Voltage Regulators:
An ideal power supply maintains a constant voltage at its output terminals under all operating conditions. The output voltage of a practical power supply changes with load generally dropping as load current increases as shown in fig. 1.
Fig. 1
The terminal voltage when full load current is drawn is called full load voltage (VFL). The no load voltage is the terminal voltage when zero current is drawn from the supply, that is, the open circuit terminal voltage.
Power supply performance is measured in terms of percent voltage regulation, which indicates its ability to maintain a constant voltage. It is defined as
The Thevenin's equivalent of a power supply is shown in fig. 2. The Thevenin voltage is the no-load voltage VNL and the Thevenin resistance is called the output resistance Ro. Let the full load current be IFL. Therefore, the full load resistance RFL is given by
Fig. 2
From the equivalent circuit, we have
and the voltage regulation is given by
It is clear that the ideal power supply has zero outut resistance.
Example-1
A power supply having output resistance 1.5Ω supplies a full load current of 500mA to a 50Ω load. Determine:
- percent voltage regulation of the supply
- no load output voltage.
Solution:
(a). Full load output voltage VFL = (500mA ) (50Ω) = 25V.
Therefore,
(b). The no load voltage
