Series Voltage Regulator – Working Principle

We assume that the voltage across a zener diode remains constant.

i.e. DVZ = 0.

In all cases, we indicate load resistance by RL

Series voltage regulator circuit diagram
Series voltage regulator circuit diagram
  • Assume supply voltage increases by DVS

Current through resistance R is IR = (VS-Vz )/R

or, DIR = DVS / R                                (Equation 1.1)

Also IR = IB + IZ ;  or, DIR = DIB + DIZ ;            (Equation 1.2)

From Equation 1.1 and Equation 1.2, an increase in VS increases base current IB and zener current IZ. Since collector current IC = ß*IB so IC also increases.

We know IE = IB + IC;

or,          (since IB is very small compared to IC)

DIE = DIC

As IC increases, IE will also increase through load RL, thus voltage output VO = IE*RL tends to increase.      (Effect 1: VO tends to increase)

Using KVL in output circuit, VO + VBE – VZ = 0; where VBE = base emitter voltage, VZ = zener voltage

DVO = -DVBE        (Equation 1.3)

Equation 1.3 suggests, an increase in output voltage VO decrease base emitter voltage VBE. As decrease in VBE causes decrease in IC , and conversely IEthrough RL , hence output voltage VO tends to decrease.             (Effect 2: VO tends to decrease)

Effect 1 and Effect 2 neutralise each other and VO is constant.

Opposite happens when VS decreases.

  • Assume supply current IS increases by DIS (keeping VS constant)

IS = IC + IR; or, DIS = DIC + DIR         (Equation 2.1)

IR = IB + IZ; or, DIR = DIB + DIZ         (Equation 2.2)

From Equation 2.1, IS increases IC (also IR). Also this is evident as from Equation 2.2, that increase in IR increases IB and hence increases IC.

As IE˜IC so IE increases through RL. Hence output voltage VO = IE*RL tends to increase.

(Effect 1: VO tends to increase)

Remaining analysis is similar to previous case. VO tends to increase decreasing VBE (like Equation 1.3). This decreases IC, consequently decreasing IE and this tends to decrease VO.

(Effect 2: VO tends to decrease)

Effect 1 and Effect 2 neutralise each other and VO is constant.

Short circuit protection

To prevent short circuit i.e. to prevent an excessive high flow of current, the following arrangement is made.

Short circuit protection
Short circuit protection

A very small resistance RSC is connected in series with the load. The base emitter terminals of BJT Q2 are connected across this RSC resistance. When a high current flows across the load, an appreciable amount of voltage is developed across RSC. Hence base emitter voltage of Q2 increases, collector current of Q2 increases, so IB is shunted away from Q1. As IE˜IC= ß*IB , hence IE decreases and a large current flow is prevented.

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Shunt Voltage Regulator – Working Principle

A zener diode forms an integral part of any voltage regulator. Before we go ahead, we know, that in all cases, the voltage across a zener diode will remain constant. i.e. ?VZ = 0. In all cases, we indicate load resistance by RL.

Regulator using zener diode only

Regulator using zener diode only
Regulator using zener diode only
  • Across RL we have: VO­ = VZ = ILRL                                            (Equation 1)
  • From current law: I = IZ + IL                                                          (Equation 2)
  • From KVL along indicated path: VS = I*R + VZ                             (Equation 3)

Equation 1 tells that output voltage VO will always be constant = VZ.

Assume two cases:

  • Assume supply current I change by dI
    From Equation 2: ?I = ?IZ + ?IL
    From Equation 1: ?VZ = ?ILRL ; or, ?IL = 0 (since ?VZ = 0)
    Thus ?I = ?IZ. This shows that excess current is bifurcated through the zener diode.
  • Assume load RL changes by ?RL (with VS constant)
    Output voltage VO will remain constant, but change in IL will be compensated by change in IZ
    From Equation 3: ?VS = ?I*R+ ?VZ ;      or, 0 = ?I*R + 0 ;               or, ?I = 0
    From Equation 2: ?I = ?IZ + ?IL ;            or, 0 = ?IZ + ?IL  ;             or, ?IL = – ?IZ

Thus if IL increases, IZ decreases and vice versa.

