Arjan P wrote:RealDeal wrote:I did that and it worked for a while, but I kept blowing fuses on power up. It turns out that one should use a 300mA slow-blow fuse on a US voltage N6. Since switching to that fuse I have not had any fuses blow. I believe the momentary voltage surge at power-on time was blowing the 250mA fast-blow fuses.
What I don't get is, why increase the value AND go to slow-blow? I mean, it makes sense to not use a fast-blow fuse exactly because of the surge at power-on, but a 250 mA slow-blow fuse should be more fitting. Half the voltage (230 > 115), double the current (125 > 250).
Hi Arjan,
If it interests you, I suggest you see
https://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf for further explanation.
Two things wear out the fuse (non-fault conditions):
1. The steady-state current (causes self-heating and slowly boils away atoms from the fuse element). This sets the rated current.
2. The in-rush current at power on (a short duration current surges that exceed the rated current). These are too
slow short to blow the fuse, but cumulative cycles boil off atoms and weaken the fuse element. This sets the blow-time characteristic.
To avoid nuisance blows (due to quick wear out), the designer selects the blow time and current rating based on downstream devices' damage thresholds and protection. Each fuse has a time-to blow characteristic that is a function of its type (fast, slow) and its rated current. Slow blow increases the risk of damage, so usually requires additional protection or higher damage thresholds for downstream devices.
Hopefully this helps answer your question.
Edited to clarify item #2