Equivalent thermistor for Cisco switch PSU

[jjne]

Hi,

I have a Cisco 3750 switch with a blown power supply. I think I've traced the fault to a blown bridge rectifier, but one of the NTC thermistors has also blown by the look of things.

It's an SCK037 which I've been having difficulties locating a fair-priced replacement for.

I think the 3D-15 looks to be near enough an equivalent to this part (same zero power resistance, operating temperature and current, and similar other values), but not having too much knowledge of these components I'm not sure.

Can anyone confirm if this would be a suitable replacement?

Thanks!

[Curious.George]

Re: Equivalent thermistor for Cisco switch PSU

Hi,

I have a Cisco 3750 switch with a blown power supply. I think I've traced the fault to a blown bridge rectifier, but one of the NTC thermistors has also blown by the look of things.

Have you sorted out why the bridge failed? I.e., do you have an idea as to how extensive the problem might be BEYOND what you are currently seeing as failed components?

Do you really need an L3 switch? Might it be easier to just pickup a similar (# ports) switch and save the headaches of chasing failuress?

[R_J]

Re: Equivalent thermistor for Cisco switch PSU

Looks like it should work, thermal dissipation constant is only 3mW lower but I dont think that should be an issue

[jjne]

Re: Equivalent thermistor for Cisco switch PSU

Have you sorted out why the bridge failed? I.e., do you have an idea as to how extensive the problem might be BEYOND what you are currently seeing as failed components?

Do you really need an L3 switch? Might it be easier to just pickup a similar (# ports) switch and save the headaches of chasing failuress?

I am just currently at the point where I am collecting parts I know to be faulty. The issue I think was caused by a surge a couple of months ago. The parts identified so far are inexpensive; if the fault turns out to be something more serious than the basics on the primary side (it's mostly through-hole parts and I will dig deeper in due course) then ultimately I'll move it on. The switch does work with an alternate power supply.

I do need a L3 switch, yes, as I am a network engineer. I use basic L2 managed fanless switches from cheaper vendors for my home network - this is for my lab.

I'm not in any mad rush to get this working again as I have spares, but the power supplies cost more than the switches used so I have decided to try to isolate the cause of the dead short on the rectifier (which follows the rectifier when the part is removed), but as the short isn't present when the rectifier is removed and all passives (and the two switching transistors) all check out fine it may be a simple fix. The thermistor measures ok but has a large crack in the casing so it has to go.

It's worth a shot - worst case scenario I stuff the switch itself. But it's just a 12v power supply, nothing complex albeit it does put out 17A.... I may try connecting an ATX supply to the switch board if I can't fix the current supply.

[jjne]

Re: Equivalent thermistor for Cisco switch PSU

Looks like it should work, thermal dissipation constant is only 3mW lower but I dont think that should be an issue

Thanks for confirming my thoughts - I will buy a couple in.

[Curious.George]

Re: Equivalent thermistor for Cisco switch PSU

I am just currently at the point where I am collecting parts I know to be faulty. The issue I think was caused by a surge a couple of months ago. The parts identified so far are inexpensive; if the fault turns out to be something more serious than the basics on the primary side (it's mostly through-hole parts and I will dig deeper in due course) then ultimately I'll move it on. The switch does work with an alternate power supply.

I do need a L3 switch, yes, as I am a network engineer. I use basic L2 managed fanless switches from cheaper vendors for my home network - this is for my lab.

Fair enough. Often we get focused on trying to fix what we have instead of just replacing with identical/equivalent. Switches are (almost) simple commodity items (unless you've written some software to interact with the management interface).

I'm not in any mad rush to get this working again as I have spares, but the power supplies cost more than the switches used so I have decided to try to isolate the cause of the dead short on the rectifier (which follows the rectifier when the part is removed), but as the short isn't present when the rectifier is removed and all passives (and the two switching transistors) all check out fine it may be a simple fix. The thermistor measures ok but has a large crack in the casing so it has to go.

So, it looks like the bridge is the real problem? I assume it lies directly on the mains so any surge just exceeded the ratings of the diode(s) in the bridge?

It's worth a shot - worst case scenario I stuff the switch itself. But it's just a 12v power supply, nothing complex albeit it does put out 17A.... I may try connecting an ATX supply to the switch board if I can't fix the current supply.

Yeah, I was amazed at how much power switches consume just for the fabric itself! The days of (relatively low power) "hubs" are long past -- esp with more advanced switch functionality.

[I've been designing a custom 96 port PoE, PTP switch, here. 2KW power supply. ]

Your "replacement" problem is the form factor of the switches enclosure, of course. (If you were a few thousand miles closer, I would keep my eyes open for something compatible, here)

[jjne]

Re: Equivalent thermistor for Cisco switch PSU

Fair enough. Often we get focused on trying to fix what we have instead of just replacing with identical/equivalent. Switches are (almost) simple commodity items (unless you've written some software to interact with the management interface).

Indeed, most people can get away quite happily with a $10 8 port switch, or even the switch built into their broadband router. My needs are a little more complex than that on my home network (needs to be CLI managed, have gig ports, VLANs etc) but they are commodity devices nowadays as you say, and there's no need for anything more than a $200-new (much less used) TP Link or Netgear for that. I would perform basic maintenance even on cheapo devices mind, it's more fun than replacement (within reason!).

