Likely specs of blown NTC - Page 1
Forum section: electronics / beginners

author: giosif
author_rank: Frequent Contributor
timestamp: August 16, 2020, 09:30:34 am
content: Hi,I have a broken 1000W computer power supply which I am trying to repair and first thing I discovered is that the NTC / thermistor is blown.As you can see from be photos below, I tried peeling the heatshrink tubing off the NTC in search for some markings, but there is no chance of finding any.So, I am now trying to identify a suitable replacement NTC.Any suggestions how I should approach this, please?If it matters, the NTC is located in the circuit straight after the mains input, on the live side.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 16, 2020, 10:08:48 am
content: Are you able to provide the details of the power supply? Manufacturer / model? Some pics of the circuit board perhaps?Determining an appropriate replacement without knowing the specs of the original NTC is going to require some more knowledge of the circuit it is protecting. Namely the total capacitance present, as well as operating voltage and current.

author: giosif
author_rank: Frequent Contributor
timestamp: August 16, 2020, 10:20:13 am
content: Sure.It is a XFX XPS-1000W-XTS / Seasonic Platinum MK3 and you can see it in the photo below (courtesy of kitguru.net), right behind the live wire.I don't know about the total capacitance of the entire supply, but the bulk caps on the primary side are two, each of 390uF.

author: Dabbot
author_rank: Regular Contributor
timestamp: August 16, 2020, 12:08:15 pm
content: From the specs of the power supply and the information you supplied on the bulk caps:Max total capacitance, assuming parallel and 20% tolerance (390 * 2 * 1.2):936uFMax continuous current (From the power supply specs):12A (120V), 6A (240V)Max stored energy in caps assuming 240V supply (340V rectified):54 JoulesSo you're looking for an NTC:- Rated for a steady state current of 12 amps- Rated for a surge of 54 joules or moreA problem I see is the NTC in the picture is about 16mm to 17mm diameter, and the NTCs I'm finding within the above constraints are all larger than this. I suspect the manufacturer has specified something smaller while I'm left to speculate based on worst case numbers above. I really suggest trying to identify the actual part rather than coming up with a suitable replacement based on the above.

author: giosif
author_rank: Frequent Contributor
timestamp: August 16, 2020, 04:34:26 pm
content: Thank you for the information!Yes, the dimensions of the initial NTC made me wonder too.Also, on a summary check, the rest of the power supply seems to be ok (i.e. no shorts that I could detect), so I'm wondering what toasted the NTC in the first place (could it be because it was under-specced?).I will try to contact the OEM and ask for the actual part number, but not holding out too much hope.

author: julian1
author_rank: Frequent Contributor
timestamp: August 16, 2020, 09:29:23 pm
content: I believe its a common design - to have the NTC shunted off after power-up, so that it imposes no unwanted series resistance. You can see what looks like a relay right next to the live wire, in proximity to where you suggest the NTC is. It may be that something else in that chain - either driving logic, or the relay has failed, so the NTC was never properly turned off in normal operation, and that led to NTC also failing due to heat over time.

author: Benta
author_rank: Super Contributor
timestamp: August 16, 2020, 09:34:02 pm
content: Ain't no NTC. It's a MOV for transient protection. Power supply will work perfectly well without it. Did you have a thunderstorm?

author: giosif
author_rank: Frequent Contributor
timestamp: August 16, 2020, 11:53:57 pm
content: Quote from: julian1 on August 16, 2020, 09:29:23 pmHmm... You have a point here.I will check for any clues in support for this scenario.

author: giosif
author_rank: Frequent Contributor
timestamp: August 17, 2020, 12:01:02 am
content: Quote from: Benta on August 16, 2020, 09:34:02 pmI am pretty sure it is an NTC, for the following reasons:1. It is in series with the live part of the mains and, with the component out of the circuit, no part of the supply gets mains power.So, it must be a conductive component, under normal circumstances.2. There is actually a MOV which is not visible in that picture and which does go across live and neutral, as expected.3. The markings on the board where the NTC is installed mention "TR001", which I'm pretty sure stands for thermistor (funnily enough, for the actual MOV, the markings on the board state "ZNR001", but I digress).

author: james_s
author_rank: Super Contributor
timestamp: August 17, 2020, 12:38:57 am
content: I would take the calculation someone else did above and then look for the closest you can find that is in the physical size needed. Chances are they cut some corners and used a smaller than ideal NTC.I suspect you have bigger problems though, the NTC is likely a symptom of a larger fault rather than the cause. Check the input rectifier and chopper transistor to see if they are shorted. I have not had a lot of luck repairing ATX power supplies, they tend to be very high density, completely undocumented and just all around a pain to work on given their low replacement cost.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 17, 2020, 04:01:23 pm
content: that's more likely the 275 volts varistor parallel, protection for overvoltages.almost any varistor for 275V, big enough, will do.usually fuse is dead when this one shorts.ask if you need help with some varistor codesee for example JVR-7N241K datasheetregards,pierre

author: giosif
author_rank: Frequent Contributor
timestamp: August 17, 2020, 06:30:08 pm
content: Quote from: james_s on August 17, 2020, 12:38:57 amI did think about the issue with the NTC being a symptom rather than the problem but, with all checks so far, nothing else seems to be wrong.That said, I was not able to check any of the ICs, but plan to do so once I install a replacement NTC.

