Replacing unknown NTC Thermistor in Corsair CX400

[Stefan Payne]

Re: Replacing unknown NTC Thermistor in Corsair CX400

100A of inrush allowed by ATX 2.2 spec at 230V? Wouldn't that cause most circuit breakers in the EU to trip? IIRC, they don't even allow as much amps as the Americas!

Yes, they would.

Normal Ratings are B16 breakers wich allow only 3-5 times the nominal value.
Wich means that everything over 48A is crap and trips breaker (hello Sirfa!!)
And max is 80A.

However there is also the possibility to install a C16 breaker that allows 5-10 times the nominal value (=80-160A), but depending on the length of the cable it's not always possible.

And they are expensive as hell. At least double the price of a normal B16.
I've paid about tripple what a normal B16 breaker whould have cost....

That sounds like much but a B16 breaker cost as little as ~2€.
And I've paid ~9€ for one (Hager) C16...

In fact, the higher normal-voltage nations seem to prefer smaller-diameter wiring and use higher voltage to make up for the copper limitations, IIRC...

First 1,5mm² copper wire is the norm here in Europe (German)
That is somewhat between AWG15 and AWG16...

Second: Normal breakers for that (as far as I know) is 16A with the B characteristics.
Although I've heard that 13A is the new norm and 16A is only allowed under certain conditions.

Third: In some regions like the old Communist Germany 10A are also used - with Aluminium wire. I can't say anything about that, never seen it, that's only what I've heard...

[budm]

Re: Replacing unknown NTC Thermistor in Corsair CX400

100A of inrush allowed by ATX 2.2 spec at 230V? Wouldn't that cause most circuit breakers in the EU to trip? IIRC, they don't even allow as much amps as the Americas!

In fact, the higher normal-voltage nations seem to prefer smaller-diameter wiring and use higher voltage to make up for the copper limitations, IIRC...

If the inrush duration is less than the trip curve of the circuit breaker then the circuit breaker will not trip.


Fuse curves:
https://cdn.badcaps-static.com/pdfs/...c9cccf3656.pdf

[RJARRRPCGP]

Re: Replacing unknown NTC Thermistor in Corsair CX400

If the efficiency is marginal, perhaps that watt or two is the difference between 80Plus or not...?

http://www.tomshardware.com/reviews/...s,2746-18.html says CX400 had 63.5A inrush current on 240v, that implies ~4Ohm cold NTC?

100A of inrush allowed by ATX 2.2 spec at 230V? Wouldn't that cause most circuit breakers in the EU to trip? IIRC, they don't even allow as much amps as the Americas!

In fact, the higher normal-voltage nations seem to prefer smaller-diameter wiring and use higher voltage to make up for the copper limitations, IIRC...

[Agent24]

Re: Replacing unknown NTC Thermistor in Corsair CX400

If the efficiency is marginal, perhaps that watt or two is the difference between 80Plus or not...?

http://www.tomshardware.com/reviews/...s,2746-18.html says CX400 had 63.5A inrush current on 240v, that implies ~4Ohm cold NTC?

[redwire]

Re: Replacing unknown NTC Thermistor in Corsair CX400

I think you're right, the heatshrink sleeve is probably about safety; heating the NTC more would lower losses but who cares about a watt or two lost here.

The cheese-cloth (fire) test means spitting sparks out the vents is not OK, so I notice you don't always see a heatshrink cover on inrush limiter, MOV in Channel Well Technology (CWT) PSU's. Mostly silicone glue if it is a spacings issue.

Regulatory agencies don't permit heatshrink over MOV's/inrush limiters because of melting, flammability and dielectric rating - but not a worry here because of this metal enclosure. As long as the flames stay inside.

The ripple current through the part has high peaks and yet you want the average current to be in spec.
SCK10 are rated for about 330uF at 240VAC. Another SCK datasheet

CX600M 600W; APFC 270uF; two of SCK015 (10mm, 1R cold, 5A)
RMX650i 650W; APFC 680uF; one of SCK035 (10mm, 3R cold, 5A)

[Agent24]

Re: Replacing unknown NTC Thermistor in Corsair CX400

Note OP's power supply had heat shrink around it, this I believe is done to keep the NTC hot despite airflow from the PSU fan. If the NTC is a bit cool, larger voltage drop during run.

Wouldn't the higher Vdrop just cause more heating and balance out the cooling from the fan anyway?

My guess was that the heatshrink was a safety idea in case of failure, as happened here. Like how they use glass fuses with heatshrink around them, instead of getting ceramic ones.

[budm]

Re: Replacing unknown NTC Thermistor in Corsair CX400

VI characteristic chart showing the the Vdrops, Current, Pd, ETC.
As you can see from the graph for 504
@4A rated current the resistance is a little < 0.20 Ohms, Vdrops is about 0.70V, the power dissipation a little > 2W

[redwire]

Re: Replacing unknown NTC Thermistor in Corsair CX400

You usually start with your max. inrush current and the cold resistance of the NTC. To not blow the fuse or kill the bridge rectifier. 10ohms is what I'm used to seeing.
Next is the max. load current, so PTC does not burn up. This is a size thing. Bigger is better but slower to heat up.

Note OP's power supply had heat shrink around it, this I believe is done to keep the NTC hot despite airflow from the PSU fan. If the NTC is a bit cool, larger voltage drop during run.

[Agent24]

Re: Replacing unknown NTC Thermistor in Corsair CX400

There is http://www.ametherm.com/inrush-curre...t-limiter.html
I don't know if that helps though.

I think it would require extensive knowledge of the PSU characteristics?

[stj]

Re: Replacing unknown NTC Thermistor in Corsair CX400

is there any info for calculating the right NTC for a specific run-load??

[Agent24]

Re: Replacing unknown NTC Thermistor in Corsair CX400

Yes it's a 10mm NTC.
Interestingly, a similarly rated Enermax Liberty 400W uses an SCK 053.

I've put the spare SCK 054 in, and it seems to be working OK so far.
Hard to check full load though as I don't have a load tester to be sure of the load.

[budm]

Re: Replacing unknown NTC Thermistor in Corsair CX400

What is the diameter of that NTC? 10mm?

SCK 054 looks to be about right.
You can put that 054 in and run at full load and check the temperature and Vdrops on that 054 to see what you get.

[Agent24]

A few days ago, something in my lounge started making a loud "phut" noise when plugged into the wall, and sometimes a smell like a hot glue gun was present in the room momentarily.

I tracked it down to the NTC thermistor in my Corsair CX400. It has cracked and burnt. It measures 50kOhm+ cold... strangely, the PSU still seemed to work fine the whole time (?), so shorted MOSFETs or such seem unlikely!


The problem is that the damage to the thermistor has resulted in the part number becoming unreadable, save the "SCK" on top.

Does anyone know the value of the thermistor used in the CX400, or can advise on how to pick a suitable replacement? I did find a thread on the Corsair forums: http://forum.corsair.com/v3/showthread.php?t=112161 but nobody answered.

How similar does a replacement have to be? I do have some pulled thermistors from scrapped PSUs. An SCK 054, a CT 5D-11, and a DSC 8D-11.