Ok, so what exactly is a "Destruct-o-mizerTM"?
It's a device that I invented for my students to use in experiments with 1/4 watt resistors (and sometimes other items). It will safely 'test' resistors by applying about 120 VAC to the resistor while it is in series with a current limiter.
To limit current, I use a 500 watt halogen lamp, which when connected directly to 120 VAC has an operating resistance of about 30 ohms, and a current of about 4 amps. If a 3000 ohm resistor is placed in series with the halogen lamp, the resistor will have almost the full 120 VAC across it, but the current will not exceed about 4 amps, even when the resistor starts to smolder, ionize, and flare up. A 3000 ohm 1/4 watt resistor will experience almost 5 watts, which means that it won't last long. By contrast, a 10K ohm resistor will experience about 1.4 watts in open air, which it will be able to withstand for quite a while.
| For Destruct-o-mizer housings, I use
surplus dot-matrix printer boxes. Follow this link for some
construction photos and tips.
I printed these labels on bumper sticker stock. |
 Inside the case shown above: lower left - current limiter, middle - test clips, near right - red test light, right middle - lid-down switch and rod, back right - fan & main switch. Underside at right - recessed momentary test switch. |
Close-up Photos
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| Testing Resistors
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Middle values smoke first. Higher values only get warm. Even higher values have no apparent effect. Very low values may not flash, but instead pop like a fuse. |
A flashing resistor. |
EXPERIMENT POSSIBILITIES
One experiment that I did in an 8th grade science class involved scientific
inquiry. They did not know what a resistor did, but they could see that
they were all the same size, and had the same appearance except for the color
stripes. I had random value resistors in a large bowl. Colors were
recorded on the board while students took turns testing. After a while (a
long while) they were able to conclude that only resistors with the third stripe
being black or brown would 'blow up' in the Destruct-o-mizer. Other third
stripe colors did nothing visual. Some said that the different colors made
the difference. The more astute decided that the colors meant that
something inside was different.
In my high school Electrical classes, our experiments have a more directed meaning. I've done two versions of power test experiments. In one, students learn to read the meaning of the stripes, calculate the power on the resistor, and predict the outcome before testing. As they select resistors to test, It doesn't take them long to know which resistors will quickly burn up! The other version of the experiment is similar, but has the extra step of finding how many times its rating a resistor will experience, then predict the outcome.
| Testing Other | (photos to come) |
In addition to students learning about resistors, they need to know that resistance is a property of matter, and all objects have some degree of resistance, even if it is zero in the special case of a superconductor. We measured with an ohmmeter a banana, a potato, a gummy worm, a lime, a wiener, and a pencil lead. The gummy worm's resistance was about 10M ohms. The wiener was about 2K ohms. All else was in between. Don't test a pencil lead while it's still encased in wood - it burns and makes lots of smoke.
Based on these measurements, students predicted the outcome prior to testing. I also discussed ionization and how it changes the resistance of an object. For example, testing a neon light with an ohmmeter shows a resistance too high to measure, but put enough voltage on the neon, and it becomes a conductor.
