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Voltage fluctuation on 12VDC/24VDC dual output P/S

Discussion in 'Power Electronics' started by partyanimallighting, Dec 7, 2019.

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  1. partyanimallighting


    Oct 22, 2012
    Hey wizards, I had a couple non-functioning LED PAR's with blown power supplies which I replaced. I checked out the faulty power supplies and I am did not see anything faulty except for a scorched resistor (23ohm??), highlighted in yellow in the image so I replaced it but the output voltages still fluctuate, the 24V side between 9V and 18V and the 12V side between 6V and 9V. The caps, coils and other components also seem to be fine so I was hoping I could get some assistance with my limited electronic knowledge. I've attached an image of the power supply, front and back (back image reversed) with the caps and some other components labelled. I hope someone out there will offer some assistance.

    Attached Files:

  2. Martaine2005


    May 12, 2015
    D3192E6F-B317-4D1C-91D8-01073AC48EB5.png Looks like you have some dry solder joints on that board.
    Also looks like some cracked solder joints bottom left of board too. Might be worth re-flowing all the joints and testing it again.

  3. partyanimallighting


    Oct 22, 2012
    Thanks Martin. I'll do a full reflow on all components in the morning, test and report back to the forum.
    Martaine2005 likes this.
  4. partyanimallighting


    Oct 22, 2012
    Tried the reflow. Same result. Fluctuating low voltages at output stage.
  5. Bluejets


    Oct 5, 2014
    Looks to me like you've made an error posting your photos.
    In the first post, the red arrow should be coming from the 4 pin device at the bottom which you have marked as YG901C2.
    This would then make the burnt resistor feeding the unmarked 4 pin device which will be the switcher element with the feedback opto.
  6. partyanimallighting


    Oct 22, 2012
    I do not think there's an error. I reversed the bottom image to match the top image.
  7. Bluejets


    Oct 5, 2014

    Fairly certain you have.
    I took the reversal into account.
    As I said, you have the red arrow coming from the device on the RHS of the board but this is a dual diode.
    The 4 pin device is on the left and like I already said, appears it is the switching element with it's burnt reference resistor.

    If you can find out what that is, you may just be well on the way to fixing it.
    Sometimes you will see numbers on a "cooked" device by shining a light on it at an angle.
  8. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir partyanimallighting

    Au Contraire . . . .

    Looks like you DEFINITELY reversed some of your parts identifications.

    Specifically . . . .

    The top YG901C2 dual diode with common anode is a 5A fast switching unit which creates your lower voltage supply with the storage of its two 25VDC max electrolytics nearby . It looks to NOT be heatsinked.

    The lower YG901C2 dual diode companion . . .which is heat sink clamped . . . creates the higher voltage supply with its the three - 35VDC max E caps.
    That supply is being referenced to send back any load variance / voltage pull down back to the 4 pin POWER " switch mode power supply in a chip"
    Where, if it detects a voltage pull down, it sends a wider pulse width square wave to the power transformer primary for corresponding compensation.

    Your mismarking is being in the ALSO assigning of the YG901C2 parts designation to the 4 pin POWER " switch mode power supply in a chip" .
    Your "wheezing" power supply condition is telling us that the basic RAW DC high voltage supply and its pigtail fuse is good and HV dc is present at the LARGE HV electrolytic at mid chassis.

    Each of the YG901C2 full wave rectifiers dump into an initial E-cap and then a series ferrite cored choke ( black heat shrink wrapped) and some more E-caps , on past, for final filtering.

    With time and use and ALL of the high frequency hammering *** that those initial E-caps receive, from the strong squarewave waveforms coming into them, will start declining in filtering effectiveness and the developing of an ever increasing ESR, overheating and a consequential electrolyte pressure seepage.With a last gasp, final drying out.
    There is being such a decline in performance, such that they can no longer keep up with the needs of the power requirements. Thus the surging, while it is TRYING to fulfill a keep up level.


    If you are having almost zero techno measurement apabilities of analytical ripple and waveform situations, minimally, you could take a full leaded length ~ 1000 ufd at 35VDC new / known good E-cap and shunt across the first cap of the lower power supply and fire up to see if the surging abates to a disappearing or slowing down a bit of the surging.
    Then pull it and move thru all of the other cluster of E-caps in that area . . . one at a time.
    PLUS there is one more E-cap being involved, and that is a value that I can not read, it is being black and physically located at the 11:00 clock position from your YELLOW marked up heated resistor .
    ( And that resistor probably will be inserted series wise, within the HOT ground return of the units 4 pin " SMPS in a chip"

    You do not need to pull existing E-caps . . . just solder tack your substitute E-cap in parallel across them. TRIPLE CHECKING POLARITY MATCH.

    ALSO . . . .if the power supply has run for 5-10 minutes . . . marginal / bad caps tend to start warming up, so compare temperature between caps with a fingertip.

    Au Fin . . .
    Sometimes you will (/ CAN) see (LASER ETCHED ) numbers on a "cooked" device by shining a (VERY BRIGHT ) light on it at an (SPECIFIC) angle.
    . . . .( with the viewing help of a magnifier)

    True . . . true . . .VERY TRUE !

    Thaaaaaaassssssit . . . . .

    73's de Edd . . . . .

    A boiled egg is . . . is . . . is . . . . hard to beat.

    Last edited: Dec 9, 2019
    partyanimallighting likes this.
  9. partyanimallighting


    Oct 22, 2012
    Thanks 73's de Edd!! You guys were so correct. I flipped once when I should have flipped twice!! :(:( I'll decipher and translate the electronic jargon of which you speak, double check to be sure I understand and follow your instructions thoroughly and report back soon. Now, I am no electronics wizard, I just do a little tinkering and I do a lot of work involving LED's and their power supplies and from what I've seen, the majority of problems are caused by failing capacitors. Get back to you soon.
  10. Bluejets


    Oct 5, 2014
    Your photos are quite ok, no problem.
    Just pointing out your interpretation of which part is which, just so you are clear which device is which.

    Again, for your attention, the red arrow you use in the photo is coming from the wrong device AND as such, your marking of the switcher as YG901C2 is incorrect.
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