[time-nuts] Parallel voltage regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Oct 24 22:42:39 EDT 2007

christopher hoover wrote:
> Tom,
> Most of the oven circuits will tolerate a significant sag in potential
> during warm-up as long as the supply doesn't cut out.  I've often run OCXO's
> and Rb's off a "wimpy" bench supply when my high current bench supply has
> been otherwise occupied.
> But if you want more current, the typical way to get a 3-terminal regulator
> to put out more than spec'ed is to place a big PNP power transistor across
> the input and output.   You'll find the details in the datasheets of many
> parts (e.g. LM340).  Also, check out the National AN-103 app note.  It's old
> but it's still full of useful info.
> I refer you, at least for a laugh, to page 20 of Linear's AN-83: 
> http://www.linear.com/pc/downloadDocument.do?id=4172
> (Despite what others have posted, *some* 3-terminal regulators *can* be
> paralleled.  The LT1083 can be -- checkout the datasheet -- but, nota bene,
> in the suggested circuit the ballast resistance is on the order of 0.01
> ohms, not 0.25 ohms.  But it isn't the best idea.)
> -ch

The trouble with these circuits, as the designers will tell you, is that
they were developed when the pnp power transistors had lower fts than
their modern incarnations.
Thus some engineering development is required to cure the oscillations
that occur when a modern pnp with high ft is connected to boost the
current output.


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