Regulator using transistor and zener diode

Regulator using transistor and zener diode
Regulator using transistor and zener diode

Few points:

Correlating VO and indicated path from point X to GND: VO = VX = VZ + VBE          (Equation 1)

I = IB + IC + IL ;   or, I = IC + IL(since IB is very small)                                                     (Equation 2)

The increase in VBE causes more collector current IC to flow.

  • Assume current I increase by dI keeping VS constant (opposite analysis will take place of I decreases)?I is positive. VS – I*R = VX ;         or, 0 – ?I*R = ?VX ; (since VS is constant)
    i.e. VX = VO  decreases on increase in I.                                                                  (Effect 1: VO tends to decrease)Next, from Equation 1: ?VO = ?VZ + ?VBE ;            or, ?VO = 0 + ?VBE ;
    i.e VBE also decreases on decrease in VO
    As VBE decreases, IC decreases.From Equation 2: ?I = ?IC + ?IL
    If ?I = positive (assumed);           ?IC = negative (as VBE decrease);           so IL must increase.
    The voltage across load VL = IL*RL increases.                                                       (Effect 2: VO tends to increase)The Effect 1 and Effect 2 neutralize and VO is constant.
  • Assume supply voltage VS is increased keeping current I constantThe analysis will take place just as done previously.
    VS – I*R = VX ;    or, ?VS – 0 = ?VX ; (since I is constant)
    i.e. VX = VO increases on increase in VS.                                                                 (Effect 1: VO tends to increase)Next, from Equation 1: ?VO = ?VZ + ?VBE ;            or, ?VO = 0 + ?VBE ;
    i.e VBE also increases on increase in VO As VBE decreases, IC increases.From Equation 2: ?I = ?IC + ?IL
    If ?I = 0 (assumed);         ?IC = positive (as VBE increase);  so IL must decrease.
    The voltage across load VL = IL*RL decreases.                                                     (Effect 2: VO tends to decrease)
    The Effect 1 and Effect 2 neutralize and VO is constant.

This post is written by Sayantan Roychowdhury. If you like this article, please share it with your friends and like or facebook page for future updates. Subscribe to our newsletter to get notifications about our updates via email. If you have any queries, feel free to ask in the comments section below. Have a nice day!

Linear Voltage Regulator – Series and Shunt type

Hi friends, in this article, we will take a basic overview of a linear voltage regulator and its types. This includes the block diagram and working principles.

What is voltage regulator?

A voltage regulator prevents the varying of the voltage across a load in spite of variation in the supply. It is also used to regulate or vary the output voltage of the circuit.

Two terms:

  • Regulation: compensates for variation in the mains (line voltage)
  • Stabilization: compensates for variation in load current

However, in practice, both the terms loosely used for the same meaning of voltage regulation.

Types of voltage regulator

There are mainly two types:

  1. Series voltage regulator
  2. Shunt voltage regulator

Series voltage regulator:

A simple block diagram is as follows

Series voltage regulator
Series voltage regulator

The series voltage regulator controls variation in voltage (DVS) across the load by providing a voltage in series with the load.

A further more detailed block diagram is shown. A series regulator has its regulating unit in series with the load.

Series voltage regulator
Series voltage regulator

There is always a voltage drop in the regulating unit (VR). The output voltage VO (or VL) is:

VL = VS – VR

Series voltage regulator usually has a negative feedback system. If load voltage (VL) tends to fall, smaller feedback decreases controlling unit resistance thereby allowing more current to flow in the load (VR decreases) and increasing VL. Opposite happens when VL increases.

Shunt voltage regulator

Block diagram is as follows

Shunt voltage regulator
Shunt voltage regulator

Shunt voltage regulator controls the voltage across the load by varying the current flowing through the load (IL) and through the regulating unit (IR).

A further detailed block diagram is shown below. A shunt regulator has its regulating unit in parallel to the load.

I = IR + IL

The stability in the voltage across the load RL is brought about by ensuring a steady current flow through it.

When the current across RL tends to increase, regulating unit prevents it by allowing the excess current to flow through it. Since current I is constant, IL decreases.

Same happens when current IL tends to decrease. Regulating unit prevents it by decreasing current flow (IR) through it, thereby increasing IL.

This article is written by: Sayantan Roychowdhury

Tags: LM317, adjustable voltage regulator, zener diode voltage regulator, voltage regulator 7805, vrm, avr, ldo.

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