PREPARED BYV.SANDHIYA

LECT/ ECE

Zener diode is a voltage regulator device because it is able to fix the output voltage at a constant value (DC voltage).

RS is to limit the zener current, IZ so that it is less than the maximum current, IZM (to avoid the zener diode from broken).

+

VS-

+

VS-

+VZ-

+VZ-

RS RS

RL

A simple regulator circuit A regulator circuit with load resistance

How to determine whether the zener acts as a regulator or not??◦ Use Thevenin Theorem◦ See example

If VTH<VZ, regulation does not occur.

Referring to zener I-V charateristic curve, if the voltage across the zener diode zener is between 0-VZ, the zener diode is operating in the reverse bias region, thus it DOES NOT functioned as a regulator.

VTH must at least the same value as VZ (VTH VZ) so that the diode CAN function as a voltage regulator because it is operating in reverse breakdown region.

Reverse biased region

Reverse breakdown

region

In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage.

Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation.

Three types of Zener analysis◦ Fixed VS and RL

◦ Fixed VS and variable RL

◦ Variable VS and fixed RL

- Fixed VS and RL

The applied dc voltage is fixed, as the load resistor.The analysis :1. Determine the state of the Zener diode by removing it from the network and

calculating the voltage across the resulting open circuit.

2. Substitute the appropriate equivalent circuit and solve for the desired unknowns.

- For the on state diode, the voltages across parallel elements must be the same.

VL=VZ

The Zener diode current is determined by KCL:

IZ = IR – IL

The power dissipated by the Zener diode is determined by:

PZ = VZ IZ - For the off state diode, the equivalent circuit is open-circuit.

- Fixed VS and Variable RL

+Vs-

Rs

Step 1- get the RLmin so that zener is on.

LS

LL RR

VsRV

ZS

ZSL VV

VRR

min

- if RL ≥ RLmin, zener diode ‘on’, so that VL=VZ

Step 2: Calculate the IZ using KCL: 2 condition1. If RLmin , then ILmax and IZmin because of constant I1 2. If RLmax, then ILmin and IZmax

minmaxconstant1maxmin orLorZ III ; Izmax taken from data sheet Izmin = 0, if not given

S

ZS

R

VVI

1

minmax

L

ZL R

VI

minmax

L

ZL I

VR Where and or

- Variable VS and fixed RL

+Vs-

Rs

Step 1- get the VSmin so that zener is on.

LS

LL RR

VsRV

L

ZSLS R

VRRV

)(min

if VS ≥ VSmin, zener diode will ‘on’, so that VL=VZ

Step 2: Calculate the IZ using KCL: 2 condition1. if VSmin , then I1min and IZmin because of constant IL 2. if VSmax, thenI1max and IZmax

constantmaxmin1maxmin LororZ III ; Izmax= Pzmax/Vz

whereL

LL R

VI

S

ZS

R

VVI

min

min1ZSS VRIV max1maxand or

The 78xx (also sometimes known as LM78xx) series of devices is a family of self-contained fixed linear voltage regulator IC. The 78xx family is a very popular choice for many electronic circuits which require a regulated power supply, due to:

i. ease of use and ii. Low cost.

17

18

LED for emitter Air as barrier for isolation Phototransistor for detector

Transformer is similar, but only for AC Optocoupler can be used for DC

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Incandescent lamp◦ Much slower response time than LED◦ Can filter out high frequency noise◦ Lower lifespan than LED however

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Optocoupler’s bandwidth - determines the highest signal frequency that can be transferred through it

Typical opto-couplers with a single output phototransistor may have a bandwidth of 200 - 300kHz, while those with a Darlington pair are usually about 10 times lower, at around 20 - 30kHz.

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Darlington Pair

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The other main type of optocoupler is the type having an output Diac or bilateral switch, and intended for use in driving a Triac or SCR.

Examples of these are the MOC3020 and MOC3021. Here the output side of the opto-coupler is designed to beconnected directly into the triggering circuit of the Triacwhere it’s operating from and floating at full 120/240 VAC

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A simple circuit with an opto-isolator. When switch S1 is open, LED D1 is off, so Q1 is

off and no current flows through R2, so Vout = Vcc.

When switch S1 is closed, LED D1 lights. Phototransistor Q1 is now triggered, so current

flows through R2 Vout is then pulled down to low state. This circuit, thus, acts as a NOT gate.

Vcc