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Question about SMPS repair

Discussion in 'Power Electronics' started by Journey11, Sep 3, 2020.

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  1. Journey11

    Journey11

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    May 23, 2018
    Hello All,
    I have a repair job here for a smps power supply that should be pretty straight forward. The SMPS comes out of a Canon CS-1 Digital Retinal Camera ... Canon no longer has replacements available and suggested to the owner to seek out a guy like myself to troubleshoot.

    After 3 hrs of inspecting and checking for faults with DMM, I found nothing obvious but did find a power resistor that was suspect. I removed since the labeling was hidden to see just exactly what it was and sure enough (out of circuit and labeled 100 Ohms) it was open.
    But here's the kicker ... this is no ordinary power resistor. After a quick search on Part# ... It comes from KOA Speer Electronics and is a 100 ohm fusable resistor ... it has a thermal fuse in it and it has gone open.
    The ratings for it are obviously the resistance at 100 ohm ... current rating @ 10A ... power rating as far as I can tell from datasheet is 1.6 watt, which is very odd due to the physical size of the part. From experience, judging by the size it looks more like a 10W or maybe even 20W. (photo included)

    Now another odd thing is that where it lives in circuit ... there is another identical part and they are IN SERIES on a power rail fed by a transistor into a small transformer ( the smallest of 3 primary side). Not sure at this point what this one is for but the two larger ones I'm pretty sure are the main switching transformers. This supply of course has one primary but two separate secondaries.

    Now my first response was to replace that resistor with a 10W 100 ohmer but that was before I found out it was fusable and only rated for 1.6W.
    So here's my question ... can I replace it with a standard power resistor say rated at maybe 2 or 5 or even 10 watts ? Since there is a second fusable in series ... I'm thinking it should work and still have that remaining fusable as a fail safe.
    (EDIT) If it's ok to just mount a standard power RES, my biggest question is what power rating ? As I said at first impressions, it looks like at least a 10 watter BUT datasheet says 1.6W ...

    Here's the caveat ... if there is another issue either on the primary or secondary that caused the 1st fusable to go open then obviously the second back up fusable will go as well BUT ...
    VIsual inspection reveals NO signs of over heating ... clean as a whistle.
    So my hunch as to why it failed, is it was a bad part right from the production line ... not quite up to operating specs and over time just failed ... I'm HOPING anyway.
    (EDIT) One other thing that MAY have caused the fuse to go could be something that was plugged INTO this supply ... ie. the camera ( I doubt it ) the lights ? (maybe)
    As far as I can tell/guess, there isn't much on this thing that would require lots of current, but the supply looks to be rather robust.
    I worked for almost 6 years as troubleshooter for a medium sized electronics manufacturer so my job was to fix production line failures after running through the test jig. I came across many many times that brand new parts were faulty right from the supplier. Our QC and QA protocols were that we tested EVER pcb that we sold. But I found out that component mfger's don't always test EVERY component ... sometimes it one every 5 or 10 or some such number so they don't test them ALL.
    Hence, some bad components get through and bla bla bla ...

    (EDIT) Almost forgot, the original part is End of Life and so far have not been able to find another mfger with a fusable failsafe in it that matches ratings.

    SO will very much appreciate some advice and/or comments on this.
    For size comparison ... a canadian/american quarter is beside the component.

    100 Ohm Fusable sm.jpg

    (EDIT) I decided to include a pic of the SMPS unit itself here.
    Quick description ... I photo shopped in some colored dots on components in question.
    Power resistor with red dot is remaining good fusable 100 ohm that is in series with the one where you can see the open footprint also marked red dot. Transformer with red dot is connected to the 2 resistors ... transistor (hard to see) colored yellow dot is the switch that more than likely turns power on to that smaller transformer. You can also see how this unit is set up. Top section is primary, two bottom sections are the 2 secondaries. 4 connectors at very bottom run to whatever needs power and only 3 of the 4 are used. The 6 IC's near the bottom bridging the two secondary sections are Opto-Isolators ... not sure why they would do this but ...

