And here is some more interesting info from a recent post
over at the Lenovo forum:
I measured a lot with my multimeter and found out, that the small pin in the (135W)connector is connected to mass with ~0,1Ohms. (That is about the resistance of the cable....)
When testing the 90W adapter, the resistance was infinity. (no connection!)
So i opened the Lenovo-tip of the 90W adapter. And i was nearly shocked. The middle pin of the connector is not even soldered to the circuit board! And on the circuit board inside the Lenovotip there is already a conductive path to mass!
you only have to:
1. open carefully the (deconnected!) tip (not really easy, but you can buy a new one pretty cheap if you failed...)
2. solder the tip to the board
3. solder together the bridge on the circuit board to mass
4. check all pins for right connectivity and test with a multimeter if there are any short circuits!!!
5. put together the plastic of the tip
I give no guarantee for working!
For the pros: The "bridge" on the circuit is probably for a resistor in (pretty small) SMD! Perhaps someone can try several values and look if the notebook recognizes a 90W adapter without freezing the machine.
There's also an internal mod mentioned (earlier in the same thread) to make the W510 think that all adapters attached are the 135W -- it involves shorting two pins on the power-supply connector under the keyboard. A few people tested it. However no-one has tried it on a W520, and I suspect the mod, if it doesn't blow a vital circuit, might end up making the W520 think it's connected to the 135W even with the 170W plugged in. That could limit functionality (as well as void warranty, etc.). But it's clear now that the resistance of the central pin in the power tip is the key.
PS I'm not an electrician and don't really understand what the "mass" referred to is. Also an SMD? And how a "circuit board" could fit inside the tip. Finally the plastic is pretty solid, and there are no obvious ways to "open" it!