troubleshooting humidifer board. Not enough voltage.

Discussion in 'The Projects Forum' started by DustinGebhardt, Feb 21, 2011.

  1. DustinGebhardt

    Thread Starter New Member

    Feb 21, 2011
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    0
    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?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    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.
     
  3. DustinGebhardt

    Thread Starter New Member

    Feb 21, 2011
    6
    0
    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?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    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.
     
  5. DustinGebhardt

    Thread Starter New Member

    Feb 21, 2011
    6
    0
    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 . . . . :D
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    #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
     
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