PSU emitting RFI (Pics inside). Ideas why?

Discussion in 'General Electronics Chat' started by fourier, Jan 28, 2013.

  1. fourier

    Thread Starter New Member

    Jul 11, 2011
    6
    0
    OK, came across this power supply which was emitting RFI right across the lower spectrum (0.3 MHz past 2 MHz)...

    CE labelled PSU, but obviously of poor quality.

    took it apart and these are the pics

    [​IMG]


    [​IMG]


    now I'm not an electronics expert, but from the heat marks on the PCB, I think it maybe caused by a faulty Thyristor.... but as I said I'm not an expert, so looking for people ideas/reasons why this was happening. :)
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I am pretty sure that there are no thyristors in there. Other than that, it seems to have some filtering on the mains side, not so much on the secondary side, anyway I´d say that the noise it emits is a feature, not a bug. Also, how exactly do you measure that noise?
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    Does it still work?

    This looks like a SMPS so it it is not likely that the main switching element is a thyristor, but a transistor. They all emit RF more or less.
     
  4. fourier

    Thread Starter New Member

    Jul 11, 2011
    6
    0
    it was a power supply for a piece of electrical equipment that when replaced with a different PSU, cleared the noise altogether.

    sorry, presumed it was a Thyristor when I saw the three pins (Black component with the heat sink round it, to the left of the L3368... sticker )

    Telecoms tech here, so was measuring it with a Spectrum Analyser attached to the copper pair entering the premises to which the noise was being induced onto.
     
  5. fourier

    Thread Starter New Member

    Jul 11, 2011
    6
    0
    also, was running really hot, and failed when it was unplugged and re-plugged back in
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The coloring on the bottom of PCB means it wasn't working as designed, overloaded, or had inadequate circulation.
     
  7. fourier

    Thread Starter New Member

    Jul 11, 2011
    6
    0
    ok, what seems to have happened is the electrolytic capacitor has failed (not overly obvious in the photo but you can see the bulge at the top of it, the black one on it's side)

    the PSU then run hot, and emits RF,

    would this cap be part of the filtering/smoothing circuit ?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    well, what else would it be there for?
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Alternatively, perhaps it wasn't designed to be working.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Good call, that slightly bulged cap is the HV DC filter cap, right after the bridge rectifier.

    Considering the amount of heat markings shown on the PCB it has had a lot of use and all 4 electro caps should be replaced. The pair of caps on the DC output should be "low ESR" rated caps.
     
Loading...