Nodal%20analysis

1
Lecture 13 Power supplies, dependent sources,
summary of ideal components
Reading Malvino chapter 3, 4.1-4.4 Next 4.10,
5.1, 5.8 Then transistors (chapter 6 and 14)
2
Topics

• Today
• Inductors and transformers
• Power supplies
• A new type of ideal device, the dependent source.
• Summary of ideal devices

3
Inductors

• Any coil of wire will produce a magnetic field
  when current flows through it
• The magnetic field holds energy.
• If the current is changed, the magnetic field
  will change, and therefore the energy stored will
  change.
• Since the power must come from the circuit, this
  creates a voltage

4
Symbol for and ideal inductor

• The symbol for an inductor is similar to that of
  a resistor, but with loops

5
Transformers

• A transformer is a couple of coils of wire which
  transfer power from one to the other by a
  changing magnetic field.
• By having different numbers of windings, or turns
  of wire, a transformer can step up or step down
  an AC voltage.

6
Transformers
7
Transformer symbols

• The symbol for a transformer is a pair of the
  same loopy lines used for inductors, but close
  together. If the inductor has a core of a
  magnetic material, it is shown as a couple of
  lines between the coils.

The number of turns in the coils will Be written
nearby.
8
Ideal transformer

• The voltage across the secondary of the
  transformer (the output windings) is
• But this only works for changes in the
  voltageand therefore for AC only

9
Ideal transformer

• The transfer of power can increase or decrease
  the voltage, and the current changes as well.
• Remember
• so if it is ideal
  (lossless))

10
Half-wave rectifier

• A single diode can be used to take an alternating
  current, and allow only the positive voltage
  swing to be applied to the load

R
11
An AC input is sinusoidal
12
The diode blocks the negative voltages
13
Full-wave rectifier

• If we add an additional diode, it does not pass
  current at the same time as the first diode, but
  the load is now disconnected during the negative
  half cycle.
• What if we could flip the connection and use the
  negative half wave?

R
14
Full-wave rectifier

• The result is called a full wave rectifier

R
15
Full-wave rectified voltage
16
Filtering

• A transformer and a full wave rectifier will
  produce a voltage which is always positive, but
  varies with time
• In order to power electronic devices, we need to
  smooth out the variations with time.
• Another way to look at this is that we need to
  store energy temporarily while the input voltage
  changes sign.

17
Power supply filter capacitor

• If we add a capacitor in parallel with the load,
  it will charge up when power is available from
  the voltage source, and then it will slowly
  discharge through the load when the diodes are
  off.

R
18
Full wave rectified, with filtering
19
Ripple
The result is a DC voltage, with some residual
variations at twice the frequency of the AC
power. The variation is called ripple.
20
Regulated power supply

• In order to produce a power supply without
  ripple, we will need to use an active device
  called a regulator

Constant voltage power supply to load
R
21
Ideal devices
Wire Current in current out No voltage
differences
Resistor
22
Ideal devices 2
Inductor
Ideal diode Reversed bias ? no current, open
circuit Forward bias ? no voltage drop, just like
a wire
23
Ideal devices 3
V1 -
V2 -
Transformer
24
Ideal devices 4
Voltage source Voltage given, current can be
anything Note the voltage could be given as A
function of time

Current source Current given, voltage can be
anything Note the current could be given as A
function of time
25
Ideal devices 5
V1 -
V2 -
Dependent Voltage source Voltage given as a
multiple of another Voltage or a current,
current can be anything

V1 -
Dependent Current source Current given as a
multiple of a different current or voltage,
voltage can be anything