Looks like I may need a little help from the PSU experts (or anyone really! ) I have a Corsair CX750M (Model 75-002019) that I picked up for free about 4 years ago. This is the PSU:
So here’s what’s strange about this one: it appears to work normally (normal output voltages) and any PC is stable with it. However, after a while (typically 10-20 minutes, but time can vary depending on the load), there is a sweetish smell of burned magnet wire insulation emanating from the PSU. Upon opening the PSU after this happened (many times), I was able to confirm the smell is coming from the output toroid inductor, which is insanely hot at that point. Attached at the end of the post are pictures of the whole PSU, just for reference. However, this picture probably shows it all:
Note the insulation on the top of the toroid is very easy to chip. A few temperature tests showed me it easily went past 100°C, if not even 150°C.
Funny thing is, I used the PSU for a good while as a “test” PSU for non-critical hardware (typically only for very brief tests, so that the output inductor couldn’t overheat too much.) I just couldn’t understand how that burned output inductor was still letting the PSU work… but somehow it did.
Now here is the big question: did the output toroid inductor overheat because it was defective? Or was there another component that’s causing it to overheat? Or perhaps the toroidal core was improperly selected for the task?
While I was searching for pictures on the internet to see if I can find an image that shows what the core of a working CX750 looks like, here is something even more interesting I found:
https://linustechtips.com/topic/8908...ng-and-repair/
^ Apparently, it’s a thread on LinusTechTips forums that looks to have been started in 2018 with some people chiming in as late as 2019 and 2020, all with the same type of failure: burned output toroidal inductor on the CX750M (but also a few other PSUs that use this same CWT platform.).
According to jonnyGURU there, the issue was due to a manufacturing defect with the output inductor on early units and was supposedly fixed on latter units. I don’t know how true that is. However, going by the info in that thread, the digits before the zeroes in the S/N of my CX750 should indicate the manufacturing date code. In my case, that’s 1537, which would be 37th week of 2015 – just one week later than the O/Ps. I got this Corsair PSU in the Spring of 2017, so that’s likely when the failure occurred. Person I got it from stated his PC would work most of the time, but then start cutting out after very long sessions. And he said the smell was really worrying him.
Now, I haven’t really done an in-depth test of the components in there. But I did a quick check on the “Sync Card” – the little daughterboard that sticks out right behind the inductor. This board holds the MOSFETs that are used for the 12V rail synchronous rectification. Picture in link form (hi-res):
https://www.badcaps.net/forum/attach...1&d=1632104825
So far, everything checks out OK… or at least none of the MOSFETs are shorted and all of their Gate driver resistors are OK, along with a few other parts I checked while in there.
Here is also a high-res picture of the secondary side PCB of the PSU (just in case anyone needs it or wants to trace connections.)
https://www.badcaps.net/forum/attach...1&d=1632104825
Since I can’t seem to find anything wrong so far, I think a good starting point would be to replace/rewind that output inductor, and then go from there to see how the PSU behaves. I *think* the core of this inductor should be black in color on all sides, as it appears to be very similar to this ThermalTake M850W PSU. Basically, both of these are Channel-Well Technology / CWT units (PU-Q platform, if I’m not mistaken.) From what I understand, when these ferrite cores go above a certain temperature, they may loose their specs. So, rewinding the old core will likely result in the windings to burn again. Therefore, I need to find a replacement core.
The big question is: where and what specs do I look at to find a proper replacement toroid core? Obviously the windings are custom to the PSU. But that doesn’t worry me as much, as I will just copy the design of the old core. What I have learned from previous posts on BCN (big thank you to PeteS on that matter, specifically) is that many of the cheap gutless PSUs (but also a few good ones) use Micrometals “26” -type core material for their output inductors – those yellow cores with white paint on one side. Then there’s Micrometals 52 -type core: light green with blue paint on one side. This one is typically seen on many OEM PSUs and often used in place of -26 cores on the better-built cheapo PSUs brands, due to having very similar permeability specs, but lower losses. So where does that put these toroids with black cores? From my search online, I found the following PDF uploaded on one website:
https://www.badcaps.net/forum/attach...1&d=1632105150
According to that, the core should be a “-45” type.
This seems to agree with the diagram on the product finder on this Micrometals page:
https://micrometals.com/products/product-finder/
However, could anyone confirm if my findings are correct?
