Dell Latitude Power Supply

Dell Latitude Power Supply Unit (PSU)

I bought a couple of Dell Latitude D620 laptops and I found I could not use generic power supply units (PSU) because these computers have a special connector with the two power supply contacts but also a third signalling connection. I decided to investigate.
The first interesting thing is that the computer itself says it requires 19.5 V at 4.62 Amps (90 W)

but the power supply pack supplied with it says it supplies 19.5 V at 3.34 Amps (65 W).

Dell power supply

The computer says it requires adapter model pa-10 or pa-12. The PSU says it is "model pa-1650-05d2, 65 W AC adapter PA-12 family". Another similar adapter I have says it is model hp-OQ065B83. It seems these power supplies are used with many different models of Dell laptops including Latitude, Inspiron, Dimension, etc.
I found this page dealing mainly with Dell Inspiron 8600 but also other models and where I find that many people have problems with the 65 W power supply which are resolved by using the 90 W PSU. It seems the way it is supposed to work is that the computer knows what PSU is connected and will limit its power usage. If the more powerful PSU is used then it will run at full blast and charge the battery at the same time but if the less powerful PSU is used then it will not charge the battery while the computer is in use and will wait until the computer is turned off.
The computer and the PSU communicate through the third conductor and if you connect a generic PSU with no third cable then when the computer starts up it will display an error message saying "The AC power adapter type cannot be determined. This will prevent optimal system performance". It seems the computer assumes a very low capacity PSU and will not charge the battery and will throtle back the speed of the processor to conserve energy.
I decided to do some tests of my own even if very limited due to the limititation of the means and tools at my disposal. Maybe someone with better tools can do better.
I determined that

The signal line gets voltage (3.26 V) from the computer, not the PSU.
Computer and PSU exchange data only once when the PSU is first connected, even if the computer is off, not during normal computer use.
The signal line is pulled down by the computer for a number of pulses (about 25) and immediately after each pulse the PSU pulls the signal line down for a shorter duration. This is very difficult for me to see on my 'scope so it is mainly guesswork. I would need a 'scope with memory to determine this with any certainty.

It seems there is some intelligent circuitry at both ends, computer and PS, and both need to be working correctly for the computer to recognize the PSU model. If the PSU has a problem it might not be recognized but if the computer end has a problem then it will not recognize any PSU.
One possible cause of problems is the signal line not making a good connection because the pin is bent or broken. This should be relatively easy to fix. Another, more serious, problem happens when the 19.5 V wire is shorted to the signal wire due to an aging cord. Then the signaling circuitry at the computer is fried and the computer will no longer recognize any PSU.
Dell power supply

Because of the signalling I cannot use a generic 12Vdc power supply and I am forced to use a 12Vdc to 220Vac converter and then the regular Dell PSU. At 12 V it uses about 3.5 A if the battery is fully charged but about twice as much if the battery is charging.

Autor: Alfonso Gonzalez Vespa