Digital Electronics

MARWARI COLLEGE, RANCHI

(AN AUTONOMOUS UNIT OF RANCHI UNIVERSITY FROM 2009)

- Prakash Kumar, Dept. of CA

-Raju Manjhi Dept. of CA

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Digital Electronics

Power supplies:

Basic block diagram of power supply- 

TRANSFORMER: The transformer steps up or steps down the input line voltage and isolates the power supply from the power line.

The RECTIFIER section converts the alternating current input signal to a pulsating direct current.

However, for this reason a FILTER section is used to convert pulsating dc to a purer, more desirable form of dc voltage.

 The final section, the REGULATOR, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or input line voltages.

How this signal is altered -within each section of the power supply. In view fig- B , an input signal of 115 volts ac is applied to the primary of the transformer. The transformer is a step-up transformer with a turn’s ratio of 1:3. You can calculate the output for this transformer by multiplying the input voltage by the ratio of turns in the primary to the ratio of turns in the secondary; therefore, 115 volts ac3 = 345 volts ac (peak-to-peak) at the output. Because each diode in the rectifier section conducts for 180 degrees of the 360-degree input, the output of the rectifier will be one-half, or approximately 173 volts of pulsating dc. The filter section, a network of resistors, capacitors, or inductors, controls the rise and fall time of the varying signal; consequently, the signal remains at a more constant dc level. The output of the filter is a signal of 110 volts dc, with ac ripple riding on the dc. The regulator maintains its output at a constant 110-volt dc level, which is used by the electronic equipment (more commonly called the load).

REGULATED POWER SUPPLY

Regulated power supply is an electronic circuit that is designed to provide a constant dc voltage of predetermined value across load terminals irrespective of ac mains fluctuations or load variations.

A regulated power supply essentially consists of an ordinary power supply and a voltage regulating device, as illustrated in the figure. The output from an ordinary power supply is fed to the voltage regulating device that provides the final output. The output voltage remains constant irrespective of variations in the ac input voltage or variations in output (or load) current.

Power Supply Characteristics

 There are various factors that determine the quality of the power supply like the load voltage, load current, voltage regulation, source regulation, output impedance, ripple rejection, and so on. Some of the characteristics are briefly explained below:

1. Load Regulation – The load regulation or load effect is the change in regulated output voltage when the load current changes from minimum to maximum value.

2. Minimum Load Resistance – The load resistance at which a power supply delivers its full-load rated current at rated voltage is referred to as minimum load resistance. 

3. Source/Line Regulation- the filtered output of the bridge rectifier is almost directly proportional to the ac mains voltage. 

4. Output Impedance – Even though the external load resistance is varied, almost no change is seen in the load voltage. An ideal voltage source has an output impedance of zero.

5. Ripple Rejection – Ripple is equivalent to a periodic variation in the input voltage. Thus, a voltage regulator attenuates the ripple that comes in with the unregulated input voltage.

Zener regulator

The constant reverse voltage Vz of the zener diode makes it a valuable component for the regulation of the output voltage against both variations in the input voltage from an unregulated power supply or variations in the load resistance. The current through the zener will change to keep the voltage at within the limits of the threshold of zener action and the maximum power it can dissipate.

Series and shunt regulation

A voltage regulator is used to regulate voltage level. When a steady, reliable voltage is needed, then voltage regulator is the preferred device. It generates a fixed output voltage that remains constant for any changes in an input voltage or load conditions.

Linear Regulator

Linear regulator acts like a voltage divider. In Ohmic region, it uses FET. The resistance of the voltage regulator varies with load resulting in constant output voltage.

Advantages of linear voltage regulator

·         Gives a low output ripple voltage

·         Fast response time to load or line changes

·         Low electromagnetic interference and less noise

Disadvantages of linear voltage regulator

·         Efficiency is very low

·         Requires large space – heatsink is needed

·         Voltage above the input cannot be increased

Series Voltage Regulator

A series voltage regulator uses a variable element placed in series with the load. By changing the resistance of that series element, the voltage dropped across it can be changed. And, the voltage across the load remains constant.

The amount of current drawn is effectively used by the load; this is the main advantage of the series voltage regulator. Even when the load does not require any current, the series regulator does not draw full current. Therefore, a series regulator is considerably more efficient than shunt voltage regulator.

Series Voltage Regulator Circuit 

Shunt Voltage Regulator

A shunt voltage regulator works by providing a path from the supply voltage to ground through a variable resistance. The current through the shunt regulator is diverted away from the load and flows uselessly to the ground, making this form usually less efficient than the series regulator. It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits wherein the wasted current is too small to be of concern. This form is very common for voltage reference circuits. A shunt regulator can usually only sink (absorb) current.