    SMPS Canon Retinal Cam sm.jpg


    Moderators note : cropped images and placed them inline
     
    Last edited by a moderator: Sep 4, 2020
  2. Bluejets

    Bluejets

    4,275
    908
    Oct 5, 2014
    SMPS units are anything but straightforward when you know next to nothing about them.
    Power resistor looks like 5w to me.
    Difficult to say whether it should be fusible or not given the photo is taken a long way out to be able to make out anything.
    Also a shot of the other side of the board would be helpful.
    If you have a link to the part, why not provide it here..??

    Common failures in these units are usually associated with bad caps, shorted out switcher mosfet or switcher ic or dodgy close by components.
     
  3. Journey11

    Journey11

    62
    0
    May 23, 2018
    Hi Bluejets,
    Thanks for popping in to chime in ...
    I have had some experience with SMPS and agree with you they are not the easiest to repair or even understand operation. Luckily I learn fast and have a fair understanding as I have repaired a few in the past and did some schooling to get a grip on their operation.
    Yes my first impression of the power resistor was 5 watt too however, I did explain that the datasheet for this part indicated it as 1.6 watt but this doesn't make sense to me and it also (in no uncertain terms) indicated it IS a FUSABLE device.
    I'll post some more photos top and bottom ... and I'll post a link for the datasheet as well
    As I mentioned above, I spent 3 hrs with a DMM hunting for shorts and bad components ... all the transistors seem fine ... there may have been one that had a wonky reading so I still need to revisit that after I find it again.
    So as far as this resistor goes ... it is OPEN and the other resistor sitting right beside the blown one is IDENTICAL part and is in series in this section of circuit ... so I'm thinking the reason for two in series is so there is a back up fuse.
    Again my 1st inclination is to just replace the blown unit with a standard 100 ohm power resistor and I'm leaning towards using at least 5 watt, maybe even over rate it and put 10 watt. In fact the only part I have been able to source (so far) that will fit the footprint is a 10 watt and it's still going to be a funky fit.
     
  4. Journey11

    Journey11

    62
    0
    May 23, 2018
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    10,588
    2,360
    Nov 17, 2011
    Not the brightest idea to put "fuses" of the same rating in series - usually one uses fuse ratings space by a factor of 1.5 to 2 to achieve selectivity.
    In this case there is no selectivity. Possibly the same resistor was chosen simply to minimize the number of different components.
    I think you may tray to get the SMPS running by replacing the defect resistor. Use the series test lamp method to limiot power dissipation in case the SMPS has another defect.

    However, the resistor/fuse burnt probably for a reason. There may be another yet undetected defect within the smps. Since you say there are no signs of heating, one can at least exclude a permanent overload. That would have left signs on th ePCB. U suggest you check all the electrolytic capacitors for capacity and possible short circuits.
     
  6. Bluejets

    Bluejets

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    908
    Oct 5, 2014
    Tried to take another look at your images you posted originally but there appears to be some problem as it comes up as an error.

    Electronics Forums - Error
    You do not have permission to view this page or perform this action.

     
    Last edited: Sep 4, 2020
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    10,588
    2,360
    Nov 17, 2011
    I tried to put the images inline - for some reason it doesn't work :(
    Not even in this new post.

    @eerob , @eeadam , @Ian : could one of you have a look into this issue, please?
     
  8. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    @Harald Kapp , I have downloaded the images and cropped them.
    Uploaded again and placed them inline.

    Bertus
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    10,588
    2,360
    Nov 17, 2011
    Funny, the same thing I tried, also with cropping, but I failed. A browser issue? I use chrome, which one do you use?
     
  10. bertus

    bertus Moderator

    1,158
    439
    Nov 8, 2019
    Hello,

    I am using Firefox on my linux system (opensuse tumbleweed).

    Bertus
     
  11. Journey11

    Journey11

    62
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    May 23, 2018
    Hi Moderators,
    Thanks for pointing out this issue with my posted pics. Not sure if it was something I did wrong.
    I just resized for file size limit and uploaded. Maybe is it best to just leave them as attachments ?
     
  12. Journey11

    Journey11

    62
    0
    May 23, 2018
    HI Harold,
    Thanks for jumping in on this one ...
    Ya about the fuses having same rating ... doesn't make much sense to me either.
    If the 2 resistors were to minimize number of parts .... then why not just put one 200 ohm ?

