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.
(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 ...
Moderators note : cropped images and placed them inline
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.
(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 ...
Moderators note : cropped images and placed them inline
Last edited by a moderator:
