Can you bend PTC fuse's flat so that they lie on the PCB? - Page 1
Forum section: electronics / beginners

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 03:45:13 am
content: Hi everyone,As the question says. Can this be done or will it affect the fuses thermal performance and make it trip prematurely? I want to use a PTC fuse in my design but I have clearance issues.Cheers

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2021, 09:20:43 am
content: Why not use a surface mount part, which is flat to start with?

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 09:29:37 am
content: Looking at stock on digikey, it seems any that are rated over about 5A and with a low enough resistance have to be radial type.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 22, 2021, 09:46:52 am
content: At higher voltages and/or currents, through-hole is the only option.As long as they are bent clear of the body resin to prevent cracking, and you don't bend them over a hot part I don't see any issue.Ask yourself if the mounting method can cause heat to conduct in or out of the fuse more than if in free air. e.g. use a foam pad and not glue if you need to secure it.Polyfuses are not precision devices, so a small change in thermal environment isn't going to be a big deal

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 11:12:39 am
content: Awesome, thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2021, 01:28:44 pm
content: It'll mind a whole lot less than a chip fuse over (or not) an inner ground pour, heh.Mind that those things take a very long time to open (100s ms). Keep this in mind if you are combining them with any other protective features (e.g. crowbar).Tim

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 01:42:24 pm
content: Wait? Are you saying chip fuses are thermally actuated as well? Well I guess all fuses are, but are you saying that a board with a bit of heat in it (in my case a 100mm2 board with around 3W of waste heat) can derate a fuse to the extent that it will actually blow early?

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2021, 02:11:07 pm
content: PTC resistors are more temperature sensitive compared to real fuses because they trip at a much lower temperature. A fuse has to reach the melting point of the wire used to break, whilst a PTC resistor just has to get to the point when the current is really low.Surface mount parts are better thermally coupled to the PCB, than through hold devices, so will be more affected by the temperature of the board.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2021, 02:43:01 pm
content: A 10x10mm board at 3W sounds like it'll be well over the operating temperature of the PTC to begin with..?And yes, mind the derating of those. They don't open at very high temperatures; every degree of ambient closer to that point, is less current it will open at.Which can be handy, say to modestly extend the safe working range of a current-limited switch: a PTC in series with the collector of a BJT current source, acts as a thermal foldback limit, and with the fuse very close to the transistor (preferably poured around with copper to increase thermal coupling), its own heat accelerates the fuse's operation.Tim

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 09:54:14 pm
content: Quote from: T3sl4co1l on August 22, 2021, 02:43:01 pmAh, I see. Interesting stuff. And yep, had a bit of a moment there and forgot how dimensions work. I meant 100mm x 100mm. And even then, it's probably more like 2W

author: julian1
author_rank: Frequent Contributor
timestamp: August 22, 2021, 10:13:23 pm
content: I was looking at through-hole PTC fuses recently, and was really surprised by their temperature sensitivity.Eg. Polyfuse 60R300 is nominally rated at 3Ahttps://www.farnell.com/datasheets/692303.pdfUnless I am reading it incorrectly, hold current is 3A at 23C. But 2.43A at 40C, and 2.16A at 50C.So placement with respect to fans, heatsinks, and proximity to board surface etc will all be an influence.

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 10:44:09 pm
content: Quote from: julian1 on August 22, 2021, 10:13:23 pmYeah, that's kinda why I asked the question, I was wondering if anyone had any experience with how well a bent over PTC is thermally coupled to the PCB. Seems like the only way to know for sure is to buy one and try it out.

author: julian1
author_rank: Frequent Contributor
timestamp: August 22, 2021, 11:16:20 pm
content: I think I would be surprised if 3W (P. typ) being reflected/radiated off the board surface and with less ventilation didn't send over 50C.They just seem like a really blunt instrument.

author: logancane
author_rank: Regular Contributor
timestamp: August 22, 2021, 11:19:11 pm
content: Quote from: julian1 on August 22, 2021, 11:16:20 pmHmm, yeah. Like only good for applications where they really aren't going to be constrained in any way (i.e. they need to be allowed to stand up and be mounted on a relatively cool part of the board, I think I'll just give them a miss.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2021, 04:27:54 am
content: Ah yeah that makes all the difference, 100mm sq. with a couple watts is negligible, and in still air and ordinary ambient, it will operate quite nominally.Tim

author: james_s
author_rank: Super Contributor
timestamp: August 23, 2021, 05:33:28 am
content: Why don't you try it with a piece of protoboard and the PTC fuse you're considering using? I'd agree with what others have said, that it probably won't cause any problems, but sometimes there's no substitute for actually playing with the part in question and developing a feel for how it behaves.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2021, 07:35:52 am
content: Any glue or potting compound drastically changes a PTC's characteristics and that extra "heatsinking" can even make it not work.This is a problem if OP glues or tacks the PTC to the PC board. I did a design various styles potted with many tests and experiments.

author: logancane
author_rank: Regular Contributor
timestamp: August 23, 2021, 06:22:03 pm
content: Quote from: james_s on August 23, 2021, 05:33:28 amSeems like the only way. I wanted to order everything at once and maybe I still can but I'll just need to get a few different sizes with the same packaging.