Rebirthing the Surface pro 2

I have a old Microsoft Surface Pro 2.
The main motivation to get it going is that I would like a computer at home so I don't need to cart my laptop back and forth each day.  This surface pro has Microsoft office on it.

In the bag with it were two chargers.  I tried each in turn and on neither of them would the light on the plug come on and so I had no way of knowing if it was charging.  I decided in the end to cut one of the leads off and use a lab power supply.

My previous post on this is here
 https://thomsoneu.blogspot.com/2014/06/microsoft-surface-power-plug-pinout.html#comment-form

SUMMARY
I think the surface pro charger is foldback current limiting.

There is a web page here that shows how to cut the charger open
https://forums.surfacetip.com/forums/topic/cracking-open-surface-pro-3-charger/

WISHES
A schematic of the inside of the charger.
To really understand what the blue wire does.
To understand when the charger turns on the LED.



When I cut the lead I saw 4 wires.
Braid is GND
Red is +12V
Blue is centre pin.
Yellow is LED control

Surface Pro 2 - Charge plug wiring schematic.

The Surface Pro 2 can somehow control how much current it draws and sometimes it goes over the 3A maximum current that this power supply can deliver and that triggered the foldback current limit.











The microsoft power supply states that it can supply 3.6A.
I also connected the Surface Pro 2 to different external power supply that could supply 7A at 12V.  (No photos of that).

It seems the blue wire might be a voltage feedback to the power supply from the Surface Pro 2 that tells the power supply how much current the laptop is taking.

V       Current   Blue Wire
12.9    2.8A        0.46V
12.9    3.4          0.5V
12.9    2.2          0.273
12.9    2.0          0.288   

I reconnected the original Surface Pro 2 charger but with an extra LED of my own and left the leads exposed so I would monitor the voltages.

It seems the real power supply has the same fold back problem with the voltage dropping from 11.8V down to about 8V in pulses.

V        Blue wire  LED      Screen
11.8    0.0            OFF     84% plugged in Not Charging.
9.8
8.?
7.95
It pulses down.  It does not stay down.

If I was going to cut a plug again I would just slice it down the back

Once I knew that the yellow wire was the LED I could apply 5V and see the LED light.  I don't know yet what voltage  the power supply uses to light the LED.

It's hard to get a decent closeup with my phone.  Gets all pixelated.