Why does a physically larger fuse have greater voltage loss? - Page 1
Forum section: electronics / projects

author: bdlow
author_rank: Newbie
timestamp: March 26, 2023, 06:43:28 am
content: MIDI, MEGA and ANL fuses abound in the automotive world; as the name of the first two suggest, the MIDI is physically smaller than the MEGA, which in turn is smaller than the ANL.I'd have expected physicallysmallerfuses to have (slightly)greaterpower loss than larger fuses - that is, a MIDI fuse to have greater voltage drop than a MEGA fuse of the same rating. The rationale being that smaller contact area/etc would lead to higher resistances in those non-fusing elements, thus greater overall losses. I would have expected the actual fusing element to be basically the same for all fuse sizes, assuming the same materials.However Victron's data sheet says otherwise:https://www.victronenergy.com/upload/documents/Datasheet-Midi,-Mega-and-ANL-fuses,-and-fuse-holders-EN.pdfe.g. MIDI fuse 100A = 60 mV (= 6 W @ 100A); MEGA fuse 100A = 90 mV (= 9 W @ 100A)Littelfuse specs broadly agree: 100A MIDI = 60mV; 100A MEGA 87mV (suspect Victron probably OEM Littelfuse)https://www.littelfuse.com/products/fuses/automotive-passenger-car/high-current-fuses.aspxThe only explanation I can come up with is that a physically larger fuse would also be a better heatsink, drawing heat from the fusing element that would lower its effective fuse response. That would mean the physically larger fuse (more accurately, the one with the larger 'heatsink') would need to have a smaller fusible element to get the same fuse response as the smaller fuse; ergo higher resistance across the fusing element and thus greater voltage loss. Or maybe not ?Anyone intimately familiar with fuse design?The conclusion I draw from the above is to prefer smaller fuse sizes, i.e. don't go physically larger until you have to. e.g. MIDI over MEGA over ANL.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2023, 07:08:04 am
content: The heat sinking thought make absolute sense. The simple designs and similar technology would trip the fuse at about the same temperature of the fusing element. With a larger physical size it would be normal to expect more power needed to reach that temperature and thus more voltage. There can still be details in the design to effect the tripping power: one could still have a large size but better thermal isolation and there could also be a lower tripping temperature before the element breaks (e.g. more stress or a lower temperature material).

author: alm
author_rank: Super Contributor
timestamp: March 26, 2023, 07:18:00 am
content: I'd guess a combination of heat sinking and the heat capacity of the larger wire. A larger heat capacity needs more power to heat up in the same amount of time.

author: Alti
author_rank: Frequent Contributor
timestamp: March 26, 2023, 08:41:07 am
content: Quote from: bdlow on March 26, 2023, 06:43:28 amOf same construction and standard - no. Usually the bigger the fuse housing, the more power is dissipated at thermal equilibrium.There are industrial fuses that have exceptionally high power dissipation for the size at rated current and also there are fuses that are optimized for reduced power loss and still fit big housing. But in 12V automotive there is no need for such unusual requirements. Maybe in EV but not 12V automotive.

author: bdlow
author_rank: Newbie
timestamp: March 26, 2023, 09:11:14 am
content: Seems like the 'heatsink' explanation is deemed reasonable, and yes can see that a designer could optimise for particular applications.It is certainly counter-intuitive though - when it comes to most 12V stuff, usually bigger is better ;-) (bigger cables, bigger switches, bigger relays, etc; budget excepted)

author: Siwastaja
author_rank: Super Contributor
timestamp: March 26, 2023, 12:42:03 pm
content: I think it's pretty intuitive that keeping a larger piece of material near its melting point consumes more power than a smaller piece.

author: jonpaul
author_rank: Super Contributor
timestamp: March 26, 2023, 01:09:24 pm
content: Unfamiliar with the models mentioned,Main difference in voltage rating, eg VOM 1000V rated totally different characteristics than 250VAC or a 32VDC fuseAlso, fast, medium, dual, slow blo have totally different R, VdropSee Littlefuse or belFuse catalogue and applications notesJon