Design Con 2015

Everyday household scope uses

-May 23, 2014

Whether you have a home lab or not, a scope has some everyday household uses.

Recently, the dishwasher that came with my 1928 house (no, I don't think the machine was quite that old though) appeared to be on its last legs. Even judicious jiggling of the timer-selector was failing to give the desired wash and rinse. The racks and innards were not looking so pretty either. It was time.

The replacement unit was to be quiet, and of decent pedigree. Not top-of-the-line or anything – just something reliable and functional. The low-end range of a certain well-regarded German manufacturer was on sale. I went for it.

It arrived one chilly January day, and I was eager to get it up and running, though, bucking the popular trend, I did not video the unboxing. I extracted the machine from its cardboard cocoon, gathered up my tools, and went to work.

After the expected few hours of struggle with plumbing, electrical, and wood, I was relatively uninjured, and the dishwasher was installed. I pressed the start button. It made noises. Water flowed. Valves clicked. The motor drive flexed its muscles. That's when my hands flew to my ears.

Emanating from the bowels of the machine was a piercing, high-pitched whine. I immediately conjectured, "Defective motor PWM drive!" I continued playing with the machine, starting and stopping it, letting it run a while. Washing dishes. Nothing changed.

(Full disclosure: I confess to being a tad more sensitive to noises of all sorts than the average person. Maybe it comes from being a musician. But I wasn't making this up. It was literally painful. And though I'm not a doctor, nor do I play one on TV, it wouldn't surprise me if this level of sound is actually harmful.)

As I was waiting for a service call, my musician side decided to have another quick listen. "Sounds like about 10kHz," I thought. How could I check? Aha! I'm no motor expert, but I do know they use magnetism.

I hauled my lightest scope up from the basement lab (yes, even the lightest still needs hauling), set it up beside the dishwasher, and simply placed a probe on the floor near the motor. Sure enough, it picked up more than enough EMF to register a good (though not exactly clean) scope display. Even the varying PWM was easily discernible. And guess what. As closely as I could measure by counting graticule divisions, it was exactly 10kHz.

The service person arrived. He wasn't sure if he heard it at first. We let it run. I covered my ears. He walked around (standing waves made the sound louder or quieter in different areas, just like RF). Finally, yes, he could clearly hear it. As there wasn't much he could do at the time, I asked him if he could check other machines. He could, and did, and let me know that they made the same terrible noise.

I sure hope your company doesn't engineer things this negligently. What would you do if it did?

Needless to say, that machine was returned. The new one has a PWM drive, too. I've heard it – once or twice. It's probably 15 or 20dB lower than the German-engineered one! Even at that low level, I find such engineering negligent.

What do you think is going on here? Are manufacturers too cheap to splurge on drive transistors that can switch at 20kHz or 30kHz? Would a dog be bothered at that frequency? Could the motor coils be potted or impregnated to suppress vibration? (I'm reminded of some power toroids I once ordered. The first batch hummed at 60Hz. Not too loudly, but noticeably. I changed the spec to add epoxy potting to the donut-hole for the next batch. Dead-quiet.)

What household uses do you have for your scope? And what badly engineered appliances have you had to deal with?

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