Tag Archives: circuits

Playing with Transformers & Building an AC to DC Converter

My one remaining IB Physics HL student was alone in class today, so I made a quick switch on plans to do one of the required activities in the syllabus: building a full-wave rectifier circuit. This was also an opportunity to play with transformers (no, not that kind of transformer) which I presented last class as a really useful application of electromagnetic induction:
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I did almost nothing during the whole class period, aside from giving instructions and asking questions. With wires, resistors, and other circuit parts all over the place, this was not a well organized lab, but that doesn't matter. Here's what we did:

  • Measure the AC voltage and frequency coming out of the supply
  • Connect the supply to one of the transformer coils, and place the other coil inside. Measure the voltage and frequency on the secondary coil. Comment on the number of coils on the primary and secondary. Observe the effect of inserting an iron core into the transformer.
  • Predict what would happen if the two coils were switched, then measure to compare with prediction.
  • Build a half-wave rectifier circuit with a diode and resistor using the secondary coil as a supply. Measure the voltage wave form.
  • Build a full-wave rectifier circuit, and measure the voltage signal across a resistor. Check this out:
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    This yielded a nice ripply DC signal:
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  • Connect a capacitor in parallel with the resistor.
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    As we had hoped, the ripples are almost gone!:

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In the final step, I connected a 5V regulator to the output and showed that this would be a way to make a cell phone charger if we had the right USB connector.

This is quite possibly the most satisfying and successful 'let's build something' lab I've done with students. It fit neatly within a class period, including the clean up time. Granted, things would be different if there were multiple groups going through the process, but I'll just ignore that fact because it's a Friday.

On not getting in the way.

Today continued a run of some great class time working on electric circuits. Our whole class period (85 minutes) consisted of looking at the following six circuits:

Circuit 1

Describe what you would expect to happen when this circuit is connected.

A student blurted out this would be a short circuit, so things would get hot. Everyone immediately agreed (WHY DID YOU BLURT THAT OUT!). Still time to save it; I ask why it is a short circuit?

Answer: Because the path through the wire is shorter than traveling through the resistors.

I smirk.

Circuit 2

Groans from students...then a more refined answer about differences of resistance between the two branches of the wires.

Circuit 3

Tell me anything you can tell me about this circuit. If you see something to calculate, calculate it. Build the circuit on the PHET circuit constructor and show that your calculations confirm what happens in the simulator.

Insert student-centered-learning opportunities and fantastic conversations between students here. Students seeing a difference between their answers and what the simulation is telling them causes conflict that they help each other to resolve. Some students bring up the term 'parallel', which I've never said in class. Others don't understand what that is, so there is some fighting. One student describes qualitatively what should happen and then shows he is correct in the simulation, no calculations. Furthermore, this student usually is one that gets anxious when there are limited formulas to cling to, which is the norm in my class.

Circuit 4

Repeat the same procedure. Calculate what you can calculate. Build the circuit in the simulator and verify. Explain away differences, or see if there is something you are missing.

Continued progress in recognizing this is a combination of series and parallel resistors, but I don't make a big deal out of this. A couple students look up formulas and discover the idea of finding equivalent resistance (which I have never mentioned to them). This helps, but the simulator telling them what the correct answers are is key. They are compelled to get the simulator's answers through calculation - it's almost as if they feel the simulator is cheating by giving them the answers, so they must understand how to get it on their own. Eventually, they are convincing each other why they are right.

Circuit 5

Same as before, tell me anything you can tell me about this circuit. If you see something to calculate, calculate it. Build the circuit on the PHET circuit constructor as a last resort and to show that your reasoning has led to a correct analysis of the entire circuit.

This time the students hit a wall. Some continued finding equivalent resistance and the battery current, but weren't sure how to find the current through the 10 ohm and 30 ohm resistors. One reasoned it would split proportionally, and confirmed the answer using the simulator. Another measured the voltage across one of the 10 ohm resistors using the simulated voltmeter, measured the current, and then calculated the voltage difference across it. Repeating for the other resistor, they figured out the voltage difference across the parallel resistors, which then led to a current calculation. Again, I only had to tap students in the right direction - the rest was them helping each other.

Circuit 6

Try to analyze this circuit completely using what you have learned today. Once you are convinced you have accounted for all voltage differences and all current, build it in the simulator to confirm your answers. Find a way to calculate the power used by the 20 ohm and the 10 ohm resistors separately - look it up if you want.

They did a fantastic job of figuring this out - some very quickly and quantitatively. One student that oftenstruggles with concepts figured out how the current between the different branches would compare, and reasoned which ones would have the greatest voltage difference across them.

Then I started lecturing about the equivalence of electrical power and mechanical power, and the magic disappeared. They stopped talking and returned to compliance mode. I saw that happening, so I stopped. Anything I could do at this point would only ruin what was quite possibly a perfect learning experience for them.

