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T540p power supply question.

T430-T490, T530-T590 Series
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Quardah
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Posts: 92
Joined: Sat May 09, 2015 1:52 pm
Location: Ahuntsic, Montréal, Québec

T540p power supply question.

#1 Post by Quardah » Tue Jun 20, 2017 9:45 pm

Hi all. This is a question for y'all electric enthusiasts.

I know running a lower wattage AC adapter can be damaging for the computer but i have a question regarding this.

I know that all the T540p model ship with a 90W power supply adaptor regardless of the chip's power consumption (35W or 47W). But the problem is that it's a really heavy cumbersome brick to carry around (about three times the weight of a 65W ac adapter!)

I presume Lenovo ships de facto the 90W in prevision of the worst case scenario happening; when you actually need 90W.

Now i would like your advice as of carrying instead my 65W power supply with me on my daily basis. Yes, before knowing this was potentially dangerous for the laptop i acquired a 65W from kijiji locally simply to carry one and have one at home at all time (This is what i had with the previous T420).

Consider the following :
- My T540p has a 35W chip (i7-4600M).
- I removed the HDD (using m.2 ssd).
- Running under Linux, the second GPU (the 730M) is off on demand (using Primus).
- Screen is only 1080p (not the 3k screen).
- Bluetooth is disabled in BIOS (might save some Watts).
- I rarely use the CD drive bay which i think sucks a lot of wattage, and when i do, usually the GPU is off.
- I used a 65W PSU daily for the last couple months, and nothing happened.

Here this guy says it's dangerous only once the laptop asks for too much power. Is this true?

I'd like to know if you think i should be ok with a 65W? And if possible, is it safe to even go lower with a 45W brick instead? (The brick is even much smaller)

Also, do you know if there's a way to do a power assessment for laptops? Is it linear? Like if it's 90W MAX but i have a 35W chip instead of 47W does that mean i can never go over 78W because they planned ahead of me?

Thanks.
Daily driver : P16s AMD Gen1 Ryzen 7 6850u Pro - Arch Linux.
HTPC : T540p 3K Screen - i7-4600M - Windows 11.
Have : X120e, X24.
Had : T420 - 240 - IBM i Series type 1171

Thinkpad4by3
Senior ThinkPadder
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Posts: 2670
Joined: Sun Aug 09, 2015 9:25 am
Location: N. Bellmore, ny

Re: T540p power supply question.

#2 Post by Thinkpad4by3 » Thu Jun 22, 2017 8:50 am

If you want to check power, you can use a kill a watt. Check the power consumption on the 65 watt and 90 watt adapters. If 65 reads 65+, the laltop draws 65+ watts. The 90 watt adapter should be 10% higher than the actual laptop because of the internal circuitries.
Thinkpad4by3's Law of the Universe.

The efficiency of two screens equally sized with equal numbers if pixels are equal. The time spent by a 4:3 user complaining about 16:9 is proportional to the inefficiency working with a 16:9 display, therefore the amount of useful work extracted is equal.

Quardah
Freshman Member
Posts: 92
Joined: Sat May 09, 2015 1:52 pm
Location: Ahuntsic, Montréal, Québec

Re: T540p power supply question.

#3 Post by Quardah » Thu Jun 22, 2017 10:45 am

Thinkpad4by3 wrote:
Thu Jun 22, 2017 8:50 am
If you want to check power, you can use a kill a watt. Check the power consumption on the 65 watt and 90 watt adapters. If 65 reads 65+, the laltop draws 65+ watts. The 90 watt adapter should be 10% higher than the actual laptop because of the internal circuitries.
Thanks.

I'll try to acquire one of these. This is the EXACT thing i need.
Daily driver : P16s AMD Gen1 Ryzen 7 6850u Pro - Arch Linux.
HTPC : T540p 3K Screen - i7-4600M - Windows 11.
Have : X120e, X24.
Had : T420 - 240 - IBM i Series type 1171

Quardah
Freshman Member
Posts: 92
Joined: Sat May 09, 2015 1:52 pm
Location: Ahuntsic, Montréal, Québec

Re: T540p power supply question.

#4 Post by Quardah » Tue Jun 27, 2017 11:08 am

Hi all.

To answer my own question (after i tested it)

The T540p refuses to charge if you plug in a 45W power supply.

It will charge with a 65W nonetheless.

Possible that it would refuse to charge with a 47W chip on a 65W power supply.

The more you know!
Daily driver : P16s AMD Gen1 Ryzen 7 6850u Pro - Arch Linux.
HTPC : T540p 3K Screen - i7-4600M - Windows 11.
Have : X120e, X24.
Had : T420 - 240 - IBM i Series type 1171

Thinkpad4by3
Senior ThinkPadder
Senior ThinkPadder
Posts: 2670
Joined: Sun Aug 09, 2015 9:25 am
Location: N. Bellmore, ny

Re: T540p power supply question.

#5 Post by Thinkpad4by3 » Tue Jun 27, 2017 11:20 am

Quardah wrote:Hi all.

To answer my own question (after i tested it)

The T540p refuses to charge if you plug in a 45W power supply.

It will charge with a 65W nonetheless.

Possible that it would refuse to charge with a 47W chip on a 65W power supply.

The more you know!
Its refusing to charge because to charge, the entire system needs to be powered and charge the battery. Woth a 47W system, all 45W (+2 from battery) go to the system and none to the battery. If you turn off, then 0W go to the system and 45W go the battery.
Thinkpad4by3's Law of the Universe.

The efficiency of two screens equally sized with equal numbers if pixels are equal. The time spent by a 4:3 user complaining about 16:9 is proportional to the inefficiency working with a 16:9 display, therefore the amount of useful work extracted is equal.

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