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9/29/2004EE 42 fall 2004 lecture 131 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)

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9/29/2004EE 42 fall 2004 lecture 132 Topics Today: Inductors and transformers Power supplies A new type of ideal device, the dependent source. Summary of ideal devices

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9/29/2004EE 42 fall 2004 lecture 133 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:

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9/29/2004EE 42 fall 2004 lecture 134 Symbol for and ideal inductor The symbol for an inductor is similar to that of a resistor, but with loops

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9/29/2004EE 42 fall 2004 lecture 135 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.

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9/29/2004EE 42 fall 2004 lecture 136 Transformers

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9/29/2004EE 42 fall 2004 lecture 137 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.

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9/29/2004EE 42 fall 2004 lecture 138 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

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9/29/2004EE 42 fall 2004 lecture 139 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))

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9/29/2004EE 42 fall 2004 lecture 1310 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

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9/29/2004EE 42 fall 2004 lecture 1311 An AC input is sinusoidal

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9/29/2004EE 42 fall 2004 lecture 1312 The diode blocks the negative voltages

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9/29/2004EE 42 fall 2004 lecture 1313 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

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9/29/2004EE 42 fall 2004 lecture 1314 Full-wave rectifier The result is called a full wave rectifier ~ R

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9/29/2004EE 42 fall 2004 lecture 1315 Full-wave rectified voltage

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9/29/2004EE 42 fall 2004 lecture 1316 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.

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9/29/2004EE 42 fall 2004 lecture 1317 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

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9/29/2004EE 42 fall 2004 lecture 1318 Full wave rectified, with filtering

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9/29/2004EE 42 fall 2004 lecture 1319 Ripple The result is a DC voltage, with some residual variations at twice the frequency of the AC power. The variation is called ripple.

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9/29/2004EE 42 fall 2004 lecture 1320 Regulated power supply In order to produce a power supply without ripple, we will need to use an active device called a regulator ~ R Constant voltage power supply to load

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9/29/2004EE 42 fall 2004 lecture 1321 Ideal devices Wire: Current in =current out No voltage differences Resistor

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9/29/2004EE 42 fall 2004 lecture 1322 Ideal devices 2 Inductor: Ideal diode: Reversed bias no current, open circuit Forward bias no voltage drop, just like a wire

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9/29/2004EE 42 fall 2004 lecture 1323 Ideal devices 3 Transformer +V1-+V1- +V2-+V2-

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9/29/2004EE 42 fall 2004 lecture 1324 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

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9/29/2004EE 42 fall 2004 lecture 1325 Ideal devices 5 ~ Dependent Voltage source: Voltage given as a multiple of another Voltage or a current, current can be anything Dependent Current source Current given as a multiple of a different current or voltage, voltage can be anything +V1-+V1- +V2-+V2- +V1-+V1-

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