Download UNIT-V-LIC

PREPARED BY
V.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).
RS
RS
+
+
VS
+
VZ
VS
-
-
-
A simple regulator circuit
+
VZ
-
A regulator circuit with load resistance
RL

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 0VZ, 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
breakdown
region
Reverse
biased
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
Rs
+
Vs
-
Step 1- get the RLmin so that zener is on.
VL 
RLVs
RS  RL
RL min 
RSVZ
VS  VZ
- if RL ≥ RLmin, zener diode ‘on’, so that
VL=VZ
Step 2: Calculate the IZ using KCL: 2 condition
1. If RLmin , then ILmax and IZmin because of constant I1
2. If RLmax, then ILmin and IZmax
I Z min or max  I1constant  I L max or min
Where
V  VZ
I1  S
RS
and
I L max 
; Izmax taken from data sheet
Izmin = 0, if not given
VZ
RL min
or
RL max 
VZ
I L min
- Variable VS and fixed RL
Step 1- get the VSmin so that zener is on.
Rs
+
Vs
-
VL 
RLVs
RS  RL
VS min 
( RL  RS )VZ
RL
if VS ≥ VSmin, zener diode will ‘on’, so
that VL=VZ
Step 2: Calculate the IZ using KCL: 2 condition
1. if VSmin , then I1min and IZmin because of constant IL
2. if VSmax, thenI1max and IZmax
I Z min or max  I1min or max  I Lconstant
where
V
IL  L
RL
; Izmax= Pzmax/Vz
VS min  VZ or
and I

1min
RS
VS max  I1max RS  VZ
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





LED for emitter
Air as barrier for isolation
Phototransistor for detector
Transformer is similar, but only for AC
Optocoupler can be used for DC
18

Incandescent lamp
◦ Much slower response time than LED
◦ Can filter out high frequency noise
◦ Lower lifespan than LED however
19


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.
20
Darlington Pair
21
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
be
connected directly into the triggering circuit of the Triac
where it’s operating from and floating at full 120/240
VAC
22
Vcc






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.
23