    I was already intending to use the light bulb method as I mentioned somewhere above about the cause that blew
    this fuse. Yes, could be another failed component downstream. I also mentioned in the 1st post, I already have spent 3 hrs looking for/testing with DMM for shorts and other wonky readings on all the transistors and all the main larger diodes. All caps tested good with ESR meter, no shorts either. (Note: they were still in circuit, so maybe for ESR test I should pull them out ? I have read that caps can be ESR tested in circuit but maybe not ? )

    Replacing the bad resistor with a standard non fusable was my 1st inclination as the 1st step. The only thing that me up was the power rating because the original datasheet rating for original part was 1.6 watts ... It sure doesn't look like it and the given current rating at 10 amps would support the notion that it's probably more like 5 or even 10 watts. I'm just going to go overkill and put a 10 watt to start with ... I think that's a safe bet..

    Thanks again for your input mate !
     
  13. Bluejets

    Bluejets

    4,275
    908
    Oct 5, 2014
    You cannot test the caps in place on the board.

    Can you show the other side of the board..?
    Preferably the top 1/3 section....
     
  14. Journey11

    Journey11

    62
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    May 23, 2018
    Hey there Blue ... yep can do/did.

    SMPS backside sm.jpg
     
  15. Journey11

    Journey11

    62
    0
    May 23, 2018
    So I replaced 100 ohm fusable with standard 100 ohm 5 watt and it worked .... for a while. Apparently after I powered it up at least 3 times and had the unit up and working ... I called the client to pick up and it DIED as soon as it went out the door. I suspected that the OTHER fusable would blow and it did. I used the light bulb trick to initially power the unit up and there wasn't even a hint of a glow from the bulb ... so what does that mean ? No huge current draw ?
    So my 1st thought is that the light bulb trick is NOT a reliable test. I powered up the WHOLE unit with the bulb ... so even if something attached to the supply was faulty it SHOULD have lit the dang bulb up and it didn't. But the supply blew the second fusable resistor and still no sign of any other faulty component(s).
    Already PULLED 3 caps out and one MOSFET and still no indication of a fault.
     
  16. Martaine2005

    Martaine2005

    2,967
    817
    May 12, 2015
    The light bulb trick is actually a current limiter.
    It’s an excellent method for powering a unit with a short. No light, no short. Depending on the units current draw, the bulb might light dim. But a short is full on like turning your light switch on in the house.
    Incidently, what bulb did you use?.

    Martin
     
  17. Journey11

    Journey11

    62
    0
    May 23, 2018
    Understood ... Using 100 watt incandesent ... so in my case here ... no shorts because light didn't even glow a bit. Rather curious that with no large current draw ... these fusible resistors popped, especially the second in the series connection after I replaced the 1st with a standard resistor.
    The fuse is thermal so I would have to assume heat from somewhere that caused the fuse to blow. These two components were right up against the large heatsink.
    Either that or they were engineered to have a very specific lifetime so the whole unit would have to be replaced as the power supply is end of life and the fusible resistors are end of life and a new machine costs $16,000 .... Canon et al are very cunning for doing crap like this.

    I think I'm going to draw out a schematic .... minus all the complicated control crap ... it would be a really big job to suss all that crap out and hope to be 100% accurate.
     
    Last edited: Sep 12, 2020
  18. Bluejets

    Bluejets

    4,275
    908
    Oct 5, 2014
    Your end of life suspicions are just a foofee.

    Caps etc will age but the rest is basically rubbish.

    As Martaine2005 says, the series test bulb is an excellent test tool for finding shorts without blowing fuses.

    You have some other component faulty to cause the unit to drop out.

    Digging around in the area of the switcher as I previously said will more than likely cure the problem.

    These sections work their ring off so expect problems there.

    BTW the fusable resistor blows when current is excessive.
     
