[time-nuts] Parallel voltage regulators

christopher hoover ch at murgatroid.com
Wed Oct 24 21:42:31 EDT 2007

Tom Clifton <kc0vsj at yahoo.com> wrote:
> A question for those that might know (or have an
> opinion)...  I have in hand an LPRO rubidium reference
> that requires 1.7 amps at 24 volts while the oven
> warms, dropping to 500ma while it runs.

> Can I parallel three or four 7824 TO220 style 1 amp
> regulators with a quarter ohm half watt equalizing
> resistor on the output of each one?

> At maximum load there would be a quarter volt drop
> across the resistors and the LPRO is stated to be ok
> running on 16 to 32 volts.


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: 


(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.)


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