T420 throttling when dedicated graphics are active

[fleming164]

I don't know if this has been covered (I can't seem to find anything in search).

I have a T420 with the NVidia GPU but when the GPU is active the CPU throttles itself to 0.80Ghz. the throttling happens immediatly and doesn't revert even when the CPU has cooled down to ~50 degrees. Is this a thermal issue that replacing the thermal place could fix? Or Is this just a feature of the Optimus machines to stop them overheating?

[dr_st]

This sounds like a firmware/software bug. But to be sure - are you running with a functioning battery? Is the power supply attached? Is it 65W or 90W+

[fleming164]

Never even crossed my mind about whether it was a 65W or 90W adapter! Just swapped to a 90W and it's working fine now!

Thanks!

[rkawakami]

Lenovo 20V power adapters have a signal pin inside the barrel (that pointy pin in the center). It's connected to a resistor (or not) so that the laptop can identify what type of power supply is connected. Since your laptop has discrete graphics, it requires the use of the 90W adapter in order to obtain full performance. For some strange reason, the Lenovo engineers chose to have NO resistor mean 90W, which I believe at the time the 20V adapters were introduced, was the highest power adapter available. Using genuine Lenovo adapters should be safe as they will self-identify and the Thinkpad will adjust accordingly.

If an aftermarket adapter does not follow this resistor standard, in other words, if it has none, it may be considered by the laptop as a 90W unit, even if it's actually rated lower. If the adapter is actually 65W but the laptop thinks it's using a 90W, that can lead to overstressing the adapter.

[dr_st]

If an aftermarket adapter does not follow this resistor standard, in other words, if it has none, it may be considered by the laptop as a 90W unit, even if it's actually rated lower.

Yep, that's exactly how some of those fake 65W are.