That would be the voltage regulator and if thats gone out then you're electrically screwed until you replace it... the 2 times I've put in a new alternator they've come with new voltage regulators as well so I never really troubleshot it any futher to exhonerate the alternator or voltage regulator. Sometimes changing the alternator and not the VR will work for a short period of time, but they generally die again within a week... month at the most from my experiance. The VR's are cheap, and the last time I got an alternator with a VR it was about $100. Bolted and plugged in and have been rockin and rollin ever since.

Good Luck!