The BuzzBox

2022-06-14 12:13 by Ian

As an object lesson in electricity, my youngest boy and I built a high Wattage transformer box with a trigger. It is named BuzzBox because while operating under full load, it emits that 60Hz buzz that usually means a fire is about to start.

Transformers

I don't ever throw away microwave ovens. Their guts are just too much fun to make trouble with. The single most useful part in a microwave is the high Wattage "microwave oven transformer" (MOT, for short). The symbol for this transformer is shown below. Some MOTs have secondary taps for low-voltages to run the control electronics, and a grounding lug on the core. But those are not depicted in this symbol, nor will they be germane to our purposes.

Transformers are analogous to other physical systems that trade-off between (force and distance) or (speed and torque), but keep power constant (minus losses). In this case, the quantities being traded are Voltage (pressure) and Amperage (flow rate). The ratio of the conversion is equal to the ratio of the windings around the primary to that of the secondary. PRI/SEC ratios larger than 1 will reduce voltage and increase current, and ratios smaller than 1 will increase the voltage while reducing the current.

This is a 120V primary, and so the winding ratio is something near 1/17 to generate the >2kV needed to drive the magnetron. But since this machine will be used by children, I'd prefer the risk of simple heat and fire, versus instant death from high-voltage arcs at half an Amp. So in this case, we want high amperage, rather than high voltage.

I sawed out the secondary coil and replaced it with 2 turns of #1/0 battery cable. Winding that thick cable was quite difficult, even with the finely-stranded cable that I knew I'd want to use. After winding the new secondary coil, I plugged it into the wall and measured a bit over 2 measly volts. I didn't care to measure the current at the secondary (yet), but from the 10A it is pulling from the wall, the math tells me it should be well more than 500 Amps (unless I'm suffering from inordinate losses with the new secondary coil). 20cm of 14ga solid copper wire goes from room temperature to incandescent nearly instantly, and melts in about a third of a second. 12ga wire lasts a few seconds.

This picture of the inside of the open box shows the re-wound transformer, and the 12V parts that I added to support the case fan (right) and trigger relay (screwed to the inside of the top plate).

Construction notes

Compared to the numerous other boxes I've built with 0.25" T6061 plate, I tried something new with this box (which is why the post is tagged under "Metalcraft"). I wanted free-flow of air, but not fingers, and wanted it to be minimal effort. So I replaced most of the vertical walls with expanded aluminum fan grating. To compensate for the diminished shear strength, I used some aluminum tubing with counter-sink cuts into both plates. Four long bolts run co-axially through the tubes, and thread into the bottom plate.

I left enough length for the bottom-ends of the bolts to serve as the posts onto which the nylon feet are screwed.



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