I have very little idea about electronics. Today I received a replacement charger for my laptop as the old one had been broken, it did not come with the plug attachment. I went to set it back up today and I found I had 2 different plug attachments, one had a 3 amp fuse and the other had a 5 ampère fuse.
Click here to see the techincal specifications of the laptop charger in questions.:
Which of the plug sockets is the correct to run off? The 3 ampère or the 5 ampère?
3.5A at 18.5V is the rated output at the DC end, with a power of approximately 65W. At the AC end, ignoring losses, the equates to a current of about 0.28A (assuming UK mains voltage of 230V). So, the 3A fuse would be best for protecting against faults.
It should be noted that the amount of power your laptop actually draws from the charger depends on the laptop itself, not the charger. Chargers are only rated to provide a certain amount of power, so in your case, 65W (at the rated 18.5V and 3.5A). If your original charger had a higher power rating then your new one, it may heat up more then you expect (this is why chargers are only rated for a certain amount of power). Do keep this in mind, because if you are infact using one rated for a lower power, it may break faster (due to the heat).
Now, what comes out of the wall is proportional to the the voltages. Assuming a 120VRMS mains frequency, the power drawn from the wall (assuming the laptop draws the equivalent of 65W at 18.5V) would be 3.5A * (18.5V/120V) = 0.54A. The current drawn is lower as the supply voltage increases (Joule's law), so if you're in the UK, it would be even less than that. If you want to be safe and account for losses in the charger, tack on an extra 15-20%.
With respect to the fuse, with that current value we calculated, the current drawn should never exceed 3A. Why? If you were to use 3A from the wall, and assuming your mains voltage is 120VRMS, the maximum power that could be supplied (assuming no losses) would be P = IV = (3A)(120V) = 360W which is way more then your laptop could possibly use. Indeed, if it was drawing this much, then there would be a huge problem (either with the charger or your laptop).
TL,DR - use the small fuse. No laptop charger should ever draw 5A (regardless of where you live).
I assume the fuse fits into the wall plug, not the laptop cable end? In that case I'm even amazed that they'd supply a 5A fuse, as the power supply shouldn't draw more than an amp.
Start off with the lower ampere one, you wont damage anything by having lower amperes to the device (too many is where things fry) and the fuse is helping protect overloads. Amps is the force of the electricity pushing, watts is how wide the pipe is, and volts is how much work can it do - they are all related to one another.
(Disclaimer: I'm from the west and am not very familiar with UK plugs, but our electricity is mostly the same - except for 50Hz vs. 60Hz.)
Considering that the current draw is 3.5 amps, the 5 amp fuse would be a good bet - 3 amps might be too low and burn out. I'd note the 'standard' fuse for that kind of plug is 13 amps, and nearly every device i've seen uses that, 30A (for airconditioners) or no fuse at all (euro plug)
In addition if the charger is double insulated, or the connector from mains into the charger is 2 pinned, then your fuse rating will not matter at all - there would be no connection between the fuse, and well, anything at all.
