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I have a thermal fuse that is blown (open circuit.) Any textual markings it might have had are worn away, but I'm wondering if the yellow color on the one end is significant. In looking for possible replacements, I've seen yellow, black, tan, etc., but I have yet to find anything that says anything about what the color means, if anything.

Would this color possibly indicate the temperature at which the fuse blows? More specifically, can I replace a yellow ended one with another yellow ended one and have it work as expected?

The circuit board is from a lighted fiber-optic Christmas tree that uses a 12V halogen bulb, so I would think just about any volt/amp ratings on a thermal fuse would be within tolerance. Just trying to figure out the temperature it was designed for.

enter image description here

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  • \$\begingroup\$ Do you have a good explanation for why the thermal fuse operated in the first place? Is it just going to happen again anyway? Maybe you should short out the fuse and see if the unit is still getting excessively hot before you try replacing the thermal fuse. NOTE: obviously shorting a thermal fuse is something that must be done temporarily only and with caution and close observation since the fuse is a protection and safety device. \$\endgroup\$
    – user57037
    Commented Jan 15, 2022 at 20:04
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    \$\begingroup\$ @mkeith It's got a 50W halogen and a fan to cool it. The fan runs (I did short around the fuse to test.) My guess is maybe blocked airflow at some point, or perhaps just the age of the thing. Do thermal fuses deteriorate over time? \$\endgroup\$
    – Dave H.
    Commented Jan 15, 2022 at 20:11
  • \$\begingroup\$ That sounds reasonable. I don't know if they deteriorate over time, but I think they consist basically of a switch that wants to open (spring loaded) and is forced to remain closed by wax with a specific melting point. Maybe the wax could deteriorate over time and allow the switch to open. \$\endgroup\$
    – user57037
    Commented Jan 15, 2022 at 20:54
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    \$\begingroup\$ Note that resoldering the thermal fuse will be challenging. The soldering temperature will trigger the fuse. Normally wires are crimped onto these. \$\endgroup\$
    – Kartman
    Commented Jan 15, 2022 at 23:31

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There does not appear to be a standardized color code. Maybe within individual manufacturers. For example, Cantherm's datasheet for similar products shows multicolored epoxy sealants but does not disclose their coding.

I can easily find clear photos of thermal cutoff with different marked Tf temperatures from 90°C to 100°C with yellow coding.

That said, if it was me, I'd feel fairly confident (given fire extinguisher and insurance situation) in replacing it with a 90°C unit, but not much more- the PCB appears to be FR1 or FR2 paper-based phenolic good for around 100°C depending on grade.

Your choice is up to you- this is a safety-related device so the only 'official' answer can be to replace it with the exact OEM type, and if that is not possible to dispose of the unit properly.

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    \$\begingroup\$ I had also thought about replacing the 50W halogen bulb with a 7W LED drop in replacement to help mitigate any heat issues. \$\endgroup\$
    – Dave H.
    Commented Jan 15, 2022 at 20:04
  • \$\begingroup\$ Halogen bulbs necessarily run at very high temperatures (like 230-250°C envelope) so they can be inherently hazardous, depending on the construction. Recall that the equivalent of 233°C appeared in the title of a famous book about a dystopian future. \$\endgroup\$ Commented Jan 15, 2022 at 20:08
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    \$\begingroup\$ Ah, yes. Celsius 233. Read that one in High School literature. :-) I do recall from previous experience with these halogen spot lights, that they get very hot. That was part of my urge to find an LED replacement. \$\endgroup\$
    – Dave H.
    Commented Jan 15, 2022 at 20:15

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