README.md Thinkpad 700C Power Supply Board

I've reverse engineered the power board from a Thinkpad 700C. The P/N is
35G4785, and the FRU number is 48G3712. A very similar or identical board is
also present in the Thinkpad 700.

The schematic isn't perfect. Open a bug report if you find a mistake. Not all
components were identified. A number of parts did not have designators on the
silkscreen, so I assigned them new ones starting at 300.

The circuit board has four layers and an aluminum core, presumably for heat
dissipation. Troubleshooting

There is a single aluminum electrolytic capacitor on this board, C18 (47uF 6.3V)
and, at least in my machine, it failed and leaked electrolyte everywhere. This
is a problem because the electrolyte likes to puddle in nearby vias, and the
acid in the solution etches away and can break the connection.

C43 and C19 are polymer electrolytic capacitors and are much more reliable than
C18. Operation

This is a very complicated design. I've added some notes to the schematic to
hopefully make it easier to understand. Basically there is an input protection,
power ORing circuit, and a set of four DC-DC converters. Two of them produce the
main 5V rail--one of them from the tiny standby battery, the other from the
battery+AC adapter. The remaining two generate two additional voltages. Several
additional power rails are derived from these three but are gated by series-pass
transistors, presumably for handling all the various power saving modes.

There is voltage monitoring for both overvoltage and undervoltage. Overvoltage
causes a crowbar circuit to slam the AC adapter+battery voltage to zero.
Undervoltage causes a power good output to deassert.

A pretty complicated circuit handles the power turn on and turn off, with many
control lines coming from the motherboard.

Resistors with the "VR" prefix are laser-trimmed resistors on the circuit board
itself. Using some sort of test fixture, the factory would measure critical
voltage using a series of test points and make adjustments by firing a laser at
these resistors. IBM Thinkpad Power Adapter 48G8942

I've also reverse engineered a compatible power adapter, FRU 48G8942. This is
designed to output 10 to 20V at 2.20A or 3.38A. Design

The power adapter is a relatively straightforward design. IC051 is a
primary-side switching power supply controller that produces 20VDC on the
secondary side output. IC151 is a dual op-amp chip that regulates either voltage
or current, as determined by D151. If the current is below the regulation limit,
then the voltage control loop regulates the output voltage to 20V. If the
current limit is reached, then the power supply lowers the voltage to limit the
current.

There are two possible values for the current regulation, controlled by the
state of the "INPUT SIGNAL" pin on the power plug.

    Floating or high: 2.2A Pulled low: 3.38A

The Thinkpad laptop pulls this pin low when it is operating, and allows it to
float when it is powered off or in standby mode.

Unlike modern laptops, this early Thinkpad power supply also acts as the battery
charger, so it tries to charge the batteries with a constant current. If the
laptop is operating, it is also using current, so the charger has to increase
the current to compensate and maintain the same charging current.