Any way to identify a failed (simple/cheap) thermistor?

[ja1870]

Hi all,
Just presuming it is a thermistor, and NTC, as when contacts are jumped (so zero resistance), the currently *not-moving* 12V case fan revs up to full speed.
By eye (FWIW) about 1.75mm x 2.5mm, and no writing noticed.
This is from an external 3.5" disk drive enclosure with a cooling fan, and the "thermal probe" - as it was called in the original installation manual - is held against the top-side of the HDD near the spindle.
Hoping to simply replace it with a close-enough component.
Thanks.
Edit: after some further random visual-type web-searches, more definitely looks like it is a "bead-type" thermistor

[redwire]

Re: Any way to identify a failed (simple/cheap) thermistor?

Looks similar to a Japan-made, older Semitec part.
Blue body and white marking would be a 103AT-2 10kΩ@25°C, B=3435K, see Radio Shack 271-110.
You can also use a potentiometer in place of the thermistor to get a ballpark on its 25°C resistance.

[ja1870]

Re: Any way to identify a failed (simple/cheap) thermistor?

Much appreciate the response. Very interesting thought re: the potentiometer.

(Was actually wondering whether the white tip was OEM marking or perhaps some sort of failure indication.)

[redwire]

Re: Any way to identify a failed (simple/cheap) thermistor?

These thermistors rarely fail, it's mechanical failure like a crack or tear from bent leads that ruins them. You can measure the one you have with an ohmmeter and see if it is working and the wiring is good.
It might be that the temp sensor works fine but it's the fan circuit that is lazy upping the speed. They are usually not great designs.
Is that text writing on the edge, looks like 504 or something?
These are penny parts and the only one with a blue body and white mark is Semitec 10k.

[ja1870]

Re: Any way to identify a failed (simple/cheap) thermistor?

Certain it is the Semitec as you suggested. Already ordered.

No writing, so probably jpeg compression artifacts. And infinite resistance on the thermistor.

Don't laugh too hard because I'm no pro...but went to the basement to extract a dusty old 150-in-One RadioShack Electronics Project Kit. Used a DMM to note the resistances at the potentiometer number-marks; wired it in, and found the fan went on at 5-6 kΩ.

According to the spec sheet for the Semitec 103AT, that corresponds to between 40° and 45°C which make sense in this context.

Do still prefer this 15-year-old enclosure because of the additional features and connections, but suspect the design was more for overheat protection rather than precise temperature regulation. By ear, the fan was pretty much either on or off. And the case already has a heat-sink which attaches to the HDD.