Besides core type, we also need to know the size. In the case of my CX750M, the output toroid measures about 35 mm for the outer diameter (OD) without the windings. The hole diameter (or inner diameter, ID) of the toroid is about 20 mm. And the thickness of the toroid is about 11-12 mm. That 35 mm OD converts to about 1.377 inches… so perhaps we can say it’s a 1.3” OD inductor? I’m almost certain, for example, that the output inductor for this BFG Tech BFGR550WGSPSU ATX power supply goes by the part number of T130-26 (meaning it has 1.30 inches OD and uses -26 core material.) Putting the two side by side for comparison (CX750 core is one on the right):
… I think the CX750M toroid is also a T130 part. Does that mean I should look for a T130-45 part? Yes/No? This is the T130-45 datasheet, if that's of any help:
https://www.badcaps.net/forum/attach...1&d=1632105150
Let’s assume the above is correct for a moment. Now the question becomes, where can I buy a replacement toroid core like this? I already found a replacement for the T130-26 toroidal core for the BFG PSU here. However, I don’t see this seller (or any other place online I’ve looked at so far) to have T130-45 cores in stock. Moreover, since that seller seems to be selling T130-26 cores in bulk, I do wonder if I could get away with using that in the Corsair CX750M PSU… or two core in series, if one isn’t enough to get a high enough inductance? If not, I also wonder if a T131-26 toroid core (that the seller also has in stock) would work. T131-26 appears to be the same thing as the T130-26, but with smaller ID / thicker core basically, so I imagine it produce higher inductance too (provided I can even fit the windings on it.) It's datasheet is attached here:
https://www.badcaps.net/forum/attach...1&d=1632105150
The only reason I mentioned the T131-26 is because going back to that PDF above, the T131-26 appears to have a very similar Al (permeability) value to the T130-45 core (116 vs. 105). The initial permeability (u_i) seems to be a bit lower (75 vs. 100), but perhaps not dissimilar enough to cause problems?
Alternatively, I also found this toroid core on Digikey:
https://www.digikey.com/en/products/...002701/8599642
It’s not a Micrometals part, but rather made by Fair-Rite Products Corp. The part number is 5961002701 and Digikey P/N is 1934-1595-ND (should the link above ever stop working for some reason.) Initial permeability on this one is 125 and Al is listed as 140. But when it comes to inductor specs, I don’t have any idea which parameters are important here and which are not… or how much I can deviate with these.
So if I can somehow find a good replacement core for the output toroid inductor, I think I should be able to wind my own inductor. Of course, the big question is, once more, if I have properly identified the old core… and if so, would any of the inductor cores I pointed to above actually work? Any advice would be greatly appreciated here!
And yeah, I know… it’s an older PSU that probably isn’t worth fixing, as it isn’t anything special. However, I would like to take it as a fun challenge.
So here’s what’s strange about this one: it appears to work normally (normal output voltages) and any PC is stable with it. However, after a while (typically 10-20 minutes, but time can vary depending on the load), there is a sweetish smell of burned magnet wire insulation emanating from the PSU. Upon opening the PSU after this happened (many times), I was able to confirm the smell is coming from the output toroid inductor, which is insanely hot at that point. Attached at the end of the post are pictures of the whole PSU, just for reference. However, this picture probably shows it all:
Note the insulation on the top of the toroid is very easy to chip. A few temperature tests showed me it easily went past 100°C, if not even 150°C.
Funny thing is, I used the PSU for a good while as a “test” PSU for non-critical hardware (typically only for very brief tests, so that the output inductor couldn’t overheat too much.) I just couldn’t understand how that burned output inductor was still letting the PSU work… but somehow it did.
Now here is the big question: did the output toroid inductor overheat because it was defective? Or was there another component that’s causing it to overheat? Or perhaps the toroidal core was improperly selected for the task?
While I was searching for pictures on the internet to see if I can find an image that shows what the core of a working CX750 looks like, here is something even more interesting I found:
https://linustechtips.com/topic/8908...ng-and-repair/
^ Apparently, it’s a thread on LinusTechTips forums that looks to have been started in 2018 with some people chiming in as late as 2019 and 2020, all with the same type of failure: burned output toroidal inductor on the CX750M (but also a few other PSUs that use this same CWT platform.).
According to jonnyGURU there, the issue was due to a manufacturing defect with the output inductor on early units and was supposedly fixed on latter units. I don’t know how true that is. However, going by the info in that thread, the digits before the zeroes in the S/N of my CX750 should indicate the manufacturing date code. In my case, that’s 1537, which would be 37th week of 2015 – just one week later than the O/Ps. I got this Corsair PSU in the Spring of 2017, so that’s likely when the failure occurred. Person I got it from stated his PC would work most of the time, but then start cutting out after very long sessions. And he said the smell was really worrying him.
Now, I haven’t really done an in-depth test of the components in there. But I did a quick check on the “Sync Card” – the little daughterboard that sticks out right behind the inductor. This board holds the MOSFETs that are used for the 12V rail synchronous rectification. Picture in link form (hi-res):
https://www.badcaps.net/forum/attach...1&d=1632104825
So far, everything checks out OK… or at least none of the MOSFETs are shorted and all of their Gate driver resistors are OK, along with a few other parts I checked while in there.
