1. Reading: Malvino chapter 3, 4.1-4.4 Next: 4.10, 5.1, 5.8
Lecture #13 Power supplies, dependent sources, summary of ideal components
Reading: Malvino chapter 3,
Next: 4.10, 5.1, 5.8
Then transistors (chapter 6 and 14)
9/29/2004
EE 42 fall 2004 lecture 13

2. Topics Today: Inductors and transformers Power supplies
A new type of ideal device, the dependent source.
Summary of ideal devices
9/29/2004
EE 42 fall 2004 lecture 13

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:
9/29/2004
EE 42 fall 2004 lecture 13

4. Symbol for and ideal inductor
The symbol for an inductor is similar to that of a resistor, but with loops
9/29/2004
EE 42 fall 2004 lecture 13

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.
9/29/2004
EE 42 fall 2004 lecture 13

6. Transformers
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EE 42 fall 2004 lecture 13

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.
9/29/2004
EE 42 fall 2004 lecture 13

8. Ideal transformer
The voltage across the secondary of the transformer (the output windings) is:
But this only works for changes in the voltage—and therefore for AC only
9/29/2004
EE 42 fall 2004 lecture 13

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))
9/29/2004
EE 42 fall 2004 lecture 13

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
9/29/2004
EE 42 fall 2004 lecture 13

11. An AC input is sinusoidal
9/29/2004
EE 42 fall 2004 lecture 13

12. The diode blocks the negative voltages
9/29/2004
EE 42 fall 2004 lecture 13

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
9/29/2004
EE 42 fall 2004 lecture 13

14. ~ Full-wave rectifier The result is called a full wave rectifier R
9/29/2004
EE 42 fall 2004 lecture 13

15. Full-wave rectified voltage
9/29/2004
EE 42 fall 2004 lecture 13

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.
9/29/2004
EE 42 fall 2004 lecture 13

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
9/29/2004
EE 42 fall 2004 lecture 13

18. Full wave rectified, with filtering
9/29/2004
EE 42 fall 2004 lecture 13

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.
9/29/2004
EE 42 fall 2004 lecture 13

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
9/29/2004
EE 42 fall 2004 lecture 13

21. Ideal devices Wire: Current in =current out No voltage differences
Resistor
9/29/2004
EE 42 fall 2004 lecture 13

22. Ideal devices 2 Inductor: Ideal diode:
Reversed bias  no current, open circuit
Forward bias  no voltage drop, just like a wire
9/29/2004
EE 42 fall 2004 lecture 13

23. Ideal devices 3 + + V1 V2 - - Transformer 9/29/2004
EE 42 fall 2004 lecture 13

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
9/29/2004
EE 42 fall 2004 lecture 13

25. ~ Ideal devices 5 + + V1 V2 - - + V1 - 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
9/29/2004
EE 42 fall 2004 lecture 13