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November 14, 2019

Ultrasonic Humidifier Repair – The Unoscillating Nebulizer

Filed under: Uncategorized — admin @ 5:01 pm

One of the items that was brought in by a friend to a recent Repair Café event in Campbell River was a non-functioning portable humidifier, a Honeywell model HUD-200. I didn’t have time to get to it during the Café afternoon, but was able to fix it afterward. These devices are based on an ultrasonic “nebulizer”, basically a piezoelectric element in a high frequency oscillator circuit generating ultrasound waves in water, and thereby causing droplets to form and flow out in an airstream from a fan. The symptom was that water was flowing through the device, but no moisture was being created.

the nebulous humidifier

Disassembly took a bit of work: about 8 Philips screws under the unit and two on the side on a moisture level potentiometer held the exterior case together. The problematic nebulizer was a bit harder to get at; it was housed inside a snap-together metal shield underneath the ultrasonic transducer at the top and side of the unit.

humidifier internals


Removal of the shield and 3 screws holding the board plus transducer to the plastic case made the nebulizer circuit board accessible.
I drew a quick circuit diagram. The circuit uses a probe of the resistance through the supply water in the pool for the transducer to turn on the power oscillator when sufficient water is present, a form of safety interlock, so I “fooled” it for a quick dry test by simulating the water with an 18K ohm resistor across the probe. I used an oscilloscope to examine the output voltage at the transducer; the patient turned out to be flatlined; no output ultrasound. However, close inspection of the circuit board revealed a suspicious looking solder joint at capacitor C5 that is in series with the transducer. I resoldered that, tested again, and behold, a strong 1-2MHz signal at the transducer.
the Nebulizer is Alive!

Nebulize on!

Reassembly and testing with water confirmed that the device is fixed. Nebulize on!

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