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WDH-930DAH Dehumidifier won't power on - 5v SMPS issue

Discussion in 'Troubleshooting and Repair' started by tjfs, Dec 4, 2020.

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  1. tjfs

    tjfs

    4
    0
    Dec 4, 2020
    I am repairing a dehumidifier which won't power on, and I've isolated the problem to missing 5v to the CPU. I found the diode which rectifies the 5v rail before the 7805 rectifier was short. Surprisingly, when I remove the bad diode and power up, I have 7.5Vac at the transformer output, so it would seem the shorted diode didn't damage anything else. With no power supplied to the unit and a 9v battery connected to the input of the 7805 via a 48 ohm series resistor the CPU powers up and displays information on the LCD, so that seems to prove the circuitry downstream.

    The SMPS is a simple design with a LNK625 driving a small E16 transfomer with a feedback winding and two secondaries. Both secondaries produce 7.5Vac. One output is rectified, smoothed and feeds the 7805, the other output is rectifed, smoothed and goes .. nowhere. I'm tempted to steal the diode from this output as it appears the same type. Probably best not to just bridge the output across as the second output winding might be lower current. The transformer is marked HS20411/E16 CQC however I can't find any information about it.

    The dehumidifier is Chinese manufactured and was sold in the UK by B&Q. The power board is labelled D2514-790.

    Any thoughts appreciated.

    Thanks
     
  2. 73's de Edd

    73's de Edd

    3,064
    1,299
    Aug 21, 2015
    Sir tjfs . . . . . ( tha'ts just for sure ?)


    This sounds like the need of good revealing photos of both sides of the PCB relating mainly to the the LNK
    OR ! ! ! !, instead, just a mere . . . .
    Consulting this particular manufacturs ap for your LNK power device and find your MAIN winding is generating an ~ 5V @1A+ voltage supply, while its other winding is being used for feedback sensing for the chips regulation demands and is using but a mere 100 V @ ~150 ma switching diode on that DC feedback loop.
    Whereas, the main supply winding is using a hefty axial leaded 1N5822 Schottky unit.
    Now, if you are looking for a FREEBIE ! . . . .you might consider the cannabilization of a computer power supply and using a good one of its usual hefty set of heatsinked dual diode units therein. Then you either just use one diode junction of that set or use a jumper, as to then be using both junctions and an added life extension . . Mean Time BetweenFailures . . . . of 2 millenia.

    HOW DEY' (LNK) DOES IT . . . .

    upload_2020-12-4_14-20-25.png





    73's de Edd . . . . .

    Every time I use the dirty word "exercise", I have to then wash my mouth out with chocolate, afterwards.



    .
     
  3. tjfs

    tjfs

    4
    0
    Dec 4, 2020
    I was tempted to "borrow" the diode from the other output. I have no idea why it has any parts connected to it as it's not in use.

    I have ordered new diodes and in the meantime I'm using a 9v alkaline battery feeding the 7805. The dehumidifier works and I've calculated one 9v battery should give me about a day's continuous use.
     
  4. Bluejets

    Bluejets

    4,602
    976
    Oct 5, 2014
    This sounds awfully like a linear power supply and nothing to do with smps.
    Could be mistaken ...
     
  5. tjfs

    tjfs

    4
    0
    Dec 4, 2020
    Definitely SMPS, based on LNK625. The transformer secondary produces about 8.5VAC which then goes through a Schottky rectifier to a 7805. They do this because they need a higher DC voltage to power the relays but also a regulated 5v supply for the CPU which doesn't draw much current.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,151
    2,545
    Nov 17, 2011
    Veeery unlikely. If it's a circuit along the schematic posted by @73's de Edd , both D6 and D7 serve a purpose. D7 would be the output rectifier, D6 would be the rectifier for the feedback loop.
    Sorry, you are. The chip here is a highly integrated smps controller+MOSFET.
     
  7. tjfs

    tjfs

    4
    0
    Dec 4, 2020
    There are three secondary windings on the transformer. One is for feedback and connects back to the SMPS controller chip. The second one is rectified, smoothed and feeds a 7805 regulator. The third one is rectified and smoothed but goes nowhere, there aren't any tracks or holes in the board suggesting a possible variant of the board which uses this output.
     
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