A friend asked me to attempt to find a fault in a short-lived dehumidifier that functioned well for 1 season, but failed to operate after being unused until needed again.
The sensor for the relative humidity is a thin ceramic square with interlaced finger patterns deposited on the ceramic (looks like a VCR/camcorder dew sensor). This sensor is placed inside the front grille in path of the incoming air flow (but inside a small vented plastic box). As the humidity rises, the very high resistance decreases.
Searching the model number revealed that many owners have had the same experience with this model, that the unit worked great for 1 season, then no worky.
This Goldstar DH305 is a normal looking small dehumidifier with no digital readout or other controls that would give the impression that it's a complicated appliance.. only an Off/1-9/Max pot (no integral switch) and a high-low fan speed rocker switch.
BTW, the wrap-around metal cover needs to come off to access the controller board.
When turned on, the fan runs for maybe a couple minutes, then stops. The unit will not do anything else unless turned off then back on, after which the same action repeats.
The capture container float switch isn't a problem.. it works normally and turns on the red panel LED to show that the bucket needs emptied, indicating that the unit isn't going to operate until it's emptied.
As stated earlier, the Off/On is just a pot, and the varying resistance turns the controller circuit on, and the 1-to-Max setting will determine the amount of run time.
The controller circuit looks fairly simple until the solder side of the board is seen.. yep, another example of unnecessarily complicated design, IMO.
The underside of the board has about 30% of the area populated with surface mount components, including a 87C809 8-bit microcontroller.
After determining that the pot, switch and other sensors are apparently working (PTC, NTC components).. and the ciruit board soldering looks reasonable, I forced the unit into operation by huffing moist breath onto the humidity sensor, the way one does the Haaaaah on their eyeglass lenses to clean them.
So, the unit is capable of operating normally, and I checked that the controller relay, not the temperature protector, was cutting off the compressor. This isn't an expensive appliance (maybe $140 new), but since it's still functional, I'll attempt to return it to "fairly normal" operation (gold star, better than it was/is now).
The voltage across the humidity sensor is approximately 4V AC when dry (well, that's relative to the ambient humidity), and the unit goes into operation when the moisture causes the voltage to drop to approx 0.085V (85mV), which may not be accurate if there is a delay involved for some (unknown to me) reason.
I'm contemplating cleaning the sensor a couple of times (although it appears to be perfectly clean), checking the cicuit for a resistor that's drifted high in value.. or adding a parallel resistor of maybe 2-5 Megohm across the sensor (to get the voltage level down to a reasonable range.
Information I have, regarding cleaning and calibration of humidity sensors cautions against using certain chemicals such as ammonia, alcohol, formaldehyde.. and suggests only using distilled water and drying in a clean area (or perchloroethylene for contaminated sensors).
This effort could be somewhat simplified if I were using a calibrated RH meter.. I have these, but they haven't been calibrated for over 6 years, so I'm not confident of their accuracy. The Solomat manual includes a saturated salt reference method but I haven't tried it.
An alternative option could be to install a mechanical strip-type sensor with an integral microswitch.. I've seen old versions which have set-point adjustments, but I don't think I have any.
-- Cheers, WB .............