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Laptop adaptor power rating interpretation Answered

Hi all. My laptop power adapter is rated as follows. -Input:100-230 ac volt, 1.6A -output: 18.5 V DC, 3.5A So what does this mean and how important is to follow?



21 days ago

Jack A Lopez, thank you very much for explaining in detail and with exsmple
Now I understood concept of the power ratting.i can use this know ledge for other home appliance
I was wondering that how sdspter is capable to output 3.5 A while its input current current capacity is only 1.6 A.

So 1.6A is meant only for adapter right?

Thank u once again la.

Jack A LopezSangpoSangpo

Reply 21 days ago

Electrical power is the product of voltage and current multiplied together.


As an example, the maximum power output from you adapter brick, is just the output voltage (18.5 V) multiplied by the maximum current (3.5 A)

Pout(max) = Vout*Iout(max) = (18.5 V)*(3.5 A) = 64.75 W

If I do the same calculation, with the numbers given for the input side, I get:

Pin(max) = Vin*Iin(max) = (100 V)*(1.6 A) = 160 W

I guess what I am saying is, it is not surprising the number for input current is small. I would expect it to be small since it is on the side with higher voltage.

Actually, I can explain that another way.

I expect the power flowing into the adapter, Pin, to be strictly larger than the power flowing out, Pout.

Pin > Pout

Or, another way to say that is, I expect some of the power that flows in will be converted to output power, and the rest will be converted to waste heat, which also is measured in units of power.

Pin = Pheat + Pout

And the reason I expect this to happen is because energy, which is what power (in watts=joules/second) is actually measuring, is one of those things that does not magically appear or disappear. It just changes from one form to another.


Actually, the number 1.6 A, for the input side of your adapter seems almost like it is too high. Like if you actually measured it (with an ammeter or wattmeter) it would never be that high.

There might also be some funny math due to a number called, "power factor"


Technically that is part of the math for AC power calculations.

The other thing is sometimes you just cannot trust the numbers printed on a name plate, and it is better to trust actual measurements, like the kind made with a voltmeter or wattmeter.



Jack A Lopez

Best Answer 22 days ago

The hieroghlyphic scribbles on your power adapter, are just a brief, incomplete, summary of a much bigger picture, which contains many unwritten assumptions.

You might wonder what I mean by, "unwritten assumptions". As an easy example consider an elevator, with a sign inside which reads, "MAX CAPACITY: 25"

That is kind of cryptic, no? Does it mean you have to older than 25 years old to ride this elevator? No. What it is intended to mean is that no more than 25 people should climb into the elevator car, all at one time, because it would make the elevator car weigh so much that it would strain, possibly damage, the machinery that lifts the elevator car, or produce an unsafe operation condition; i.e. possibility of the cable breaking, and everyone in the elevator car falling to their deaths.

What I mean by assumptions is, the fact that 25 refers to a number of people. Another assumption is that these are adult people, of average weight. That's what I mean by assumptions. Those assumptions are not written into the warning explicitly. They are implied.

Anyway, back to the scribbles on your power adapter.

One of the usual assumptions with power sources, is the voltage of the source stays constant, or almost constant, and the amount of current that flows varies, depending on how much current the device wants, which varies depending on what the device is doing; e.g turned on, turned off, sleeping, working hard, etc.

For that reason, numbers written for voltage (V), are assumed to be constants, and numbers written for current (A) are maximum values.

For a power adapter, the number for current on the output side is an expression of the maximum current this adapter can comfortably supply. In other words, if the laptop, or whatever tries to draw more than 3.5 A, this will make the adapter too hot to work properly, or trigger it to shut itself off for its own protection, or explode, or something.

Presumably it is an amount of current that your laptop would never use. In other words, the actual maximum current drawn by the laptop, would maybe only be, perhaps, 2.5 A, and the designers who arranged the arranged marriage (between your laptop and its power adapter) did it this way intentionally, to provide a kind of margin of safety.

BTW, In the event you lose your laptops AC adapter, and have to find a new one, that means you have to be the matchmaker for a new marriage. Choosing an AC adapter with same output voltage spec, and a output current spec greater or equal, to that of the old adapter is a safe bet.

Back to the scribbles, on the input side, the number for current (A), 1.6 A for your adapter, is again a maximum value, and it corresponds to a kind of worst case, the amount of current drawn from the AC mains (the wall outlet) when the adapter is working as hard as it can, presumably in the same worst case mentioned previously when it is supplying 3.5 A at its output, and maybe simultaneous with the mains power voltage being at the extreme low end of its range, i.e. 100 VAC.

Which brings me to the only other number left for me to comment on, is the voltage spec for the input side. That number has a hyphen (-) in it, suggesting it is a wide range of input voltages (100-230 VAC), and that means is the adapter is comfortable with AC input voltage, for all the values in that range, from 100 VAC to 230 VAC.

What happens at input voltages outside that range? Who knows, right? But these are cases not recommended by the manufacturer of the adapter. My guess is too low is safer than too high. Certainly it can survive 0 VAC, when it is not plugged in, or plugged into "air", so to speak.

I mean, back to low versus high, I expect the adapter would have a better chance of surviving the experience of being plugged into 80 VAC, versus being plugged into 250 VAC or 300 VAC.