90W power adapter

I just happened to look at couple of my 90W adapters and I noticed that while most (3) were having input rating of 2.4A, one stated input from 2.5A to 0.5A, but the last is saying 1.5A

so looking at the last one I'm thinking efficiency went up between the adapter revisions ... by a lot ?

 

The switch mode PSUs are quite efficient but the newer PSUs do have a slightly higher efficiency rating (probably a lower standby power consumption). I've never seen my T420s with 65W PSU draw more than about 66W from the mains socket.

Some of the nominal power ratings are unnecessarily conservative. 1.5A @ 110V is 165W which would leave 75W as heat if only 90W is used by the computer. However, there may be a 1.5A surge for a fraction of a second when the PSU is first plugged in.

John

 

if you're measuring 66W input with your 65W adapter then either your measuring device can not catch short increases in power consumption or your adapter does not output 65W at the time of the measurement (most likely).

In general, impulse power supplies have around 75% efficiency at most.

What I'm trying to say is for the engineers to cut down almost 1A from the input while still being able to produce the same output and in the same form factor (size) is rather impressive Either that, or the sticker lies .. lol


P.S. 1.5A at 110V is not 165Watts, it's less than that - this is AC power on the input, not DC.

 

I should clarify that I don't know if the computer is actually drawing 65W. I'm just saying that 66W is the highest I have seen being taken from the mains socket. It's plausible that the computer only needs 50W.

There are ways to improve the efficiency. One factor in PSU design is the heat dissipation. Larger, or slab-shaped PSUs have more surface to conduct and radiate heat (it's also one reason why they are usually black). As the efficiency increased then they can be made smaller, subject to enough space for the components.

Watts = volts x amps. The amps is a maximum rating. But if your older PSU actually used 2.4A at mains voltage for any significant period and passed only 90W to the computer then the remaining 174W would have to be emitted as heat. I think it would be glowing red. Did you ever try holding a 100W light bulb while it is working?

John

 

This ^ is only true for DC systems.

For AC (such as the input part of the adapter): Power=Volts(x)Amps(x)cos(phi), and the last is always less than 1. And all this happens due to the phase difference between the voltage and the current that flows.

so with 90W power supply all you know is the device will be sufficient to supply 90W at its output at the preset voltage (i.e. 4.5Amps max in this case, DC), but to do that it will take more as input considering its efficiency. So if you take 75% efficiency then your 90W adapter will draw 120Watts as input when used up to its potential, with the 30Watt difference heating up the unit. And those 120Watts means input current of more than 1.1A (at 110V) due to what I just explained. Hence you see input of like 2.4A max on the label.

which brings me to my point that dropping from 2.4A to 1.5A is impressive

 

I think it's more a case of being realistic. If the PSU used 2.4A @ 110V (whatever the power factor) for any significant period then it would get dangerously hot unless it was also the size of a real brick.

John

 

Err... just to point out, the 110V is a RMS value.

 

true

Recently I bought new(ish) 90W adapter for the GFs Dell D630 as well, and it also draws 1.5A for output of 4.62A max at 19.5V

now I'm wondering what other people's 90W adapters show on the sticker, lol.

 

I can't follow the calculations going on here, but there you are...

Input: 1.7A-0.9A for output of 4.5A. That's the 90W adapter that came with my T60 five years ago.