is it possible? MOV or varistor instead of NTC-5d 9?

[ali6x944]

hi everybody,
is it possible? MOV or varistor instead of NTC-5d 9 in an ATX power supply?

[Ian.M]

No.  A NTC thermistor is used to control inrush current.  Any type of varistor would either block current to the reservoir cap, or if it was a low enough voltage one to let current pass,overheat and die catastrophically.  You *MIGHT* get away with a low-ohm high power resistor and upgrading the diode ratings in the bridge rectifier to handle the increased surge.

[ali6x944]

will it work at high current draws? and how low u recommend the resistor value to be for 400w supply, model: P4-400W.

[ali6x944]

Could I use a PTC Thermistor instead like the SPMZ6?

[ali6x944]

See photos of the supply:

[Gyro]

No. The answer is in the name. An NTC (negative temperature coefficient) thermistor decreases in resistance as it's temperature increases.

A PTC (positive temperature coefficient) thermistor increases in resistance as it's temperature increases.

They are for different purposes. What is your problem with using the correct device?

[Yansi]

That's like asking if you could use salt instead of sugar.

Either learn how do these components work or stop guessing and pulling random component types and asking if they could be interchanged.

NTC can be replaced only by NTC.

PTC can be replaced only by PTC.

MOV can be replaced only by MOV.


NTC is a resistor with negative temperature coefficient, used for temperature measurements or inrush current limiting (high resistance when cold, warms up at current flow an decreases resistance).

PTC has a positive tempco, used mainly for temperature measurements or special types of PTCs also can be used as resettable fuses (PPTC, polyfuse, polyswitch)...

MOV (or "varistor") is a resistor with voltage dependence. The higher the voltage, the less the resistance. Used only for surge/spike voltage protection. At normal conditions and operating voltage, almost no or negligible current flows. If the voltage is higher than the rating, it will decrease its resistance several orders of magnitude and shorts the surge.

[ali6x944]

i get it now thanx everyone i really appreciate it
so based on what i read here and in Wikipedia i could replace an NTC with a regular 1w 10ohm resistor 

No.  A NTC thermistor is used to control inrush current.  Any type of varistor would either block current to the reservoir cap, or if it was a low enough voltage one to let current pass,overheat and die catastrophically.  You *MIGHT* get away with a low-ohm high power resistor and upgrading the diode ratings in the bridge rectifier to handle the increased surge.

[wraper]

i get it now thanx everyone i really appreciate it
so based on what i read here and in Wikipedia i could replace an NTC with a regular 1w 10ohm resistor

No, it  will burn. You need much higher power than 1W, 10 ohm would be too high either.

[ali6x944]

Is 5w 1 is fine...
I don't get how u know if it will fit or not so could u tell me how it is calculated please ...
So if in emergency I know how to do it.

[Ian.M]

Probably not for a lasting repair.  At 90V (10% below Japan's 100V supply, a 90% efficient 400W supply could draw nearly 5A continuously with 24.4W dissipation in a 1R resistor.

If the PSU is 220V-240V only and you know it will never see less than 200V, its a far more reasonable 1.8A and the 1R 5W resistor will probably do till you can get the right NTC thermistor *IF* the bridge rectifier diodes have a 1/2 cycle non-repetitive surge rating >360A.  If not they'll short and blow the fuse usually with no other damage so it may be an acceptable gamble in an emergency.  The odds are worse the more times power is interrupted so if power quality is bad in your area, don't do it!

[ali6x944]

If the PSU is 220V-240V only and you know it will never see less than 200V, its a far more reasonable 1.8A and the 1R 5W resistor will probably do till you can get the right NTC thermistor *IF* the bridge rectifier diodes have a 1/2 cycle non-repetitive surge rating >360A.  If not they'll short and blow the fuse usually with no other damage so it may be an acceptable gamble in an emergency.  The odds are worse the more times power is interrupted so if power quality is bad in your area, don't do it!

Thus a 1N5406 wouldn't work ?

[Ian.M]

Probably not. its only got a 200A peak rating 0ver a 8.3ms half sinusoid.   OTOH it may be better than specified and you've got a 50% chance of switching it on close enough t a zero crossing for it to survive so if it was mission critical and it only had to survive ONE powerup, I'd try it.

[Gyro]

You started this thread in back in March and you're still asking about different components that you could use to replace a failed standard NTC thermistor???   

So far you've wanted to replace it with a:
- MOV
- PTC thermistor
- 10R 1W resistor
- 1R 5W resistor
- 1N5406 Diode

Hint: A replacement NTC thermistor would work rather well. 

[ali6x944]

You started this thread in back in March and you're still asking about different components that you could use to replace a failed standard NTC thermistor???   

So far you've wanted to replace it with a:
- MOV
- PTC thermistor
- 10R 1W resistor
- 1R 5W resistor
- 1N5406 Diode

Hint: A replacement NTC thermistor would work rather well. 

I already bought NTC back then but it blew again due to a shorted diode in the full bridge rectifier thats all...
So I'm trying to fix it with an ntc with a couple of diode s will
I found 1N5406 diode , but no NTC thay were all bought earlier this month...
I need it to power on long enough to get some important files of my computer,so that's it really.

[Ian.M]

I presume it has a custom form-factor PSU otherwise why haven't you simply changed it? 

To get the data off it, you could short out the NTC anti-surge thermistor and make up a a split lead to add an external anti-surge resistor.  Simply cut the lead and take a 4 way terminal block and a kettle lead (the one like an IEC computer power lead but with a notch., and a 5A single pole switch.  Wire the Earth ad Neutral straight through to the PC, and T off the Earth to the kettle lead.  Wire the kettle lead Neutral to PC end Live, and kettle lead  Live to incming Live from the mains plug end of the lead.  Wire the switch across the kettle so it connects the Live wire in the two halves of the PC lead.   A 2000W 220V kettle is a 24R resistor when switched on, and with 2l of water in it would give you a couple of hours running time befor it boiled.  The 5A switch lets you bypass the kettle so it doesn't boil and you can carry on running.  You should close the bypass switch as soom as possible so the PC gets its full rated line voltage.

[Gyro]

I already bought NTC back then but it blew again due to a shorted diode in the full bridge rectifier thats all...
So I'm trying to fix it with an ntc with a couple of diode s will
I found 1N5406 diode , but no NTC thay were all bought earlier this month...
I need it to power on long enough to get some important files of my computer,so that's it really.

Ok, I understand your situation now. From your photos in Reply #4 it looks like an ordinary ATX PSU. It would be safer (for you) to get a replacement, even if you're not planning to keep the computer... or even borrow a PSU from a friend for a few hours?

Edit: Trying to operate the PSU with a shorted diode in the bridge rectifier doesn't sound as if it will end well, even with a kettle in series.

[Ian.M]

If you are determined to attempt repair, get another dead PSU preferably with a 500W rating but at least 400W, and hope its got a good NTC and rectifier diodes.  However a blown NTC + shorted diodes are never a good sign - its likely to have suffered catastrophic primary side chopper failure.   Have you checked for this?   

Personally, I'd take Gyro's advice, and I've repaired something like 30 or 40 PC PSUs over the years I was working in a service shop (we only bothered repairing custom form-factor ones, everything else simply got replaced).  Continuing to try stuff is probably going to end up with a blown motherboard and hard drive.  If I needed the data sooner I'd get a cheap external hard drive case USB=>whatever interface the drive has.

My advice above about a kettle in series ONLY is applicable if you KNOW the rest of the PSU is good and simply cant source the NTC.

[ali6x944]

Thanx all for the advice, good you told me or otherwise I might have lost all the data 
Thanx alot everybody , thanx alot