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: V = 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*Rincreases.                                                       (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 VAs 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 liked this article please share it with your friends. Like our facebook page and subscribe to our newsletter for future updates.

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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