Rob Thompson 2018-02-28 00:44:18
I DOUBT I’m alone in finding that automotive students often struggle the most when learning basic electricity. To address this issue, I created a series of articles with ideas for helping teach fundamental electrical concepts using some simple classroom lab activities. The first part of this series appeared in the March 2017 issue of techdirections (www.techdirections.com/past-issues.html), which starts with making sure students understand basic terms, such as volts, amps, and watts. We then talk about measurements, meters, and circuits. Previously, I mentioned having students use automotive bulbs to build and test basic circuits. Performing lab activities gives the students practice using a DMM, inspecting and testing bulbs, sockets, and wiring, and introduces troubleshooting procedures. And, because of my sadistic nature, I mix blown bulbs and faulty sockets into the selection of parts the students use to build and test these circuits. One of my favorite circuits uses an 1157 and a 194 bulb in series, Fig. 1. The students are told the 1157 bulb represents the brake or turn signal light and it is supposed to illuminate. Of course the problem is the 1157 won't light up (Photo 1). This usually makes the students try reversing the order of the bulbs and then assuming the 1157 bulb is "bad." I give the students spare bulbs and sockets so they can replace the "bad" bulbs with little effect. After some more testing, circuit resistance and current flow as shown in Photos 2 and 3, students can see that the circuit is complete and neither bulb is "bad." Checking the voltage drop of the 1157, shown in Photo 4, begins to show why the circuit acts the way is does. I find this circuit is particularly effective in helping to teach voltage drop and the effect of unwanted resistance on circuit operation. Another way to reinforce how voltage drops affect a circuit is to have students set up a series circuit with DMMs connected, to show each voltage drop (Photos 5 and 6). Photo 5 shows the voltage drop of each bulb, and Photo 6 shows each referenced to the battery. Again, having students set up and measure voltage drop this way can really reinforce what is going on in the circuit, and creates a visual image in their heads about what voltage drop looks like. Another benefit of using circuits and components such as those shown here is that as the connections and wires break, it automatically provides a supply of items that need repairing. This gives students practice cutting and stripping wire, installing terminals, and soldering pieces back together. Continuing the theme, here are some more examples of using a scope to teach electricity. Figure 2 shows a three-bulb series circuit with a PICO scope connected to show each voltage drop and the current flow. While not earth-shattering, this set up does provide a good visual of how current remains constant in the circuit relative to the voltage drops. Figure 3 shows a turn signal circuit using an old 552 flasher unit connected to an 1157 and a 194 bulb wired in an alternating-flash circuit found on many older GM products. In both Figs. 2 and 3, it is clearly visible that the bulbs require a small amount of time to heat up, which affects the current flow in the circuit. Figures 4 and 5 show the current flow through two different types of windings. Figure 4 shows current flow through a generator field coil that has been removed from a generator. This type of demonstration is great when discussing electromagnetism and CEMF. Also, it's fun to toss small tools at a charged field coil to impress students with the strength of the magnetic field that is created. Figure 5 shows current flow through a fuel injector. This type of image is nice because it shows how the movement of the core affects the magnetic field. Figures 6 and 7 show two different power window circuits. Figure 6 is a passenger window, and Figure 7 is the driver's side window with the auto up and down feature. Figure 8 shows battery voltage and current during cranking and starting on a known-good vehicle. With this one capture, you can show battery, cranking motor, and generator condition. Figure 9 shows the same vehicle during extended cranking to perform a relative compression test. Once students have built and tested a variety of circuits, they take a pass/fail test. They have to build and test a series-parallel circuit of their own design, diagram the circuit, test resistance, current flow, and voltage drops correctly with me watching them work. Once complete, they begin performing tests on school vehicles using lab worksheets (Figs. 10-12) to guide them. The main purpose of the circuit building is to get students used to using meters and taking measurements. A second benefit is they can see how series, parallel, and series parallel circuits operate. Transitioning to working on a vehicle is often where a stumbling block occurs. Students who have little experience with using a DMM usually do not transition easily to making measurements on vehicles and still confuse taking a voltage reading with measuring voltage drop. Additionally, students still may not have a good grasp of how voltage drops affect circuits on the car. To help visualize an actual circuit, I put together a brake light circuit that approximates how it would be on the vehicle. In Photo 7, the brake light circuit initially is wired up to work normally. As a class, we go through each part of the circuit, discuss the functions and operation of the components, and use a meter to make measurements. Once familiar with how the circuit works, I introduce several "bugs." Included in the bugs are damaged wires at the switch, two resistors wired in after the switch, and several poor connections at various points (Photo 8). With the circuit taped to the board, students can easily visualize the circuit and then perform tests at various points to try out diagnosing and fixing various concerns. This usually starts some of the students on developing diagnostic techniques and thinking about how to find and repair electrical problems. I've found that providing a circuit like this, and one that is removed from the car, allows students to get a much better idea of how a circuit is constructed, how it operates, and how to perform tests before being expected to tackle real problems on vehicles.
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