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This 220VAC to 24VDC 2.5A switching power supply stopped working. I've found the culprit (or at least one of the culprits) but I've no idea what it is.

It is marked TH101. I think it may be a thermistor, but I've never worked with thermistors so I've no idea. I know it's impossible to know for sure without looking at the schematic, but getting hold of that is not going to happen so I need to ask, what this component is, and given the supply's characteristics is there any generic value which would be "good enough" of a replacement?

This component is on the input side of the power supply just past the bridge rectifier:

This component is on the input side of the power supply just past the bridge rectifier

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  • \$\begingroup\$ Can you find any markings on it? I could guesstimate a value for you but knowing the original markings would be best. \$\endgroup\$
    – winny
    Commented Jun 9, 2021 at 21:20
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    \$\begingroup\$ Can you reverse-engineer how it is connected to nearby components? Is it in series with mains to power supply input (i.e. the inrush limiting NTC thermistor) or some other componet (i.e. MOV protection PTC)? \$\endgroup\$
    – Justme
    Commented Jun 9, 2021 at 21:21
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    \$\begingroup\$ @Justme I bet you 10 € it’s series NTC. \$\endgroup\$
    – winny
    Commented Jun 9, 2021 at 21:21
  • \$\begingroup\$ It's in series with one of the rectified outputs of the bridge and the bulk cap which has it's other lead to the other DC bridge output, as for markings the front face is the one that blew up, so markings are just flat-out gone \$\endgroup\$
    – Santiago Perotti
    Commented Jun 9, 2021 at 21:26
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    \$\begingroup\$ Measure the diameter and pin spacing on it. Try to match that with standard commercial offerings from for example Vishay. If you can’t get the part number from the manufacturer (unlikely) or someone else with an identical unit, I would aim for 10 ohm, 250+ Vac and as high energy rating as you can mechanically fit. \$\endgroup\$
    – winny
    Commented Jun 9, 2021 at 21:35

1 Answer 1

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It's a thermistor. Its purpose is to limit inrush current at power on. It has a negative temperature coefficient, or NTC: a high resistance on start-up, which goes down as it heats up.

The NTC value you choose will depend largely on the typical current your supply uses and the size of the input filter cap (that is, the expected worst-case inrush.)

Here's a write-up that discusses this: https://www.powerelectronics.com/community/article/21860750/how-do-you-choose-the-right-type-of-ntc-thermistor-to-limit-inrush-current-for-capacitive-applications

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    \$\begingroup\$ Keyword is NTC. \$\endgroup\$
    – winny
    Commented Jun 9, 2021 at 21:19
  • \$\begingroup\$ It was indeed an NTC, I replaced it with one i took from a power supply which I know it's bad practice but i don't have anything else on hand, the power supply now works, I'll have to leave it an entire day powering something at close to max current to check that it doesn't blow again \$\endgroup\$
    – Santiago Perotti
    Commented Jun 9, 2021 at 22:53
  • \$\begingroup\$ It'll probably be fine. One thing to watch for: is the filter cap getting hot? If so, it might be on its way to failing. When it does this will blow the fuse. \$\endgroup\$
    – hacktastical
    Commented Jun 9, 2021 at 22:56
  • \$\begingroup\$ One of the filter caps was slightly charred, Guess I'll have to replace it later, but at least I could check that the NTC was the only thing preventing the supply from working \$\endgroup\$
    – Santiago Perotti
    Commented Jun 9, 2021 at 22:59
  • \$\begingroup\$ Look for swelling on the end of the cap. If it starts bulging (especially at the top) it's going to fail. \$\endgroup\$
    – hacktastical
    Commented Jun 9, 2021 at 23:25

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