When I taught AP Physics, we spent a day on series circuits and deriving resistance formulas, a day on parallel circuits and deriving equations for parallel resistors, and then another day on analyzing circuits that have both. Before today, I had never used the term 'parallel' with my students. This time they brought it up. They now have the ability to analyze the same level of circuits as my former AP students, but this group was able to figure much of it out on their own, with no mention of memorization of formulas and no extended periods spent listening to me blabber on about how 'going through the theory helps you understand'.

There is lots I could say about this, but I think the points made are pretty clear. Let's just say that I'm really proud of my students work today.

Electric Circuits - starting at the end.

We only have a couple weeks of class left, and there's not enough time to do the traditional Physics B sequence that I've used for electricity with my seniors that asked for a non-AP physics course at the beginning of the year. Normally I do electrostatics for a couple of weeks, talk about electric fields and potential, and then use these concepts to motivate a treatment of electric circuits. I could have stretched that out, but given my freedom in pace and curriculum, I decided to switch everything around.

This year, I started at the end of my sequence to address a pretty big issue I've always seen with my students. As much as they talk about charging (mobile devices, laptops) and basic energy conservation such as turning lights off, they have a pretty fuzzy understanding of electricity and the origins of the energy they use everyday. Some of the last topics in my traditional sequence involve real voltage sources, batteries and internal resistance - the "real" electronics that you need to know if you want to actually build a circuit. You know, the actually interesting part.

There's nothing interesting in looking at a circuit and calculating what current is going through an arbitrary resistor in a given circuit.  It took me a while to come to this realization because I still have some brain cells clinging to the "theory first, application second" philosophy, the same brain cells I've been working to silence this year. These are the sorts of things I want my students to learn to do:

  • Build a charger for an iPod using a solar panel and some circuit components. What is involved in charging a battery in a way that the battery will actually charge up without blowing Nickel and Cadmium all over the classroom?
  • Create a circuit that lights up an LED with the right current so it can outlast an incandescent bulb.
  • Look at an AC adapter that isn't made for a given device, and modify it so that it does work. The fact that it only costs $5 to buy a new one is irrelevant when you compare it to the feeling you get when you realize this is not hard to do. (Thanks Dad!)
  • Generate electricity. Figure out how hard you have to physically work to run your laptop.

This is what we did on day one:

I gave them a solar panel, some small DC motors and LEGO motors, a stripped down version of our FIRST Tech Challenge robot, some lemons, clip leads, and different kinds of wire, and said I wanted them to use these tools to generate the highest voltage they could. There was also a bag of green LEDs on the table there for them to play with. There was a flurry of activity among my five students as they remembered something vaguely from chemistry about sticking different metals into a lemon, and needing to connect one to another in a certain way. They did so and saw that there was a bit of a voltage from the lemons they had connected together, but that there wasn't much there.

I then showed them one of the LEGO motors and had them see what happened on a connected voltmeter when the axle was rotated. They were amazed that this also generated an electrical potential. This turned immediately into a contest of rotating the motor as quickly as possible and seeing the result on the voltmeter. One grabbed an LED and hooked it up and saw that it lit up.

They then turned to the robot and its big beefy motors. They found I had a set of LED lights in my parts box and asked to use it. Positive results:

The solar panel was also a big hit as it resulted in us going outside. They were impressed with how "much" electricity was generated after seeing the voltmeter display over 15 volts - they were surprised then to see that it worked to turn on the LED display, but not any of the motors they tried.

At this point it was the end of the class block, so we put everything away and went on with our day.

Some of the reasons I finished the day with a smile:

  • There was never a moment when I had to tell any of the students to pay attention and get involved in the activity.  The variety of objects on the table and the challenge were enough to get them playing and interacting with each other.
  • While I did show them how to play with one of the tools (i.e. DC motor acting as generator) , they quickly figured out how they might transfer this idea to the other items I made available.
  • They made bits of progress toward the understanding that voltage alone was not what made things work. This is a big one.

The next day's class used the PHet circuit construction kit to explore these ideas further in the context of building and exploring circuits. We had some fantastic conversations about voltage of batteries, conventional vs. electron current, and eventually connected the idea of Ohm's law (which was floating around in their heads from middle school science) to the observations they made.

I was struggling for a while about how to approach electricity because I have always followed the traditional sequence. In the end, I realized that I really didn't want to go through electrostatics - I wasn't excited to teach it this time around.  I also realized that I didn't need to do so, either in order to teach my students what I really wanted them to learn about electricity.

I think this approach will help them realize that electricity is not magic. They can learn to control it. I admit that doing so can be dangerous and expensive if one doesn't know what he or she is doing. That said, a little basic knowledge goes a long way, even in today's world of nanometer sized transistors.

Tomorrow we attempt the LED lighting assignment - feel free to share your comments or suggestions!