    Last edited: Sep 12, 2020
  19. Journey11

    Journey11

    62
    0
    May 23, 2018
    Well ain't this just the kick in the ass I needed to deal with.
    I looked again at the main filter caps out of circuit and a few MOSFETS ... zero faults.
    So put it all back together with ANOTHER standard 5 watt 100 ohmer to replace 2nd blown fusible resistor and
    bench tested with NO loads BUT made it so that I could INSERT an ammeter to see where current is sitting at.
    Ammeter was inserted at the end of the two original fusibles and got a current reading of about 120mA but on the 10A setting.
    Switched to lower range and ZIP ! but output voltages seemed to be there ... 6.5, 5, 12, 1.3 etc Good DC outputs under ZERO load.
    Next step ... connected loads with ammeter insert wires still intact ... fired it up unit and it runs BUT GET THIS ... zero current flow on ammeter even WITHOUT light bulb limiter ... WTH ??? The ammeter was on the PRIMARY just before the small transformer which feeds the two main switching MOSFETS that feed the two large transformers to the secondary's.
    No over heating detected especially in the replacement resistors ... cool as a cucumber.
    My 1st inclination is that these fusibles have an engineered lifespan ... 1000 hrs according to the datasheet.
    And this was intentional so as to make the whole unit OBSOLETE when they were cooked/EOL.
    And since Canon EOLed the wholepower supply AND KOA Speers EOLed the replacement fusibles ...the $16,000 unit goes to the junkyard and Mr. Optometrist buys a new one for 16 grand.

    Given this early data ... what do Y'all think ?
    Only time will be the ultimate test to see if some other component will fail down the road ... so far there is NO component that seems to be over heating due to excess current. I have been meaning to buy a digital IR thermometer but haven't gotten round to it so I risk electric shock by using my fingers. There is one very big cap hidden in a place I haven't been able to open but I was able to disconnect it's connections and ESR test it out of circuit ... ESR reading is well below 1 and I think it's for the 12 V power for the stepper motor for the focus lenses.
    All other power to the unit ... save for the lights ... is very low power like say for the camera itself. There doesn't seem to be much else.
    So like I say above ... engineered LIFESPAN so these poor SOB's have to spend about 16 grand every 1000 hrs for a new machine.
    AND every new machine that comes out is ABSOLUTE JUNK !!! ...plastic lenses instead of good GLASS from Nikon or Ziess. This has been a trend for a while now according to the guy who sold the machine to Joe in the 1st place. Apparently, esthetics and poor quality pics are the standard these days for your RETINAL scans.
    If your eyes are failing and the quality of your eyeglasses are suffering these days ... now you KNOW why ... all these mfger's are cheaping out so badly that the equipment our healthcare providers have to work with is ABSOLUTE JUNK !!!
    Same can be said about COVid-19 ... the test for it is ABSOLUTE freaking JUNK !!! Almost ALL FALSE positives ... stop wearing your MASKS ... I have and no one has said SH*T to me about it. BUT I see that almost everyone is still falling for this crap !

    Speaking about engineered obsolecense ... it IS a very REAL thing as one has called me out on it ... I worked in manufacturing for many years ... I effin know ... I was there ... I know EXACTLY how it is done. I knew the Engineers and spoke with them about it.
    We as humans are being engineered OUT of existence ... we are to be replaced by ROBOTS and when I worked at the mfger ... There were many many many jobs replaced by robotic pick & place machines. I was let go due to the DOT.com crash in early 2000 ... we actually made Sh*T that DOES stuff ... and I still got let go ... I was the HIGHEST paid troubleshooter and they found cheap cheap cheap guys to replace me ... my final wage before I got let go was $15.50 per hr CAD.
    Pretty sad for the amount of cash I made for my company !

    Me and one other engineer pushed our company into it's very 1st ... 1 million dollar shipping month of product ... I got a raise immediately after that when it wasn't due for at least another 3 or 4 months.
    I was IN the biz and very much behind the scenes here in Canada ... I know about ENGINEERED obsolecense ... trust me.
     
    Last edited: Sep 13, 2020
  20. Journey11

    Journey11

    62
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    May 23, 2018

    WRONG WRONG WRONG sir !!!!!!!!!!!! ... the datasheet specifically SAYS it is a THERMAL FUSE . Heat will cook it before an over current or over voltage situation occurs. The resistor is RATED for 10 AMPS/250V if you read the damned datasheet ... you were the one who asked that I post it and I did ...
    And the current measured when it runs without load is about 120mA ... and then get this ... after load is connected ... ZERO current on the primary ... what do you make of that my friend ? Read my recent post after I had the unit brought back with yet another blown fusable resistor ... now that they are BOTH GONE ... it still seems to be working fine .... time will tell if somewhere down the line there is a big draw ... but I don't think so cuz there is ZERO heat from any semi-conductor and with ammeter connected on primary ... almost ZERO current draw.
     
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