Executive Summary
Currently, there is a challenge concerning the production of a constant output voltage in some electrical circuits. Devices such as fire alarms require a constant voltage supply to perform their tasks well. In case there is no regulator to stabilize the output voltage, then the variation of voltage at the input will be reflected in the output voltage. Such small changes are referred to as “ripple voltage” or electrical noise. The source of these disturbances includes control circuits and switching power supplies. Besides, their impact can worsen when one grounds an electrical power distribution system. If the ripple voltage is not regulated, it can adversely affect the working of electrical devices. This research proposes the use of a regulated power supply that can provide DC output voltage of between 1.30 Vdc and 15.0 Vdc. Admittedly, regulated power supplies are easily available and affordable. In case one has a sensitive device, the best option would be to use a regulated power supply since it will ensure that there is a correct amount of voltage supply regardless of the input.
Introduction
Primarily, a voltage regulator ensures the output voltage in a circuit is close to the desired value. It is a vital device in the construction and design of an electrical circuit since an unregulated power supply can damage some of the components in a circuit. There are different types of voltage regulators. One of them is a passive voltage regulator that can produce a voltage greater than the one that the components of circuit need. Such a voltage regulator consists of a resistor with a predefined set of performance traits. It reduces the value of the input voltage to the required output level and disposes of the excess energy as heat. This kind of regulator requires heat sinks that will dissipate the excess heat. Another type is the active voltage regulator that uses a negative feedback mechanism to control the output voltage. This regulator alters the input voltage until it reaches the desired value. Hence, a voltage regulated power supply is vital in homes since it ensures that electronic circuits have a certain power supply limit and constant DC voltage. One can regulate the DC supply by altering the voltage and current since the supply from the mains is bound to fluctuate. As a result, every active and passive electrical device has a well-defined DC operating point, also known as the Q-point. For a circuit to stable, the source of DC power must work within the Q-point. In this experiment, we use LM 1085-ADJ or LM 1086-ADJ to ensure that the output voltage is kept at the desired value of between 1.30 Vdc and 1.50 Vdc.
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Purpose of the Power Supply
LM1085-ADJ power aims to foster a constant output voltage regardless of the changes in the AC input voltage and the variations in load current.
Theory of Operation
LM1085-ADJ serves as a positive voltage regulator that uses a single NPN transistor to control the input voltage. As a result, it produces a lower output voltage. In the circuit diagram, there is a pass transistor that can also be referred to as a quasi LDO. One of its advantages is that it has a low quiescent current. More importantly, the design of LM1085-ADJ ensures that there is a minimum dropout voltage of 1.3V over temperature at full load. The input voltage can go up to 18Vdc. In the experiment, the input voltage source is the power plug whose line regulation is 17.5V at maximum current. At the transformer, the input voltage is stepped down and later supplied to the full-bridge wave rectifier that reduces the AC ripples. This voltage is then filtered by capacitor C1. Later, LM1085-ADJ develops 1.25 reference voltage (V REF ) between the output of the circuit and the terminal. Here, the voltage passes through resistor R1 and generates current I1. As a result, the constant current generated flows through resistor R2. The final voltage drop across R2 combines with the reference voltage resulting in the desired output voltage given by the following formula.
V OUT =V REF (1+R2/R1) + I ADJ
In ensuring stability in the circuit, the loop must retain negative feedback. LM1085-ADJ uses the series resistors at capacitive loads. Thereby, they introduce a zero in the loop, which increases the phase margin and enhances stability. In the experiment, solid tantalum series resistors are used. The reason for selecting these resistors is that their equivalent series resistance does not significantly vary under temperature change.
