Gas station without pumps

2014 June 4

Random topics in class today

Filed under: Circuits course — gasstationwithoutpumps @ 19:20
Tags: , , , , ,

Since students have started on their last lab, there is no more material that I have to cover, so I threw today’s lecture open for questions.  I had prepared some material on Wien-bridge oscillators, in case no one had any questions, but we filled the time with stuff they were confused about from earlier in the quarter.  In roughly the order I covered them, we talked about

  • FETs. I showed the cross-sections of nFETs and pFETs, explained the “back gate” or substrate connection and why it was tied to the source on the power FETs. I also talked about the flyback diodes and why they are needed when driving inductive loads.  This also gave me an opportunity to talk about how ignition coils on cars work.
  • PWM. I redid a lecture that had not gone over well the first time, talking about how the rectangular voltage pulses turn into up and down ramps for current in an inductive load, and how duty cycle gets converted to current level.  I still think I could do a better job of the PWM talk, but the students were feeling better about understanding how their class-D amplifiers worked.
  • I also introduced H-bridges for DC motor speed control, and showed how PWM could control the motor to turn forward or backward at different average current levels.
  • A student asked about how the gain in theirpreamp affected thefinal output loudness, so I redrew a part of the comparator function from their lab handout:
    Example of comparator output comparing a slow signal from a preamp and a fast triangle wave to get a pulse-width modulated wave.

    Example of comparator output comparing a slow signal from a preamp and a fast triangle wave to get a pulse-width modulated wave.

    I then showed how a small signal centered at the same voltage as the triangle wave would produce a 50% duty cycle, with only small fluctuations from 50% as the signal went up or down.

  • Finally, I reviewed sampling and aliasing, explaining where the beat patterns they saw in their lab came from.  I think I need to provide more on that earlier in the quarter, as they did not seem to get as much from the sampling and aliasing lab as I had hoped.

Tomorrow is the last lab (unless students request extra time in the lab to redo something next week), and I expect all the students to finish their EKG soldering.  I did remember to suggest that everyone solder a board, so that they could have one to demo to people, but we’ll see how many takers there are tomorrow.

On Friday, I’ll once again take questions, but I’ll still have the Wien-bridge oscillator to present if they don’t have anything to ask.


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