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Does somebody recognize the component labelled with "1/4"? It measures 6 mm x 2.5 mm (the white body only about 2 mm though).

It is connected between the positive pole of a 9v battery and the rest of the circuit like a fuse would be.

Is it more likely to be a traditional fuse or a PTC? What is the "1/4" label likely to mean?

enter image description here

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  • \$\begingroup\$ How likely it is to be a PTC or fuse depends on the product. But these days I'd say there's little way to guess which it is without more details. The 1/4 could mean that it is rated for quarter of an amp (but it could also mean it was made in the first quarter one year, or it made at factory or production line 1/4) \$\endgroup\$ – Puffafish Sep 16 '20 at 10:13
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    \$\begingroup\$ @Puffafish I thought maybe somebody could tell me that e.g. PTC never come in this type of package... \$\endgroup\$ – ARF Sep 16 '20 at 10:54
  • \$\begingroup\$ @ARF I don't suppose the component labels are under the components, are they? \$\endgroup\$ – Andrew Morton Sep 16 '20 at 11:48
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    \$\begingroup\$ Indeed, PTCs never come in that type of package, and fuses do. It wouldn't make sense for a PTC to be designed with that shape - surface-to-volume ratio is too low. \$\endgroup\$ – pericynthion Sep 16 '20 at 18:06
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Chances are very good it's a 250mA one-time fuse. I don't think anyone can say that resettable fuses NEVER look like that, but I can say that every fuse I've seen that looks like that is a one-time fuse, and most small SMT one-time fuses do look like that. Eg. these ones

enter image description here

Edit: I didn't notice the bel (manufacturer name) on the ends of these, as well as some other markings, until the photo was enlarged. Maybe OP's fuse has similar markings. Here is a datasheet for a fast-blow bel 0679L fuse which is similar.

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  • \$\begingroup\$ I agree for the same reason...haven't seen a PTC that looks like that. \$\endgroup\$ – DKNguyen Sep 16 '20 at 18:06
  • \$\begingroup\$ This site never ceases to amaze me! You actually did find the exact part! After you pointed out the "bel" marking I managed to find it on my part with a magnifying glass! Many thanks for your help. \$\endgroup\$ – ARF Sep 16 '20 at 19:07
  • \$\begingroup\$ Do you know by any chance whether bel and ESKA are the same manufacturer? In your first link they say that the manufacturer is ESKA but the image they use show bel fuses. I find this curious since ESKA seems to make fuses that look almost identical. ESKA Datasheet That said, my part is clearly from bel... \$\endgroup\$ – ARF Sep 16 '20 at 19:14
  • \$\begingroup\$ No, I don't know if there is any relationship between the two. One could be reselling fuses made by the other. \$\endgroup\$ – Spehro Pefhany Sep 16 '20 at 19:22
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    \$\begingroup\$ There are even SMT fuse holders for that style of fuse. \$\endgroup\$ – Ben Jackson Sep 16 '20 at 21:24
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Anecdotally, this does definitely look like an SMT fuse to me. Not a PTC.

SMT PTCs often come in a much flatter package (similar to typical SMT resistors), while fuses often tend to come in a ceramic square profile package. I am sure there are exceptions to both, but this seems to be the case for most parts I've encountered so far.

Try to explore part manufacturer's or distributor's websites and compare what fits this the best. Perhaps look at littelfuse's website here for fuses and here for polyfuses and compare what looks close to this.

The "NANO2" 451/453 series at least also do come in a 6.1 mm x 2.7 mm x 2.7 mm package, which also matches your measurements.

Some of these do indeed come in a 0.25A rating, which is what the printing likely refers to.

Fun fact, there even are sockets for these fuses! Sadly these seem to be used rather rarely; I only ever saw them on some once-very-pricy telecom boards.

Disclaimer: I have no affiliation with littelfuse, I just find their product pictures nice to illustrate this.

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