Resettable fuse equivalent to 500 mA glass fuse ?

[stuarth]

Normal fuses and PTC devices operate quite slowly, fast enough to prevent a fire, but probably not fast enough to protect semiconductor devices.

The Energiser Ultimate Lithium (Lithium Iron not Lithium Ion) 1.5V AA cells include a protective PTC in each cell to protect against the cell, or presumably several cells in series, being shorted out and causing a fire. Around £7~8 for 4 from most supermarkets. Expensive way to buy the PTC, but no postage charges!

Stuart

[Jolly 7]

If you do happen to have a picture of a dismantled lithium iron battery with its fuse, can you please share it. It would be interesting to know what rating of PTC fuse they have used.

[Tractionist]

Originally Posted by Jolly 7

@Tractionist: To the best of my knowledge, fuses buried inside the mains transformer could be hidden under insulation tape windings and are thermal fuses, not PTC types. I once had a similar problem with an open winding primary for a wall wart type adapter. I cut open the insulation tape around the primary but could not find any thermal fuse.

There was some discussion on the forum earlier this year in relation to hidden mains transformer fuses. I may have started a thread too a while ago with photos, but currently cannot locate it.

Please watch the YouTube video below from Grants TV repair. It might be helpful.

https://youtu.be/4HoHERN_-DE?si=4ezw5osnApJTpzAV

Cheers Jolly,

Much as I suspected ... a thermister 'of sorts'. I've now fixed the unit with a replacement cannibalized part - but it seems a pity to abandon/scrap a good TX with all three secondary rails intact and a blown 'fuse' on the primary eh? (I may yet cut it open ....).

[Jolly 7]

Congratulations on your successful repair. Do keep us posted if you are able to locate the 'fuse' in your scrapped transformer. I have a whole bunch of scrapped transformers myself and just cannot throw them away ( :

[stuarth]

No, I haven’t disassembled one of those Lithium AA cells. From the data sheet, I think they trip at around 2A, low enough to prevent a fire, but way above your 500mA fuse. Energiser talk about 3A pulse loads and say it’s affected by temperature, proximity to other cells, etc.

It is just there as a safety cutout to protect the battery, a bit like the consumer unit for household mains. Protecting the load would be something else, and probably dependant on the individual projects you intend to use the battery to power.

Stuart

[Jolly 7]

Originally Posted by trobbins

Sorry for all the queries - just making sure I appreciate the application.

Given the likelihood that a fault would be internal, and worstcase could be a short across the battery then a PTC should 'mitigate' the outcome, and hopefully then prompt an investigation, and as indicated above, best to replace the PTC at the same time. These are physically small parts, and I've seen them in ss amps where repeated exposure to fault currents have damaged the PTC - so somewhat easy to faultfind !

The part's Hold current rating needs to be derated due to local ambient, and any other tolerance, to avoid operational hassles. Colder temps shouldn't alter a fault outcome, but hotter ambients could cause hassles. For that RXEF range, as an example, a 500mA Hold part should be ok for 220mA nominal out to 70C.

If you use a different battery config, with a different operating current, then assessment gets a little more onerous as to what the max operating current could be, as that is what determines the PTC part choice. I'd imagine that any battery config would cause a worstcase fault current that is > 10x the part rating, so that aspect may not change.

Just be mindful that all fault scenarios may not cause a short circuit of the battery through the PTC, as some faults may include series resistance that limits the fault current to a level where the PTC may not operate as intended.

No worries. Thanks for delving into this. I might be able to obtain some polyfuses that have a hold current of 170 mA and trip at 340 mA. I can also source ones that trip at 200 mA, but I guess they are probably not suited for the current requirements of my circuit.