65W vs 90W power supply: how does it know?

[mvan]

Hi,

I have a small problem and a question.

I have an X220 with a 65W power supply of which I replaced the DC cable.
Oddly enough it is now recognized as 90W.
This is usually ok, but under combined heavy load and charging, it is overheats and shuts off.

So:
1) how does the laptop know which powersupply is attached?
Is there a circuit in the powersupply, is there something in the cable, or does the laptop measure the max current?

2) How can I make it be recognized as 65W again?

Thanks,

[dr_st]

The resistance between the center pin and the outer shell / ground is different for each PSU type. That's how it knows. Your DC cable probably matches the one from a 90W PSU.

Read this and the subsequent posts:
http://forum.thinkpads.com/viewtopic.ph ... 90#p682644

[mvan]

Cool, many thanks.

I should be able to correct this...