Applications with diodes

Half-wave rectifier

Pulses selector

2 / 9

Full wave rectifier,

diode bridge

vI>0V vI0

D1, D3 (on) D2, D4 (off)

negative half, vI

Power-supply filtering

Half-wave rectifier with capacitive filter

If the current is (can be approximated as) constant: I

tIvC C

dttitCdv CC )()( For the capacitor:

)0()(1

)(0

C

t

CC vdttiC

tv

)0(1

)( CC vItC

tv

CiC

vC

tDuring a finite time interval ,the capacitor discharges at a constant current, giving a voltage variation Cv

Power-supply filtering Half-wave rectifier with capacitive filter cont.

Lets assume that during the capacitor discharges at a constant current (when D is off )

dt

R

VI IO

because

IVv

RC

VTI

C

TI

C

tv IOO

do

dOo tIvC

Od

o IC

tv

lets approximate Ttd

o

rippleoutput ov

RC

V

fRC

VTv IIo

1

Example

V10 IV f=50Hz, 100LR 5.1v V C=?

We chose an electrolytic capacitor C =1500F/25V

What is the value of the output ripple?

How should be a new value of C if the output ripple

have to be reduce to the half?

Solve again in the case of full-wave rectification.

V5.1

fRC

Vv I

F1333100505.1

10

5.1

VfR

VC I

F1333C

Backup Supply

Zener Diode

relative regulation factor of ZD

Z

Z

Z

Z

Z

I

i

V

v

F

Z

zZ

r

rF

Regulation region of ZD

Z

dZ

V

PI maxmax

ZZ IV @

Excerpt from datasheet

Parametric voltage regulator

The parametric regulator uses as operating principle

the nonlinear current-voltage characteristic of the ZD.

];[ maxmin III VVv

];[ maxmin OOO III

];[ maxmin ZZZ IIi

OZZOO VViIV ,,,,

ORZ Iii ZO VV

ZD

OZnom

ZI

II

VVR

Parametric voltage regulator cont.

Exercise

?

mA70,V5.7V,12

R

IVv OOI

Light-Emitting Diode: LED

Behaves similar with semiconductor diodes, but:

1.5V to 2.5V forward voltage drop

in forward bias the LED lights up: red,

yellow, green, blue, white (infrared remote

control)

typical forward current: 5mA to 20mA

Current through LED?

A

K

vD

iD

13/10

Excerpt from datasheet

Problem

To the output of a circuit the voltage can be +9V, 0V, or -9V.

How can one signalize the output voltage value using two

LEDs (green for +9V and red for -9V)?

The current through the conducting LED should be 10mA.