A lab for work is a different story though. Must be Cisco, Juniper etc and must have the required features (that said most of what I do these days is virtual, but some physical kit is still useful).

So, it looks like the bridge is the real problem? I assume it lies directly on the mains so any surge just exceeded the ratings of the diode(s) in the bridge?

Yup, although I am buying in a few extra bits and pieces that are cheap, just in case (switching FETs, thermistor, making sure I have the right other diodes and fuses and the like) within a small budget so it's not left lying around too long.

Yeah, I was amazed at how much power switches consume just for the fabric itself! The days of (relatively low power) "hubs" are long past -- esp with more advanced switch functionality.

[I've been designing a custom 96 port PoE, PTP switch, here. 2KW power supply. ]

Indeed, although the 3750 is an older design now (my examples certainly are anyway) and I think newer designs are somewhat more efficient - 200w for a 24-port gig switch, even L3, is quite a bit. They do 95% of what modern replacements do, but the power draw is quite scary. Hence I would never use them for a home switch - too noisy and too expensive to run 24/7.

PoE obviously makes things even worse - 2kw for a 96 porter isn't even surprising any more. The bloody thing must double up as a heater.

Your "replacement" problem is the form factor of the switches enclosure, of course. (If you were a few thousand miles closer, I would keep my eyes open for something compatible, here)

Well I've been thinking about that. If the ATX works as a 'proof of concept' my next step would be to measure how much the switch actually draws (I only use a few of the ports at any one time) and buy in one of those metal-housed LED PSUs which come in 8a, 10a and 15a forms etc. They come in under £10 for the smaller ones and may well fit inside the case. May well be an elegant solution.

[Curious.George]

Re: Equivalent thermistor for Cisco switch PSU

Yup, although I am buying in a few extra bits and pieces that are cheap, just in case (switching FETs, thermistor, making sure I have the right other diodes and fuses and the like) within a small budget so it's not left lying around too long.

If you're confident of the "problem", its a small additional investment (considering the time and effort already expended to get to that diagnosis) to address potential/likely problems that may be "lurking".

[Nothing worse than having to repair something shortly after having previously repaired it -- even if the reasons differ]

Indeed, although the 3750 is an older design now (my examples certainly are anyway). They do 95% of what modern replacements do, but the power draw is quite scary. Hence I would never use them for a home switch - too noisy and too expensive to run 24/7.

Yup. I have a similar attitude re: my blade server (dims the lights when I power up all 14 blades). OTOH, its hard to come up with anything "comparable" to emulate that sort of functionality.

PoE obviously makes things even worse - 2kw for a 96 porter isn't even surprising any more. The bloody thing must double up as a heater.

The one big advantage is that most of that power is dissipated in other (remote) places. So, aside from the power needed to run the actual fabric (which is obviously dissipated in the switch itself) and the losses associated with the "inefficiency" of the power supply, its really not as bad as a disk array!

Well I've been thinking about that. If the ATX works as a 'proof of concept' my next step would be to measure how much the switch actually draws (I only use a few of the ports at any one time) and buy in one of those metal-housed LED PSUs which come in 8a, 10a and 15a forms etc. They come in under £10 for the smaller ones and may well fit inside the case. May well be an elegant solution.

I was thinking more along the lines of finding yet another Cisco and hoping the power supply for it was modifiable to fit in the existing switch. Unlike PC's (which have highly standardized power supply shapes and interfaces), switches are a bit more variable.

I periodically rescue power supplies from "odd" devices that I already have just to keep a cold spare on hand (e.g., the power supply for my SB2000 is larger than most PC's -- though only ~700W!) -- I'd rather "swap out" than have to replace (esp when the failure only happens when you are actually needing/using the device!)

[jjne]

Re: Equivalent thermistor for Cisco switch PSU

Re PSU from another Cisco - doubtful. This particular switch is an early 3750g which is 1.5u rather than 1u in height. This has its advantages, but a disadvantage is the PSU is a very different size/shape to the typical power supply in switches.

However, it is a very similar size and shape to the LED PSUs...

The newer ones of course are modular, and hot-swappable That's great in the field, not so good when you're trying to frig something to get an old clunker to work!

Relevant to this site, the capacitors in this thing are garbage. A mixture of Taicon and mostly Ltec. Not the worst but hardly what you'd expect from what was a very expensive device in its day. Even Netgear use Nichicon in their enterprise switches FFS.

[jjne]

Re: Equivalent thermistor for Cisco switch PSU

Hmmmmm. Alternative fix is easy...

wiki.frotmail.nl/index.php/ATX_to_RPS

Heh.

[Curious.George]

Re: Equivalent thermistor for Cisco switch PSU

Re PSU from another Cisco - doubtful. This particular switch is an early 3750g which is 1.5u rather than 1u in height. This has its advantages, but a disadvantage is the PSU is a very different size/shape to the typical power supply in switches.

Ah. I couldn't say for sure how often I'd come across something like that. I typically see pallets of network kit and, from any distance, it all just looks like stacks of 1U-ish boxes (only the colors differentiate the products!)

(Because they are so common, I rarely go looking through those pallets unless I'm actually in need of another switch!)