Lenovo Thunderbolt Dock Type 40AC

The Lenovo Type 40AC dock is a Thunderbolt 3 dock that is capable of USB Power Delivery changing.

However, getting the 40AC dock to charge over USB PD requires at minimum a 135W AC adapter, regardless of how much USB charging power is actually being used. When using a smaller AC adapter (90W in my case), the dock will function but refuse to charge. The red LED on the Thinkpad logo will continuously flash if the AC adapter is too small.

The simplest solution would be to get a 135W or higher AC adapter. Instead, I chose to buy some short (15 cm) AC adapter extension cables and modify the resistor to make my 90W adapter appear as 135W. The cable I got came with a resistor indicating 90W, so it overrides the power capacity identification resistor of the connected AC adapter. But I needed to replace the resistor because I needed something different than 90W.

The laptop I’m charging won’t come close to exceeding 90W, so I should have plenty of margin left even with a smaller-than-required AC adapter. The laptop now happily charges through the dock.

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