The listing, Dell Laptop Adaptor has ended.
Dell Laptop adaptor, 19.5v DC, 4.62 Amp, 90W, Dell Part #K2596, Model PA-1900-02D2, DP/N-U7809.
Just tested it, works well. Center pin is positive. Outside dimension of connector plug is 1/4" as far as I can tell. See tape measure. Those of you with a dead Dell adaptor will know if this is the one or not by the part # if nothing else. Of course, if your good with a soldering gun and the power requirements match and you have another pin configuration, this will work, too.