Thermal Cutoff for DC circuits - Impossible to source? - Page 1
Forum section: electronics / projects

author: mikemontauk
author_rank: Newbie
timestamp: December 11, 2021, 11:37:50 pm
content: I am seeking a thermal cutoff in an axial package to operate in the 24V DC range between 0.25 to 2 amps. I am looking for cutoff (Rated Functioning Temperature) around 80-150C.Much to my surprise, I can't seem to find ANY cutoffs with DC ratings. I found some obsolete Panasonic ones, and Bourns cutoffs that specify for battery cell applications only, but other than that, they all seem to be rated only for AC onlyCan anyone provide insight into this scarcity? It may be driven by physics principles I'm not privy to?Thanks!

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 12, 2021, 12:46:20 am
content: Well, DC tends to arc more than AC does when contacts open. This is because the arcing stops as the AC voltage crosses zero and is why you typically see a switch rated for 240VAC only be good for 32VDC. Same with fuses.If you're going to be ordering a large quantity, the manufacturer may be willing to provide this missing info.

author: bdunham7
author_rank: Super Contributor
timestamp: December 12, 2021, 02:55:53 am
content: Quote from: mikemontauk on December 11, 2021, 11:37:50 pmIf a device can interrupt mains AC at 10A, it will be fine interrupting moderate DC currents below 30-ish volts. However, that doesn't help if you need the rating on paper. I think that it is just a case of the manufacturer not wanting the bother of an additional certification when most customers don't need or want it. The same issue arises with glass fuses, some manufacturers used to self-certify them, but now you just don't see a DC rating. Or perhaps there is some other technical issue, but sustaining an arc with 24VDC @ 2A isn't it.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: December 12, 2021, 03:05:05 am
content: Several of the Schott SEfuse types are DC rated:https://www.schott.com/en-gb/products/sefuse-p1000292Alphaterm does have several types with DC rating:http://www.alpha-therm.com/Not sure about Microtherm:https://www.microtherm.de/en/home.html

author: mikemontauk
author_rank: Newbie
timestamp: December 13, 2021, 01:54:46 am
content: Hello, thank you very much. I am looking into the Schott SM072A as it appears to suit my needs well. You've been a great help. Take care.

author: coppercone2
author_rank: Super Contributor
timestamp: December 13, 2021, 04:25:26 am
content: DC switches/breakers that have 'impressive' ratings have plasma management technology designed into them. Like using a jacobs ladder to move arcs into a harmless area where they can subside. You keep the arc moving to prevent substrate heating and pitting or whatever and you try to extinguish it rapidly and to direct gasses/heat into a dissipation area (its probobly a hard core design for something that looks like a fancy womens hair do).. its a mechanical circuit for sparksYou can only do so much with 'bulk', i.e. heavy contacts, before you run into problems relating to the thermal and electrical properties of the base material (copper), being insufficient to deal with power levels at hand, so then you need to start playing with moving the plasma around. I don't know what a hard limit is but I suspect that some of the high current crazy switch designs would not be used and we do without plasma management if we found a more impressive conductor (higher thermal and electrical conductivity + metallurgically tougher). Perhaps the island of stability will bring us there in a 1000 years. It would need to have drastically different thermionic properties I think.