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# ps2 - Massachusetts Institute of Technology Department of...

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Massachusetts Institute of Technology Department of Aeronautics and Astronautics 16.682 – Special Subjects in Aeronautics and Astronautics “Prototyping Avionics” Homework #2 Out: Mon Feb 27, 2006 Due: Wed Mar 8, 2006 Topics: Power supplies Transistors Problem 1 – Transformers and Bridges In class we saw the use of a full-wave bridge rectifier to help convert an AC signal to a DC signal (with some ripple): 1.5 1 0.5 0 -0.5 -1 -1.5 0 2 4 6 8 10 12 14 Æ 0 0.2 0.4 0.6 0.8 1 1.2 0 2 4 6 8 10 12 14 There are other types of rectifiers (half-wave, for example) and other ways to implement a full-wave rectifier. Lets take a quick look at these: 1) A half-wave rectifier looks like this: T D AC- C AC+ TRANSFORMER 1 5 4 8 RL Explain using a diagram that compares the input voltage (AC+/-) and the output voltage (V across R L ) why it is a half wave. The following steps could be useful: a) Assume the capacitor C is not there: i) how does the current flow across R L when the AC is on the positive side? ii) how does the current flow across R L when the AC is on the negative side? b) Add the capacitor and show the approximate signal 16.682 “Prototyping Avionics” 1/5 Problem Set 2 SP06

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2) Another way to implement a full wave rectifier is as follows (note it is not a bridge ): AC+ D AC- C T TRANSFORMER CT 1 5 6 4 8 RL D This is a great way to understand the concept that voltages are relative and at the same time how transformers can help us: a) Since pin 6 is defined as ground, what is the AC polarity of pins 5 & 8? How about the DC polarity (after the diodes) of pins 5 & 8? b) Ignoring the capacitor, show how the current flows when the AC voltage is positive and negative c) Draw a picture of the output voltage before the capacitor, and compare it to the input voltage d) What happens when you account for the capacitor?
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