. Circuit to study voltage regulation by Zener diode If the input DC voltage is increased, the Zener current increases thereby increasing current through R s and the voltage drop across R s is also increased. The increased current flows through the diode without affecting the I L . Since Zener diode is operated in the breakdown region, the Zener breakdown voltage across the diode is nearly constant even though the reverse bias current through the diode increases considerably.
The increase in input voltage is dropped across R s and hence it is also called dropping resistance. Because of the parallel connection, the voltage across R L is also equal to Zener breakdown voltage which is taken as constant output voltage V . If the input DC voltage is decreased, the diode takes a smaller current and the voltage drop across R s is reduced. Thus, the output voltage V remains constant.
To sum up, if there is any change in input voltage, the voltage drop across R s changes accordingly. But the voltage across Zener diode or voltage across R L remains constant. Thus the Zener diode acts as a voltage regulator. - - - - - - - - Unit electronics and Communication The colour of the light is determined by the energy band gap of the material.
Therefore, LEDs are available in a wide range of colours such as blue (SiC), green (AlGaP) and red (GaAsP). Now a days, LED which emits white light (GaInN) is also available. Applications The light emitting diodes are used in · indicator lamps on the front panel of the scientific and laboratory equipments. · seven-segment displays.
· traffic signals, emergency vehicle lighting etc. · remote control of television, air conditioner etc. EXAMPLE . Determine the wavelength of light emitted from LED which is made up of GaAsP semiconductor whose forbidden energy gap is .
eV. Mention the colour of the light emitted (Take h = . × – Js). Solution E hc hc E g g × × λ λ Therefore,