a mARTIAN dIARY

A voltage regulator is a circuit that provides a precise output voltage under varying load conditions and temperature from an unknown and possibly varying input voltage”. From the definition itself it is clear that the there are three varying parameters whose effect has to be nullified: -

(1) Change in input voltage

(2) Change in load conditions

(3) Change in temperature

A perfect voltage regulator is one, which would give a constant voltage irrespective of change in any or all of these parameters. Usually practical regulators achieve regulation within tolerable limits, which are set by the circuit designer. In this article an attempt is made to explain the basic theory behind there regulators based on their general classifications after which a simple “Series Voltage Regulator” circuit is studied for a deeper understanding from a theoretical view. Finally we shall look into the “packaged” alternatives available and 2 such alternatives shall be studied which can be easily and more importantly cheaply realized at home. (LM317 and 78XX /79XX)

Based on the working principle voltage regulators can be classified into two different types-:

1.Linear, Dc or Dissipative type
2.Switching type

In the dissipative type the resistance of the active element is varied in such a way so as to provide the output with the same voltage where as switching regulator consist of switches which is turned ‘on’ and ‘off’ at a rate such that a regulator delivers average current in periodic pulses

Linear DC or Dissipative Type

This type of regulators are found in two basic configurations

1.Shunt Regulators

2.Series Regulators

A generalized diagram of a shunt regulator is shown in the figure

Here it is clearly seen that regulation is achieved by varying the parallel resistance Rp such that the drop across Rs provides the desired output voltage Vo since,

VO = Vin – IinRs

Hence in simple terms the regulation is maintained by making the source ‘see’ the same load. But here, as the current drawn from the supply as all times is the peak load current, shunt regulators are generally inefficient. A more mathematical description about the efficiency of the various regulators discussed can be found in the books listed in reference.

Here PD is the dissipated power. And , since efficiency decreases as PD increases, it can be seen that the efficiency increases if a current near to the peak load current is drawn. A simple shunt regulator can be constructed as

This circuit is very primitive in the sense that it has very poor load and line regulation. In practical circuits the zener diode is replaced by active feedback network as shown later which have better load and line regulation

Series Pass Regulator

As is obvious from the figure here regulation is achieved by varying the series resistance as directed by the change in output. Thus R­s is varies such as to provide the required output Voltage

Vo =Vin – IinRs

Practical series voltage regulator circuits contain other circuits such as output sampling, voltage reference and error amplifier.

In series voltage regulators the efficiency is improved since only the load current is drawn from the source at any given time. But since the whole load current has to pass thru the series pass element , the rating of the pass element used will have to be high compared to that used in the shunt regulator.

Now that we have had a sip of the theory behind voltage regulators; its time we have a look into a practical voltage regulator circuit

Working

The circuit of the practical series voltage regulator can be dissected into the 6 different blocks as shown in the figure. The unregulated supply is connected to the output thru a series pass element whose transconductance is the function of the output of the error amplifier. The error amplifier compares a sample of the output voltage with a precision voltage reference.

The same implemented using discrete component is shown in the figure. Here the series pass element is the 2N3055 power BJT. And the error amplifying circuit consists of a 6.1 volt zener as voltage reference. The BC107 BJT forms the core of the error amplifier. The sampling ratio determines the output voltage.For a detailed analysis of the design of the same see the references.

 

Packaged Options

Now we shall look into some of the “Packaged” options available. The voltage regulator is the most common monolithic building block available next to the Op-Amp. Even though it is possible to replace voltage regulators with Op-Amp circuits, it is easier and cheaper (!!!) to use IC voltage regulators. Because of low cost, size better regulation etc. Further more VR IC’s with a lot of extra features are available in the market today such as programmable output, short circuit limiting etc.

Now we shall look into two practical circuits which are quite cheap and easy to construct right at home.

LM 317 Variable Voltage Regulators

The first circuit that we will look into revolves around the LM317 voltage regulator. It is a popular 3 terminal adjustable regulator available in a variety of packages with power ratings up to 0.1- 1.5 A and can provide output voltages in the range of 12 – 57 V

The 317 maintains a nominal voltage of 1.25V across the o/p and adjust terminals. This is the reference voltage and does not change. This voltage is impresses on the resistor R1 which makes a current I1 = VR /R1 flow where VR=1.25 V

Now the current I2 that flows thru R2

I2 = I1 + I­adj

with Iadj~100uA(remains constant)

Vo = I1R1 + I2R2

= I1R1 + (I1+Iadj­) R2

=VR(1+ R2/R1) + IadjR2

Vo =VR(1+ R2/R1)

Where VR = 1.25V
Thus by varying R2 variable voltages can be obtained. R1 is usually taken to be 240? and should be directly tied to the IC rather that near the load for good regulation. Practical circuit will contain some noise reduction capacitors and a proper unregulated DC supply

Fixed Voltage Regulators (78XX & 79XX Series)
Having seen a variable voltage regulator we shall now look into a common fixed voltage regulator. 78XX (79XX) series of voltage regulator IC’s provide fixed positive (negative) voltages. For example 7805(7905) gives a fixed regulated voltage of 5V (-5V).We shall look into a practical circuit using 7812 and 7912 to give a dual supply of +12V and -12V.The circuit diagram is as follows
The IC is available in TO-3 and TO-220 packages. The 7812(7912) maintains a constant voltage of 12V (-12V) between the Gnd and Out pin. The centre tapped transformer is used to provide a dual supply of 12V and -12V. The output of the CT transformer is rectified using a bridge rectifier. The capacitors are provided to improve regulation and minimize noise effects. It can also current source or a variable voltage regulator.

Conclusion
Power supply is a important part of any system, and for new systems being designed selection of the correct type of power supply as per the system requirements is one of the most critical decisions of the design phase. Some of the parameters that must be considered include total power required, size, cost, efficiency etc. Trade offs exist between these parameters and these must be made according to the system under consideration. Thus knowledge of voltage regulators is essential for an aspiring electronics engineer.
References

1.Integrated Circuits – K.R.Botkar Excellent text a must have for all

2.electronics graduates. Data Sheets And Applications Notes Excellent source for sample circuits and design considerations