Diode Circuit Analysis 2 Electronics 1  Lecture 6 Diode Circuit Analysis 2 Ahsan Khawaja Ahsan_khawaja@comsats.edu.pk Lecturer Room 102 Department of Electrical Engineering

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

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 ???

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

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

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.

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.

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

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.

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

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

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.

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

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

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

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

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.

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

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

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

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.