I have an irrigator pump with a generator putting out 480 volt 3 phase. I bought a digital voltmeter on ebay.  It looks like it will work just fine, but there is one difficulty.  It requires 5VDC to operate.  I thought of just using a battery, but doubt it would last long enough.

I think I can pull 120VAC from one leg to power it, but it will not be a constant 120V until I get is to speed and locked in.  Is there a reasonably simple circuit I can buy or build to provide a constant 5VDC from a source that will vary as much as my engine driven generator will?