Smart pin voltage-to-capacity correlation for laptop power supplies

By twm
Mar 18, 2012
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  1. The "smart" pin (center pin) on laptop power supply connectors is used for the purpose of the computer being able to determine the amperage (or wattage, if you prefer) of the 18.5 volt power supply that is plugged in to it, so it does not exceed it's capacity, especially when the computer is running AND charging the battery (or batteries) at the same time. I found a thread relating to this (topic#137840), but it is closed, and it did NOT provide the answer that I was looking for, anyways.

    I believe that the pin voltage will vary between around 5-6 volts to maybe 10-12 volts. The higher the voltage, the more current (amps) the PS can deliver. If the pin voltage is outside of this range, or the center wire is broken off at the connector, the laptop will not run off of it (at least for HP/compaq laptops). I run into the broken off center conductor wire scenario regularly. I suppose that is the one that always breaks first because it is the thinnest of the wires going to the plug, from the PS.

    I use one of those 6.7 amp brick-sized power supplies on my compaq 6715b, because it has a secondary battery attached to it (underneath). But the interesting thing is that I had the PS connected to a Dell laptop once, and it would run the computer, but refused to charge the battery. When the PS was plugged into it, a message would come up on the screen stating that the type of the PS could not be determined.

    Assuming that ALL brands of laptops now use the same type of power connector, is there any sort of "standard" among manufacturers that correlates center-pin voltage with PS capacity, or is it wide open to whatever they decide?

    If there is a standard, could someone please provide me with the link to some sort of "correlation table" showing center pin voltage vs. PS capacity?
  2. Tmagic650

    Tmagic650 TS Ambassador Posts: 21,905   +203

    You are talking about AC adapters right?
  3. Geronimus

    Geronimus TS Rookie

    I had a similar question when my Hp 6910p 90 watt adapter with smart pin failed,
    I had a adapter with the right ratings but the wrong connector ( 2 pin ) laying around and was hoping to convert it.
    I read a few threads looking for info and also could not find any conclusive awnsers.
    So I took the failed adapter apart and found the three wire connections on the print board ( + 19v / ground / Smart pin ).
    I expected some complex circuitry but it was a verry simple one.
    There was a 200k resistor between the +19v and smart pin connection folowed by a small ceramic capasitor connected to ground. This is for a 90 watt adapter.
    It kinda looks lijke a simple PRESENCE Signal cicuit.
    For my laptop there are 2 different adapters available, a 65 watt and a 90 watt version, so there could be a difference in smart pin signal voltage to indicate power rating.
    I converted the adapter I had with this cicuit and it works perfectly ( so far ).
    Hope this helps.

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