Here is also a high-res picture of the secondary side PCB of the PSU (just in case anyone needs it or wants to trace connections.)
https://www.badcaps.net/forum/attach...1&d=1632104825
Since I can’t seem to find anything wrong so far, I think a good starting point would be to replace/rewind that output inductor, and then go from there to see how the PSU behaves. I *think* the core of this inductor should be black in color on all sides, as it appears to be very similar to this ThermalTake M850W PSU. Basically, both of these are Channel-Well Technology / CWT units (PU-Q platform, if I’m not mistaken.) From what I understand, when these ferrite cores go above a certain temperature, they may loose their specs. So, rewinding the old core will likely result in the windings to burn again. Therefore, I need to find a replacement core.
The big question is: where and what specs do I look at to find a proper replacement toroid core? Obviously the windings are custom to the PSU. But that doesn’t worry me as much, as I will just copy the design of the old core. What I have learned from previous posts on BCN (big thank you to PeteS on that matter, specifically) is that many of the cheap gutless PSUs (but also a few good ones) use Micrometals “26” -type core material for their output inductors – those yellow cores with white paint on one side. Then there’s Micrometals 52 -type core: light green with blue paint on one side. This one is typically seen on many OEM PSUs and often used in place of -26 cores on the better-built cheapo PSUs brands, due to having very similar permeability specs, but lower losses. So where does that put these toroids with black cores? From my search online, I found the following PDF uploaded on one website:
https://www.badcaps.net/forum/attach...1&d=1632105150
According to that, the core should be a “-45” type.
This seems to agree with the diagram on the product finder on this Micrometals page:
https://micrometals.com/products/product-finder/
However, could anyone confirm if my findings are correct?
Besides core type, we also need to know the size. In the case of my CX750M, the output toroid measures about 35 mm for the outer diameter (OD) without the windings. The hole diameter (or inner diameter, ID) of the toroid is about 20 mm. And the thickness of the toroid is about 11-12 mm. That 35 mm OD converts to about 1.377 inches… so perhaps we can say it’s a 1.3” OD inductor? I’m almost certain, for example, that the output inductor for this BFG Tech BFGR550WGSPSU ATX power supply goes by the part number of T130-26 (meaning it has 1.30 inches OD and uses -26 core material.) Putting the two side by side for comparison (CX750 core is one on the right):
… I think the CX750M toroid is also a T130 part. Does that mean I should look for a T130-45 part? Yes/No? This is the T130-45 datasheet, if that's of any help:
https://www.badcaps.net/forum/attach...1&d=1632105150
Let’s assume the above is correct for a moment. Now the question becomes, where can I buy a replacement toroid core like this? I already found a replacement for the T130-26 toroidal core for the BFG PSU here. However, I don’t see this seller (or any other place online I’ve looked at so far) to have T130-45 cores in stock. Moreover, since that seller seems to be selling T130-26 cores in bulk, I do wonder if I could get away with using that in the Corsair CX750M PSU… or two core in series, if one isn’t enough to get a high enough inductance? If not, I also wonder if a T131-26 toroid core (that the seller also has in stock) would work. T131-26 appears to be the same thing as the T130-26, but with smaller ID / thicker core basically, so I imagine it produce higher inductance too (provided I can even fit the windings on it.) It's datasheet is attached here:
https://www.badcaps.net/forum/attach...1&d=1632105150
The only reason I mentioned the T131-26 is because going back to that PDF above, the T131-26 appears to have a very similar Al (permeability) value to the T130-45 core (116 vs. 105). The initial permeability (u_i) seems to be a bit lower (75 vs. 100), but perhaps not dissimilar enough to cause problems?
Alternatively, I also found this toroid core on Digikey:
https://www.digikey.com/en/products/...002701/8599642
It’s not a Micrometals part, but rather made by Fair-Rite Products Corp. The part number is 5961002701 and Digikey P/N is 1934-1595-ND (should the link above ever stop working for some reason.) Initial permeability on this one is 125 and Al is listed as 140. But when it comes to inductor specs, I don’t have any idea which parameters are important here and which are not… or how much I can deviate with these.
So if I can somehow find a good replacement core for the output toroid inductor, I think I should be able to wind my own inductor. Of course, the big question is, once more, if I have properly identified the old core… and if so, would any of the inductor cores I pointed to above actually work? Any advice would be greatly appreciated here!
And yeah, I know… it’s an older PSU that probably isn’t worth fixing, as it isn’t anything special. However, I would like to take it as a fun challenge.
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