1. Diode Circuit Analysis 2
Electronics 1 
Lecture 6
Diode Circuit Analysis 2
Ahsan Khawaja
Lecturer
Room 102
Department of Electrical Engineering

3. AC Voltage
The output of an AC power source is sinusoidal and varies with time according to the following equation:
Δv = ΔVmax sin ωt
Δv is the instantaneous voltage
ΔVmax is the maximum output voltage of the source
ω is the angular frequency of the AC voltage

4. AC Voltage, cont. The angular frequency is
ƒ is the frequency of the source
T is the period of the source
The voltage is positive during one half
of the cycle and negative during the
other half.
Commercial electric power plants in the Pakistan use a frequency of 50 Hz.
This corresponds with an angular frequency of _____ rad/s ???

5. Resistors in an AC Circuit
Consider a circuit consisting of an AC source and a resistor
The AC source is symbolized by
Δv = ΔvR = Δvmaxsin wt
ΔvR is the instantaneous voltage across the resistor

6. Resistors in an AC Circuit
The instantaneous current in the resistor is
The instantaneous voltage across the resistor is also given as
ΔvR = Imax R sin ωt

7. Resistors in an AC Circuit
The graph shows the current through and the voltage across the resistor.
The current and the voltage reach their maximum values at the same time.
The current and the voltage are said to be in phase.

8. Resistors in an AC Circuit
For a sinusoidal applied voltage, the current in a resistor is always in phase with the voltage across the resistor.
The direction of the current has no effect on the behavior of the resistor. Resistors behave essentially the same way in both DC and AC circuits.

9. Getting DC back out of AC, Why?
Electronics Overview
Getting DC back out of AC, Why?
AC provides a means for us to distribute electrical power, but most devices actually want DC
bulbs, toasters, heaters, fans don’t care: plug straight in
sophisticated devices care because they have diodes and transistors that require a certain polarity rather than oscillating polarity derived from AC this is why battery orientation matters in most electronics
Use diodes to “rectify” AC signal
Simplest (half-wave) rectifier uses one diode:
input voltage
AC source
load
diode only conducts
when input voltage is positive
voltage seen by load
Lecture 8

10. Rectifier Circuit
The arrow on the diode ( ) indicates the direction of the current in the diode.
Because of the diode, the alternating current in the load resistor is reduced to the positive portion of the cycle.

11. A Half Wave Rectifier
Since the diode only allows current in one direction, only the positive half of the voltage is preserved.

12. Half-Wave Rectifier Circuit with Resistive Load
For positive half-cycle of input, source forces positive current through diode, diode is on, vo = vs.
During negative half cycle, negative current can’t exist in diode, diode is off, current in resistor is zero and vo =0 . 12

13. Half-Wave Rectifier Circuit with Resistive Load (contd.)
Using constant voltage model (second approx), during on state of diode vo =(VP sinwt)- Vd,on.
Output voltage is zero when diode is off.
Often a step-down transformer is used to convert 240 V- 50 Hz voltage available from power line to desired ac voltage level as shown.

14. Half-Wave Rectifier Circuit with Resistive Load
Peak inverse voltage (PIV)

15. DC component and PIV
PIV is the maximum (peak) voltage that appears across the diode when reverse biased.
Efficiency = 40.5 (Very low)

16. Doing Better: Full-Wave Rectification
The rectification process can be improved by using a full-wave rectifier circuit.
Full-wave rectification produces a greater DC output:
Half-wave: Vdc = 0.318Vm
Full-wave: Vdc = 0.636Vm 16

17. Full-Wave Rectifiers The average VDC or VAVG = 2Vp/.
A full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360º.
Note that the output frequency is twice the input frequency.
Most power supplies use full-wave rectifiers. Half-wave rectifiers see lesser applications like lo-cost power supplies.
The average VDC or VAVG = 2Vp/.
Fig 2-11 Block Full Wave

18. A Full Wave Rectifier
The rectifier we have just seen is called a half-wave rectifier since it only uses half of the sinusoidal voltage. A full wave rectifier uses both the negative and positive voltages.

19. Full-Wave Rectifier Center-Tapped
This method of rectification employs two diodes connected to a center-tapped transformer.
The peak output is only half of the transformer’s peak secondary voltage.
Center-tap
Fig 2-13 Full Wave Center Tapped

20. Full-Wave Center Tapped
Note the current flow direction during both alternations.
Fig 2-14 Center Tapped Current

21. The Full-Wave Bridge Rectifier
It employs four diodes arranged such that current flows in the same direction through the load during each half of the cycle.
Fig 2-20a&b

22. A Full Wave Rectifier 1.4V (2 diodes)
Note: Since a small voltage drop (around 0.7V) now occurs over two diodes in each direction, the voltage drop from a full wave rectifier is 1.4V.