12vdc stability problem

Discussion in 'The Projects Forum' started by gribbler, Jun 5, 2009.

  1. gribbler

    Thread Starter New Member

    Jun 5, 2009
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    Dear All
    I wonder if anybody can help-
    I have an appliance which runs off 240 vac. At its heart is a pcb which incorporates a transformer and associated circuitry to provide a 12vdc control voltage.
    The machine has failed completely and i am being quoted the majority of the price of a new machine for a replacement pcb. Before I take this step I am wondering if it might be worth trying to replace selected components on the existing board in an effort to fix the problem. The symptoms are as follows-
    The machine (a domestic appliance) failed mid cycle and appears to have gone completely dead- the lcd display and even the led "on" light do not operate. The 240 volt side seems ok but the 12vdc side is displaying particular symptoms which I am hoping might shed some light on which individual component on the board might have failed. When I measure across 12vdc terminals I get a continually repeatedly pulsing dc voltage which rises from 0 to about 2 volts in a (typically) 1/2 second period.
    I am hoping that somebody out there might be able to link these symptoms to a particular component or component type within the 240ac to 12vdc conversion circuitry so that I can start by replacing these individual components in an effort to fix the problem.
    I would be interested to hear any reservations which anybody might have about my proposed course of action (including advice for precautions) or of any alternative suggestons for fixing the problem e.g. fitting an ancillary 12v supply and disabling the existing on board unit.
    I thank you in anticipation.
    Gribbler
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It looks like the power supply is a switch mode one. Check the filter capacitors.

    Is there a fan for cooling the power supply?

    How many amps are needed by the appliance?
     
  3. gribbler

    Thread Starter New Member

    Jun 5, 2009
    6
    0
    Thank you Mik3 for your interest.
    The machine is rated at 240 vac 2,150 Watts (it incorporates some sizeable electric motors) and is protected at the plug with a 13 Amp fuse.- I haven`t found any local internal fusing protecting the board itself.

    There is no fan cooling the electronics.

    I suspect that I could quite easilly check the capacitors as you suggest- many thanks.

    Gribbler.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    63
    What does the broken power supply powers?
     
  5. gribbler

    Thread Starter New Member

    Jun 5, 2009
    6
    0
    Thanks again for your continued interest.
    As far as I can see, the 12vdc is provided for the control side of the machine. It is used to provide signal voltages for the machine push button controls, the machine user interface (a lcd display unit) and to provide the switching power for some on board relays. The machine is microprocessor controlled with an onboard chip and I think that the 12vdc also powers this although I am quite prepared to be overruled in this matter if you suspect otherwise.
    Regards
    gribbler.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If the 12V power supply is of low wattage it would be better to replace it because it won't be very expensive.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Didn't the OP already mentioned a quote for the the PCB costs the majority of the whole equipment? :confused:

    @gribbler,

    You'll need to post an image of the PCB.
     
  8. gribbler

    Thread Starter New Member

    Jun 5, 2009
    6
    0
    image of pcb in question
    cheers
    gribbler
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    R114 has some very obvious heat damage, its the blue resistor to the right of the big grey resistor. If it is not open circuit it's resistance value has probably increased as it looks like a fusible resistor.

    This is often a sign that something downstream has blown and is drawing too much power, so thats worth checking. That glass diode next to it looks orange, not red? Maybe it has overheated too.

    I would also lift one leg of R121 (100k 1W) and measure if it is still 100k. That is a start resistor and they tend to fail high resistance too. The shutdown "hiccuping" fault you have is more indicative of a PSU failure or downstream excessive load, bit it's still worth checking R121.
     
  10. gribbler

    Thread Starter New Member

    Jun 5, 2009
    6
    0
    Thank you `The RB` for your thoughts.
    I am staggered that forum members can derive so much information from observation alone, especially from such a grainy picture. I may try to take a better picture and upload it on that basis.
    I have looked at the board itself and it does seem that the body of R114 has glazed and become shiny compared to other similar items on the board.
    If something downstream has blown and is drawing too much power, as you suggest may be the case, I am not sure if I know enough about what to expect as "normal" to be able to isolate it.
    I will check the resistance of R121 tomorrow- thanks for the advice.
    If I can prove a PSU fault I might be willing to fit a (properly protected) ancillary dc supply-any comments?

    Again - thank you very much for your time

    gribbler.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    R114 looks to be pinkish in the middle, not the uniform blue that is has on the ends. That is a very typical sign of a metal film or fusible resistor that has seen too much current and got hot to the point where it discolours. Since your hiccuping symptom is also typical of overcurrent fault that is why I suggested looking downstream at what that power rail supplies. Check for fused (short cct) power diodes or power devices FETs etc. R114 supplies a big electro, you can measure across the caps legs to see if that psu rail has a downstream short (after removing R114).

    Probably the first thing to do though is determine the original value of R114 if you can still read the colour bands, remember they might be discoloured now too. It must have been a lowish ohms value to get enough current to cook it like that, so it's probably a fusible resistor (which acts something like a fuse) placed on the supply rail and is usually a couple of ohms or so. If you can work out the original value you should test the resistor to see if it still ok.

    As for connecting another psu? That's a high tech PCB there and you know it's got at least one level of fault detection because the psu is shutting down. I don't think it's going to be as easy as patching in another supply. Maybe if you had a schematic you could try, but then if you had a schematic then it's still easier to repair the psu that you have. ;)
     
  12. gribbler

    Thread Starter New Member

    Jun 5, 2009
    6
    0
    Thank you again "the RB"

    I`ve bought a second hand washing machine to act as a stopgap for the time being- (to placate the missus) whilst I continue to try to repair this item).
    I remain quite determined not to "reward" the oem supplier for the premature failure of this part by buying what I consider to be overpriced spare parts.

    In the meantime I would like to express my gratitude to all those who have taken the time to share their experience and help me out.

    Cheers

    gribbler
     
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