I was asked by a friend to look at a portable dehumidifier that had stopped working. The small roll-around unit had been working fine in the basement but had suddenly stopped running and wouldn't respond when you pushed the membrane keyboard buttons.
When I disassembled the unit on the test bench and plugged it into a power source, the microprocessor signaled a beep that it had reached the post and was running properly. The display LEDs that indicate the set point for humidity control still remained black, as well as all the indicator LEDs.
When I checked the hex buffer inverting chips, it was noted that the digital inputs to the buffer were not shifting to a good zero or one level. When I looked at the input circuit, I could see that the input buttons were not pulling the input levels to a true zero. They actually voltage divided the 5V DC through the pull up resistors, but it was not low enough through the hex inverter buffer to tell the micro to run the dehumidifier.
In reality, there was nothing wrong with the unit, it just wasn't being told to turn on. The pushbuttons were the small, silver-plated versions used in a typical computer mouse. Silver, when exposed to a damp basement, will tarnish and become a poor contact. I replaced all of the buttons with good contact types, and the dehumidifier hasn't failed since.
I wonder how many dehumidifiers have been thrown out simply because the manufacturer used the wrong kind of pushbutton for a damp environment.
This entry was submitted by Jim Ellsworth and edited by Rob Spiegel.
Tell us your experiences with Monkey-designed products. Send stories to Rob Spiegel for Made by Monkeys.
It may have just been designed cheap overseas, but it could have been designed in the desert Southwest, where humidity (and its problems) doesn't occur to people. In western Oregon, we think about humidity all the time.
It causes problems when designers/installers from dry climates work here in the summer, and don't have a clue about driving rain and 70% humidity for 6 months of the year.
On the other hand, static damage occurs to those folks often, but is not so much of a problem here.
Nadine, in most of the cases the best part is it may be only a minor or silly faults. But in a fast life mode, nobody has the time to just check atleast the basic things before calling a service person.
Anne, we had one of those non-electronic dehumidifiers and ours put in 10 years in one basement and another 20 in our Massachusetts house. It had one dial that went from OFF through LOW (humidity) to HIGH. The coils filled up with dust and required a thorough yearly cleaning with an outdoor hose, but otherwise the dehumidifier kept running and running and running. We left it for the buyer when we moved to Utah where we have no humidity problems.
Now that people note problems with small disc switches on front panels, I wonder how long our washing machine and dishwasher will last. I can't think of more humid operating environments. Perhaps the manufacturers used gold contacts.
When I went shopping for a dehumidifier several years ago, I of course looked at the cheaper ones first, which are aimed at consumers. Although they're a lot prettier than the industrial versions, they just didn't look serious: I live in a rainforest and I wanted to buy something that really works here. So I passed those by. The only electronics in my $900 portable, rollaround, commercial-grade industrial-strength, ugly gray metal Ebac dehumidifier are probably a few sensors and the LED "full" light on the front panel. More germane to this story, the only front-panel control is a mechanical rotary dial to indicate what percentage humidity I want it to keep. Although we've had to open it up and clean out the air filter every couple of years, it's been a workhorse and well worth the investment.
As an exercise in "discovery" & satisfaction for the more experienced among us, it would be interesting to find out WHO the manufacturer of this dehumidifier was, and get more info on the design team. IF the "chief engineer" of this company has an advanced degree from one of our prestigious business schools (think M.B.A. here!!), then it's no wonder that the switch design was marginal from the outset. Experienced engineers would have nixed using a pad switch w/ silver contacts, instead opting for a tad more expensive gold contact, which exhibits no oxidation properties. And, IF the engineers were recent grads, with little real world experience, the silver oxidation & migration phenomenon would have been alien to them.
Good question, Nadine. Even so, I would think manufacturers should test their products in consumer applications before sending then off into the consumer world. The last place they want to find out about problems is from their customers.
It is interesting how many companies make mistakes like this. It reminds me of the old 1960s British sports cars I had. They were really fun, but terriably unreliable. The engines were relatively heavy and overdesigned. They would last forever. It was always some switch or small part that would break. We joked that these were designed by their new engineers.
I worked for a company that made agricultural electronics. We quickly found that it's very difficult to manufacture a membrane switch that's reliable in an outdoor environment. We went through several vendors before finding one that could manufacture a robust switch. Most of the problems were from silver migration, where the silver trace in the switch panel migrates to an adjacent trace. Not surprisingly, a more robust switch also cost a little more money.
Since the problem with the humidifier was caused by a switch making marginal contact I wonder if the pull-up resistor was too stiff to work properly as the product aged?
It's scary how many manufacturers don't pay enough attention to all of the design considerations that go along with optimizing a product for its environment. This was a nice find, as noted, but a real bogus oversight on the part of the manufacturer. That kind of testing, especially given what the dehumidifier is supposed to do, isn't rocket science--just common design sense. A simple, cost-effective choice of another type of material for the push buttons would have been all that was necessary to avoid any issues.
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