# What ampère power supply must I use for my laptop charger?

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.

• The 3 Amp fuse can supply around a kilowatt of power. If the charger gets anywhere near that you're in trouble. The fuse will protect the cable from a short circuit in the charger. Otherwise it could cause a fire. Aug 12, 2011 at 17:43

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.)

• Voltage is more akin to pressure, amperage is akin to flow rate (volume per second), watts are a measure of power, watts x time (e.g. kWh) is how much work is done. See en.wikipedia.org/wiki/Hydraulic_analogy#Principle_equivalents Aug 12, 2011 at 13:40
• Also note that the flow rate (current) is directly related to the size of the pipe (resistance). The more narrow the pipe (higher resistance), the less flow (current) can pass through it. This is why a short circuit causes so many problems - it causes a path of near zero resistance, which causes the current to approach infinity (obviously not possible, but that is what the math equates to). Aug 12, 2011 at 13:44
• I can't really agree on any of the three parts of that analogy. @Red describes a much better, and commonly used, "water pipe" analogy. Aug 12, 2011 at 14:24

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.

• Would they make and adapter that burns out the fuse when you use it you think?! That would be weird. And kind of funny!
– skub
Aug 12, 2011 at 12:22
• well, i half suspect it might not need to fuse at all. Generally, the 'standard' fuse size is 15A for normal devices, and 30 amp for high current ones like airconditioners. Aug 12, 2011 at 12:35
• @Journeyman Nope, it's only 3.5A on the DC side. The AC rms current will be much lower. Aug 12, 2011 at 13:19
• 13 amps is the standard in that case. Aug 12, 2011 at 13:55
• The device in the image has a UK plug. In the UK, domestic power outlets are limited to 13A. Domestic appliances with higher current requirements (e.g. electric ovens) do not have plugs, they are permanently wired by an electrician. In the UK, double insulated devices still have a fuse in the plug to protect the cable from outlet to device in the event of an internal short between live and neutral in the device. Aug 12, 2011 at 14:08