sensor unit
In this section a basic voltage divider circuit is used as the AC and DC Sensing Unit to scale down the
input DC and AC voltages into a DC voltage in the range of 0 to 5 V. The Processor Unit can read this
scaled down voltage and calculate the actual AC and DC voltages and display them on the Display
Unit.
3.1.1. DC voltage sensing circuit
The DC voltage sensing circuit is a basic voltage divider circuit as shown in Fig. 1. Eq. (1) represents
the output voltage of the sensing circuit and is used to design the resistor values of the sensing circuit
shown in Fig. 3. The maximum voltage to be measured by this circuit is 100 VDC. Applying 100 V as
the input V, then the output V2 should not be more than 5 V and hence using Eq. (2) the voltage
across R1, V1 will be 95 V.
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