Components and Operation of DAC and Power Supply

Introduction
In order to operate, many household electronic devices fluctuating alternating current from the
mains need to be converted to constant direct current. Every electronic device has specific power
supply limit and these devices have a circuit inside them that is able to control and ensure
constant direct current is provided. The conversion is done by a regulated power supply circuit.
Testing of apparatus to determine if the components are faulty was also dealt with in details.
Procedure of troubleshooting is also explained.
background
Power supply
A regulated power supply is used to convert non-constant and unregulated alternating current
that normally comes from socket source supply to a constant direct current. This conversion
ensures that the output to electronic devices remains the same regardless of how much the input
change. Figure 1 below shows typical components of direct current power supply, which the
output waveform for every stage
Figure 1: block diagram power supply
Main components of a regulated Direct Current supply power are
a. A step- down transformer
b. A regulator
c. A filter
d. A rectifier

a. A step-down transformer
The transformer is responsible for stepping down alternating current from the mains which is
usually 230 volts to a required voltage level. This is achieved by using the turns ratio of the
transformer. The output of the step-down transformer is fed a full-wave rectifier.
b. Rectifier
A rectifier circuit used in this case is full wave rectifier and it consists of four diodes that are
responsible for the rectification process. Rectification is the process of converting alternating
current or voltage to its equivalents direct current or voltage. The output of this rectifier pulsating
direct current as shown in figure 1, and is fed to the filter
Figure 2: Full-wave rectifier bridge
A full wave rectifier of figure 2 consists of four p-n junction diodes, connected in a specific
manner. In the positive half circle of the voltage supply, diodes D1 and D4 are forward biased
and diodes D3 and D2 are reversed biased. During this cycle, diodes D1 and D4 acts as a closed
switch and will allow current conduction, diodes D2 and D3 act as an open circuit. A rectified
waveform due to this conduction thus appears at the output of the rectifier as shown in figure 1.
During the negative cycle, diodes D3 and D2 are forward biased and diodes D1 and D4 are
reversed biased, and a positive voltage appears at the output of the rectifier.
c. A filter
Waveform from rectifier is a pulsating waveform with high ripple content as shown figure 1. To
achieve ripple free direct current waveform a filter is used to smoothen the waveform. The type
filter used is the capacitor filter.

Figure 3: smoothing process
When the voltage increases the capacitor is charged, charging occurs until the waveform is at its
peak value. When the voltage (instantaneous) reduces, capacitor discharges slowly and
exponentially through connected load. Less rippled voltage output is thus obtained.
d. A regulator
When input voltage from ac mains fluctuate it is eliminated by the regulator. Fluctuation at the
output can be due to changing load current or due to external factors such as change in
temperature. All these fluctuations are eliminated in the regulator. To obtain constant output
voltage IC’s such as 78XX and 79XX series are used. Normally coupling capacitors with
capacitance in the order of 0.01uF to 10uF are connected at the input and output to eliminate
noise at the input and transients at the output.
Digital to analogue converter (DAC)
In real world, data converters are of much importance, data converters are used to convert either
analogue signal to digital or digital signal to analogue form. There are two types of converters,
digital to analogue converter (DAC) and analogue to digital converters (ADC). In this paper, we
shall deal with DAC converter. DAC is an electronic device that is used to convert digital data to
its equivalent analogue electrical signal. The digital data that are produced by several electronic
devices such as microprocessor needs to be converted to a form that easily interacts with real