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  #1  
Old 11-16-2017
Inverhyt Inverhyt is offline
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Default Replacing a Thermistor in a bad PSU

Hello!

So being the creative soul that I am, I stripped a bunch of cables on a Raidmax RX 530SS so I could use it as a DC Power Supply for testing circuits. Unfortunately, being the idiot I am I shorted it two weeks after I started using it as one. I pulled apart the supply and checked the fuse, but there's solder put haphazardly everywhere so I desoldered the fuse, and checked it. It's alright.

So I checked an NTC Thermistor as a last-ditch effort on the mains circuit (or whatever you call the input of the power supply) and it's completely open. No resistance, no continuity (but don't they not have continuity anyways???). It is wired between the ground (I assume) and positive terminal.

https://1drv.ms/i/s!AgqD9tj0PzzmlJlybyit_IgkqW72eg

So I have a strong temptation to just find any old NTC thermistor and solder it in its place, but experience tells me that that train of thought usually causes fires. It doesn't have any sort of identifier on it. How can I find what thermistor I need to replace it with?

Thank you for any help! I'll do my best to provide pictures and help.
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Old 11-18-2017
TELVM TELVM is offline
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Quote:
Originally Posted by Inverhyt View Post
... No resistance, no continuity (but don't they not have continuity anyways???) ...
NTC thermistors are resistors whose resistance varies in function of temp, the colder the thermistor the higher the resistance, the hotter the lower.

It should read some resistance when cold, then when you touch it with your fingers (warmer than ambient) you should watch its resistance decrease slowly.

Quote:
... It is wired between the ground (I assume) and positive terminal ...
Nope, the NTC thermistor must be in line with the positive/line/phase (black/brown).

If there is something resembling a NTC thermistor across line and neutral beware that might be a MOV instead.


If problem is busted up NTC thermistor, you may replace it with another functional one scavenged from a PSU of similar wattage.
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Old 11-19-2017
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Per Hansson Per Hansson is offline
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As a test you can wire in any size NTC thermistor, or even just bridge it for a test.
But if you don't have a suitable resistor putting something that will limit the current is a good idea.
You could wire a 60w incandescent light bulb in place of the NTC thermistor.
If that turns on brightly you have a short somewhere in the PSU.
This is quite likely btw when the NTC has failed.
Of course you could verify that with a multimeter as well...

Anyway, if it's not a NTC but a MOV replacing that with a wire will create a dead short, so be careful!
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