My home humidifier has been acting strangely, giving me false high readings. I did an initial check and the humidistat mini-board was not getting the required 5VDC and the output frequency was therefore too low. So I looked upstream and tried to figure out where my voltage drop was. I was only getting 3.3VDC going into the 5VDC voltage regulator. I pulled off each of the non-SMD components and checked them. C1 checked in a 2.2uF (this is a 250VDC cap). R7 was 47ohm. R10 was 10ohm. C2 was 1000uF. While I had C1 off, I check R9 and it was 330kohm.

Also note that I'm not sure about the polarity and value of C4. There are 2 polarized caps on this board, C2 and either C4 or C5 (not shown).

When I plug this into my home wall power, I get 128VAC across ACL and ACN. I get a 120V drop across R9. I get a 6V drop across R10.

What could be wrong with this system?

 

Those kinds of transformer-less supplies are inherently unsafe, as there is no isolation from mains power.

I'd remove R9, C1, R10, D1, D2, C3, C2, C4, R11, and replace them with a transformer supply with an output of around 8v to feed the regulator.

 

Thanks Sgt. Do you have a link to an appropriate transformer? I will look into this.

But what could be causing the issues with my current setup?

 

You might consider something like this "wall wart" supply; they're quite inexpensive:
http://www.mpja.com/prodinfo.asp?number=17558+PD

With your existing supply, you could have a blown diode or a bad capacitor. I really don't like to offer advice on such circuits, as they are unsafe.

 

My only issue with the wall wart is that I also need to provide 120VAC to the fans via 3 x triacs. I would have 2 wall plugs. Is there an equivalent to the wall wart that I could mount internally?

Or maybe I could hide the wall wart inside the humidifier and wire it to the incoming AC supply? I just might be able to do that . . . .

 

....Is there an equivalent to the wall wart that I could mount internally?

Or maybe I could hide the wall wart inside the humidifier and wire it to the incoming AC supply? I just might be able to do that . . . .

#2 is the easiest solution. Be sure to insulate the wires you connect to the plug blades.

#1 I do this frequently. I carefully gut old 5VDC/700mA to 2A cellphone chargers. I get them for $.39 to $0.99 at thrift stores. Your friends may even have old ones laying around. This gives you a nice, small, isolated switch mode power supply: http://www.electro-tech-online.com/general-electronics-chat/93884-not-transformerless-power-supply.html#post749571 Remember that the input side is still 120VAC.

Ken