author: giosif
author_rank: Frequent Contributor
timestamp: August 17, 2020, 06:32:15 pm
content: Quote from: perieanuo on August 17, 2020, 04:01:23 pmThanks, Pierre, but I am 100% sure this is an NTC and not a MOV.There is a MOV as well, to the left of the NTC and right behind the input spade terminals (MOV is not easily visible in the photo I provided).

author: Gyro
author_rank: Super Contributor
timestamp: August 17, 2020, 06:52:04 pm
content: Quote from: giosif on August 17, 2020, 06:30:08 pmI once caused that sort of NTC damage by accidentally repeatedly cycling an SMPS without giving it time to cool down, causing it to be hit by a series of high current surges - pretty sparks emerging were the result. I've only ever managed to do it once though.An intermittent mains supply connection could potentially cause the same effect.

author: giosif
author_rank: Frequent Contributor
timestamp: August 17, 2020, 11:13:43 pm
content: Quote from: Gyro on August 17, 2020, 06:52:04 pmThat's one of the things I learned with this occasion: the limitations / caveats of using an NTC.I will be more considerate when power cycling such supplies from now on (although I didn't damage this specific supply - I bought it in this state).

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 18, 2020, 07:14:10 am
content: hi,well, in this case, my bad.If it's the series ntc, just check the power part (rectifier, caps and switching mosfet) then put a 220v 40-60w bulb in place of the NTC and go ahead, with precaution if you can put a scope and see if switching mosfet is getting pulses from pwm regulator.ntc value is not so important but his power is.my only cases of ntc failure was when the ntc had poor soldering on one terminal, normally the fuse dies long before him is short on 300vdc lineif something is not clear from my part, just ask for detailsregards,pierre

author: giosif
author_rank: Frequent Contributor
timestamp: August 18, 2020, 06:10:56 pm
content: Thank you, Pierre!I get what you are saying with the incandescent bulb but I don't have such contraption already built.I am still hoping the manufacturer will be kind enough to give me the specs of the original NTC.With regards to the cause of the failure, I am still not 100% sure but, in the meantime, is tried measuring the resistance of the relay coil (in-circuit) and I was reading OL.Initially, I thought the relay might be dead but, after de-soldering it, I checked and the coil measures fine.Could it have been a dry joint for one of the coil contacts?Or maybe my meter probes weren't making good contact on the solder in the first place...I'm not sure.In any case, I soldered the relay back on and am now waiting for the manufacturer to respond.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 19, 2020, 07:52:43 am
content: hi,don't be lazy, just solder 2 wires to a cheap 220v/60w bulb and testany ntc with 5-10 ohm resistor will do, at least for testing, you find this quicly in your electronic components local storecourage mon grand

author: giosif
author_rank: Frequent Contributor
timestamp: August 19, 2020, 05:54:52 pm
content: Hahaha...In all honesty (and I should have explained this from the start), the reason for me being reluctant is that an incandescent light bulb fixture is probably the only thing missing in what is the mess of my home "lab".It may sound like I'm blowing this out of proportions, but I *really* need to put the place in order first.Until then, I am trying to resist adding any more clutter, even if it appears insignificant.And, yes, I've ordered a few NTCs I intend to test with, but they seem to take some time to arrive.I also like your joke about local electronic components store. Does something like this even exist anymore in this world?Maplin was the closest to that here in UK, but even it went down.Also, in US, I heard that at least some Fry's stores closed now (I could spend a day just browsing through the shelves there), so I'm not sure if there is anything left there either (not that those would be local to me anyway).

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 21, 2020, 01:39:56 pm
content: Lower power ATX power supplies (300-500 W) usually have the same NTC - 5R0 rated NTC.I havent seen 1000 W PSs inside so can't say definitely if they have lower rated resistance, and how lower?

author: giosif
author_rank: Frequent Contributor
timestamp: August 21, 2020, 08:55:09 pm
content: Thank you all for the input!In the meantime, I got the answer: 5R0, indeed, with max steady current of 10A.I've ordered an Ametherm / Sure-Gard (?Gard?) SG26 and am waiting for it to arrive.As a test, I installed a temporary 5R0 NTC rated for only 5A and the supply turns on and provides the expected output voltages (no load, though).I've also checked that the relay is switched on, after the supply starts and all looks good.I'll wait until I install the SG26 for more comprehensive testing and checks.