Thank you, Bjorn, for your time and effort to experiment. I appreciate it very much. A reminder to readers: regarding these vintage AC adapters in ThinkPadLand, the
S pin is referred to as:
S,
SIGNAL, or
INPUT SIGNAL.
Here, I modified Ray's ASCII illustration.
_____
/ S + \ OUTPUT PIN ASSIGNMENT
| G - |
+-------+
_____
/ + S \ DC INLET
| - G |
+-------+
Norway Pad wrote:operating the laptop seemed to affect the S voltage level:
First: Changing the brightness/contrast up and down changed the Signal between 20.99 to 21.06V
…
So basically, + and - was stable, while S was varying.
After I read your reported observations, I wondered if S means 'POWER GOOD'.
I thought, it could be like a canary in a cave mine. Assuming S is a place in PSU circuitry, before all of the conditioning of + output, it is of higher potential level, because filtered and regulated and such. Assuming in the event of some fault, capacitors can sustain potential of + while S drops. If S is approximately 1 volt above + : good. If S is equal or below + : bad.
Norway Pad thought, it could be a signal whose meaning is same as in Intel PC power supply (ATX, and its extensions and derived form factors). + must reach stable level, and all internal conditions okay, before asserting POWER GOOD. Maybe, without S, ThinkPad will not start. I like this idea, more than my canary idea. See also: ThinkPad T20 'Blink Of Death' BoD.
In a private message, Norway Pad wrote:If we go down the On/Off route, and use logic: Any internal failure in the adapter causes the SIGNAL to go down, and it shuts the laptop down even if 20V is still present on the main leads. It can even shut something off inside the laptop, to prevent any harmful power spikes / low voltage from the adapter to do any harm to the laptop. Before switching adapter technology got as refined as it is today, maybe they had little or no control over what the adapter did when it failed, and SIGNAL was a safety measure to prevent it from damaging the laptop?
An argument against this hypothesis comes from ThinkWiki user Cloudane, who wrote about a replacement plug hack. It seems that SIGNAL pin is not needed for operation of at least some unspecified ThinkPad model, and at least to run without charging.
http://www.thinkwiki.org/wiki/Power_Con ... _connector
Norway Pad wrote:Can the laptop change the voltage level through the S pin?
I too, wondered about this. I assume these are variable output supplies. If at least one old model used 16V from the hexagon, maybe another model used 10V? Perhaps this is pulled-down with a voltage selection device. That would explain why PSU label says INPUT SIGNAL: It is the reference potential, input used by voltage regulation circuit. I can imagine, S is not connected, not used at all, by some ThinkPads. Because it is left floating, S can vary, as Norway Pad observed rising higher than 21 V.
About power levels: twbook.pdf indicates certain models having such-and-such watt supplies, but I find eBay photographs of underside labels, marked 50 watt. I assume certain hexagon-plug ThinkPads can draw 50 watts max. when charging battery, but that is more than needed. They can operate, and perhaps charge battery more slowly, with a lower nominal power rating. They will never draw more than 50W. Is this believable?
About Norway Pad's failed bricks: I do not blame them for being overworked and under-rated. Rather, I suspect they are aged beyond design lifetime, fatigued and worn out-of-spec. Perhaps they are now damaged, perhaps fuses or something are blown. Perhaps, if capacitors were replaced with fresh newer technology parts, then failure could have been avoided.
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Can any of you verify which one is the correct one, so I can purchase one?
