Fuse size of laptop power adapator

More information required.  

Is that 20V 4.5A DC or AC?
Where are you planning on putting this fuse?  Between power adaptor and laptop?  Or on the main AC line from the wall?
And why do you need a fuse?  Laptops use the fuse or circuit breaker in the wall power supply to protect themselves rather than relying on a fuse elsewhere (this is true of probably every household appliance).

It is a power adapter of a Lenovo X1 Carbon. The fuse inside the power socket is broken and i need to replace it , Thx

then it is usually replaced with the same value

Just replace it with the same one that is blown. If with Power socket you mean the laptop's power supply, then replacing the fuse may not be a good idea. Fuses usually blow when there is something wrong with the circuit it supplies in order to prevent further damage or fire. So you would first have to make sure there is nothing else wrong with the power supply or laptop itself, and repair that first.

Actually, I replace it with 3A but it blows again. Just wonder whether the user has swapped a wrong cable. The fuse is 3A but the label in the adapter tell that it can output 4.5A...

Buy another charger or adaptor for the Lenovo.  Stop trying to fix the charger/adaptor you are playing with.

You don't specify if the 3A fuse is on the output or input side of the circuit as well.  If it is on the input side (the side from the wall) then you don't put in a bigger fuse.

This is the output marked on the power adapter of the laptop. Input current is 1.8A.

I believe the adapter change the AC to DC...

Thx

As I already mentioned, fuses blow for a reason, and that is usually because there is something wrong with the circuit. So changing the fuse won't be likely to repair the cause for the fuse having blown. You must first repair the circuit before replacing the fuse, or it will blow again right away.

The adapter does change the AC to DC.  That is the 18V 3.5A output.

The fuse can either be for the AC or for the DC.  You haven't said which.  If you don't know what the original fuse was then don't try a bigger fuse.  Fuses also vary depending on what they are designed for: quickblow, slowblow, constant current.  You don't mix the types up.

Note rindi's comment above.  He's repeated it twice.

I'll repeat it.  If the fuse is blowing constantly then there is something wrong elsewhere.  Something major.  Get a new adaptor.

i suspect that the AC adapter is bad - you can test with the notebook NOT connected if it still blows
in that case - buy another adapter

20 volts at 4.5 amps is 90 watts max output power of the adapter.  Allowing 50 percent additional wattage for voltage conversion inefficiencies means that you need to supply about 135 watts total input.
 
If you are running 120 volts mains then you need a fuse with a max if 1.5 amps.  If you are running 240 volts mains, you'd only need 0.75 amps fuse.
 
The fact that you are blowing 3 amp fuses almost certainly suggests a failure in the adapter unit... it has a short in it would be my guess.

Thx.

A quick question for fuse. From the label of the adpater,

Input  : 100 - 200V ,1.5A  50-60Hz
Output : 20V 4.5A

So the fuse size should be on Input size rather than output, correct ?  

Beside, will there b a possibility that a big power demanding appliance that requires large current and cause the laptop adapter to break the fuse ?  Thx

>>  So the fuse size should be on Input size rather than output, correct ?  <<  since the adapter provides 20V at 4.5 A = 90 W, the fuse should be on the AC side for 100 V= 100/90 = 1.1 A

>>   a big power demanding appliance that requires large current and cause the laptop adapter to break the fuse ?   <<  the fuse is normally broken by loads taken from that line; on an AC adapter for a laptop there are no other loads

The logical place to have a fuse is at the beginning or input of a circuit, as only that will cause it to blow if there is something wrong with the circuit coming after it. If the fuse were at the end of the circuit something could fail before it and the fuse won't blow, that causes fires, which the fuse is there to prevent.

A quick question for fuse. From the label of the adpater,

Input  : 100 - 200V ,1.5A  50-60Hz
Output : 20V 4.5A

Your fuse should be 1.5 to 2.0 amps... it says right on the adapter.  It doesn't have to be exact.  When you get a short circuit, current draw far exceeds the amperage of the fuse, whether 1.5 or 2 amps, and it blows.
 
If you were to use a 15 amp fuse or wrap the blown fuse in aluminum foil, then it would carry a lot more current and something else within the circuit would melt or blow up instead.
 
You can do a simple test.  Put a fresh fuse in the adapter.  Do NOT plug in the adapter to the computer or whatever else it powers.  Plug the adapter into wall power.  Does the fuse blow?  If yes, the adapter is certainly bad.  If not, plug the adapater into the equipment.  Does it blow now?  If yes, then the problem lies within the equipment, not the adapter.

that test is exactly what i posted before

Thx