Circuit Simulation in Electronics

Rick Matthews

Introduction

Students are learning much more electronics since we began using circuit simulation software in 1996. We use the package CircuitMaker.

Computer simulation of electronics has been available for years, and has proven to be accurate and effective. Most of the better such packages are based on SPICE, developed at Berkeley.

However, until recently, using such packages was a lot like learning to program. Now there are packages that hide all the details of SPICE from the user. The user pastes components onto the screen from a pop-down list, connects them together, and runs the simulation. Virtual oscilloscopes and voltmeters probe any point in the circuit.

(Click on the schematic at right to see a typical CircuitMaker display.)

What is the problem?

Most homework in electronics is design of circuits to accomplish one task or another. Through circuit design, students can pull together the concepts they have studied, and they can learn to synthesize these concepts to produce new designs for novel purposes.

Unfortunately, I have accepted for years assignments that simply would not work. Students can a few of their designs in lab. They can find out whether the circuit works. If the circuit does not work, the student can use diagnostic devices to isolate the portion of the circuit that is the source of the malfunction, and they can reexamine their understanding of that part of the circuit. However, this happens for only a small portion of the circuits they design. Most, however, were simply graded and corrected.

Why not have the students test all their circuits in lab?

• Wiring a circuit in lab takes a long time.
• Spotting wiring mistakes on a typical "breadboard" is hard.
• Sometimes the problems are due to component failure. Components fail often in instructional electronics labs. This adds to lab time.
• Safety requires that students not work alone in the electronics lab.

Why not give the students feed back through grading?

• I do not always spot problems with students' designs.
• My feedback cannot fully mimic what they would receive from diagnostic instruments, as they would be forced to rely upon outside the classroom.
• Each student needs to struggle with these ideas for many hours each week. I cannot give each student hours of individual attention each week.

Because of these limitations, my students and I developed a mutually satisfying relationship. The student would design a circuit and submit the design to me. I would usually find the circuit reasonably well designed, but with one or two flaws, which I would indicate. The student received an 87, and both student and professor were satisfied.

Nowhere in this process was there a circuit that worked. Outside academe, there is almost no use for circuits that do not work.

How do we do things now?

We use CircuitMaker.

We do not use CircuitMaker to replace the lab; we use it to make homework time more like a lab experience.

I no longer accept assignments that do not work.

• Students must test every design in CircuitMaker. CircuitMaker has virtual voltmeters, oscilloscopes, function generators that behave much like the ones they use in lab.
• Building circuits in CircuitMaker is much faster than building them in lab, and students make fewer wiring mistakes.
• The software is safe to use alone, and the components never break!
• Students quickly determine whether their circuits behave as they planned. If they do not understand something, they find out immediately.
• They usually discover their own misunderstandings. Students frequently discuss the measurements reported by CircuitMaker, and work with each other to determine what is wrong with the circuit and their understanding of its underlying principles.
• Occasionally the students need help from me. We nearly always discuss the student's design in front of CircuitMaker. Rather than telling the student the answer, I can prompt the student to measure voltages at key points, so that the student can often discover the problem on his or her own. The student will have learned not only more about the design of the circuit, but will also have learned more about diagnosing circuit problems.

The students learn electronics better.

Circuit Simulation does not replace the lab.

Instead, it makes homework time more like lab time. Students are able to observe, model, predict, and observe again as part of homework. These are the essence of science, and these activities have previously been restricted to one afternoon per week.

The difference in what the students in the class are learning was brought home to me most dramatically by a conversation I had with the teaching assistant during the second electronics lab in spring of 1996, our first semester of using this software. The TA asked, "What's going on?" I asked him what he meant. He replied, "You and I are standing here having a conversation. We have never had time to do this during electronics lab before."

As he observed, the students were all hard at work building and diagnosing circuits. In the past, the TA and I could not keep up with all the questions and need for help. This day, the questions were rare.

The students had already spent close to ten hours using CircuitMaker, and an hour with CircuitMaker is worth around three hours of lab time. (You can wire with the software much faster than you can breadboard real components.) By second lab, the students had more "virtual" lab experience than most would gain in a semester of "real" electronics labs.

In my second time through the course using simulation software, I assigned more difficult work, and the students met my expectations. Clearly CircuitMaker helps students learn.

Web resources for electronics

All the major chip manufacturers have all of their data books online. Students can download spec sheets and application notes on the latest devices.

What's more, they can download SPICE models for many new devices and import them into CircuitMaker for analysis and testing.

See the Physics 230 home page for links to such pages.

Summary

• Circuit simulation software enables students to thoroughly examine their degree of understanding of each concept.
• Every student now successfully designs every assigned circuit. I no longer grade homework. I only accept homework that is correct, because students can now determine for themselves whether their designs work.
• Homework time is more like lab time. Lab skills markedly improve.
• Students learn more

Reference

• CircuitMaker is produced by Microcode Engineering, http://www.microcode.com.