First microscope DSLR adapter, some small fuses have two wires

[ChunkyPastaSauce]

First image from my 3d printed SLR mount for a used microscope I picked up. It is sightly out as I need to extend the mount a little bit more to get proper focus and the optics need cleaning badly (looks to have sat a long time before I bought).

Everyone probably already knows this but some small fuses have two wire running the length (probably for support?)

[SeanB]

Slow blow fuse. The spiralled wire is to get an increased thermal mass but still keep the surface area large so it can stay cool with slight short term overloads. A very high current leads to rapid heating to melting, while a more moderate overload results in it slowly heating up over a few seconds to minutes to the melting point. A load under the blowing current simply results in it warming up tll the radiated heat equals the generated heat, keeping the wire elements below the melting point.

This enables it to survive a short turn on surge without blowing, but still fail open if the current is only slightly above the design current. Otherwise you have to use a regular fuse with a much higher current rating, which does not then protect against moderate overload until the device fails short circuit, or you have the fuse blowing at turn on every few times.

[ChunkyPastaSauce]

Slow blow fuse. The spiralled wire is to get an increased thermal mass but still keep the surface area large so it can stay cool with slight short term overloads. A very high current leads to rapid heating to melting, while a more moderate overload results in it slowly heating up over a few seconds to minutes to the melting point. A load under the blowing current simply results in it warming up tll the radiated heat equals the generated heat, keeping the wire elements below the melting point.

This enables it to survive a short turn on surge without blowing, but still fail open if the current is only slightly above the design current. Otherwise you have to use a regular fuse with a much higher current rating, which does not then protect against moderate overload until the device fails short circuit, or you have the fuse blowing at turn on every few times.

Makes total sense [see following post, a lot more to it!].

Looked up the fuse, it is an inrush-withstand fuse.

Referencing socfuse catalog, typical quick acting and normal acting fuses are usually solid wire, inrush-withstand/time-delay fuses have a core wire with evenly wrapped outer wire, time-lag fuses have a core wire with non evenly wrapped outer wire.

[ChunkyPastaSauce]

Slow blow fuse. The spiralled wire is to get an increased thermal mass but still keep the surface area large so it can stay cool with slight short term overloads. A very high current leads to rapid heating to melting, while a more moderate overload results in it slowly heating up over a few seconds to minutes to the melting point. A load under the blowing current simply results in it warming up tll the radiated heat equals the generated heat, keeping the wire elements below the melting point.

This enables it to survive a short turn on surge without blowing, but still fail open if the current is only slightly above the design current. Otherwise you have to use a regular fuse with a much higher current rating, which does not then protect against moderate overload until the device fails short circuit, or you have the fuse blowing at turn on every few times.

Makes total sense [see following post, a lot more to it!].

Looked up the fuse, it is an inrush-withstand fuse.

Referencing socfuse catalog, typical quick acting and normal acting fuses are usually solid wire, inrush-withstand/time-delay fuses have a core wire with evenly wrapped outer wire, time-lag fuses have a core wire with non evenly wrapped outer wire.

I found more information here http://www.profuseinternational.com/wp-content/uploads/2012/10/ProFuse-Fuse-School-Module-7.pdf and here https://www.ieee.li/pdf/viewgraphs/overcurrent_protectors.pdf

Surge withstand fuse (the fuse shown):

"A spiral wound element is used for circuits with in rush current. The spiral allows the element to expand and contract without damage under inrush conditions."

"These are compound elements that have been tightly wrapped, or one element is wound around another. The two elements are typically two different conductive materials. Each conductor has a given voltage drop at elevated current. As current level increases, the initially primary conductor can become the secondary conductor, changing the elements operating characteristics. The twisted elements also provide more element surface area than a single element, resulting in more cooling effect. A compound twisted element is similar to an “M” Effect fuse, in that it requires additional energy as the two different metals alloy during the melting process. Surge Withstand fuses are slower than Quick Acting or Normal fuses, but faster acting than Time Delay fuses"

Spiral Wound Time-Delay:

"Element is wrapped tightly around high alumina ceramic core which acts as a heat sink. Ceramic core may be either solid rod or flexible yarn. As element temperature rises with current increase, heat is absorbed by ceramic core. Transfer of heat will continue until core saturation occurs."