[time-nuts] Parallel voltage regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 25 06:39:04 EDT 2007

Don Collie wrote:
>  Hi Bruce,
>     I`ve got the National Semiconductor Corporation Voltage Regulator 
> Handbook [1982]. On page 3-3, the leftmost graph shows the LM117/217/317 as 
> having its current limit, with a junction temperature of
> 125 degrees Centigrade, at 2.25 Amps over the input/output differential of 5 
> to 10 Volts.
>     The point being, that if you use a higher current regulator, you loose 
> the advantage of
> the regulator`s current limiting, and perhaps, its thermal shutdown 
> protection as well.
>     A fuse *might* protect the semi`s down the line, but often it`s the 
> semi`s that fail before the fuse, and the peak current that might flow 
> before the fuse blows might be many times the current limit of the regulator 
> [which is nearly instantaneous], and if so, damaging, so it is wise to run 
> these regulators near their current limit, just as you would set the current 
> limit on a bench supply to just above the working current.
>     I find your use of the emotive words "inexperienced",  "gullible", and 
> "cavalier" saddening.
> Wishing you well,................................Don.

The graphs are only typical, read the actual printed specifications.
You need to be more skeptical and question your assumptions.

If you want a lower current limit regulator use an LM350.
However you have identified one problem in applying 3 terminal
regulators, the rather wide tolerances associated with the current limit

A 723 regulator can be easily designed to have a much narrower spread
for the current limit.


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