DIY ThinkPad Charger

I recently bought a used Lenonvo T440s. I got it for a good deal but there with a few issues:

  • No charger included
  • Missing trackpoint nub
  • Missing an arrow key

First thing I did after ordering the laptop was order a new Thinkpad charger. No problem; but while browsing Ebay looking for a new key cap and trackpoint nub, I found generic laptop charger to Lenovo slim charging adaptors. Knowing I have a few spare old chargers, and that I would want a second for my new laptop, I ordered a few from China. After they arrived however, I knew they weren't going to work so great.

Adaptor and old charger Adaptor and old charger

Digging Into the Adaptor

The Toshiba charger I had was very loose in the adaptor and the connection was flaky at best. Then, knowing I had two adaptors, I figured it couldn't hurt to dig into one and see if I could wire it up directly. So I went at he adaptor with a sharp knife and cut away all the over-moulded rubber.

Adaptor free of all wores and rubberAdaptor free of all wires and rubber

While digging I found and promptly broke a resistor built in. After some google-fu, I found myself on the ThinkWiki with a chart of the resistor values for different wattages. The resistor goes between the center signal pin and outside shield (Ground).

Wattage Resistance
45W 120R
65W 280R
90W 550R
135W 1K
170W 1.9K

I multiplied the rated voltage and amperage of the Toshiba adaptor 19v * 4.75A = 90.25W - A 550R resistor will be perfect. After digging through my parts bin, the closest I could find was two 1K resistors in series for 500R. I figured that would be plenty close.

Putting It All Together

I took the freshly stripped adaptor and soldered a short wire to the ground pin. Then I put the two resistors between the center signal pin and the new wire I soldered. Lastly I stripped the Toshiba adaptor and soldered the positive wire to the middle pin and the negative to the top of the resistor.

All soldered upAll soldered up

To protect the joints and prevent anything from moving, I mixed up some two part epoxy and coated all the exposed joints. After ot was all set, I had a perfectly usable charger to toss in my bag.

Notes

  • Wile the Thinkpad charger is rated at 20 volts, 19 volt chargers work fine.
  • Evidently, the tolerance of the resistor isn't too critical. 50 ohms off wasn't an issue.
  • If you are more careful removing the rubber, the existing resistor can be used.
  • The adaptors can be found on Ebay by searching for female dc to Lenovo slim mine were about $2.50CAD for 2.