[time-nuts] Parallel voltage regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 25 19:36:48 EDT 2007

Magnus Danielson wrote:
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> Subject: Re: [time-nuts] Parallel voltage regulators
> Date: Thu, 25 Oct 2007 17:42:51 +1300
> Message-ID: <47201ECB.6050702 at xtra.co.nz>
>> Don Collie wrote:
>>> Hi Tom,
>>>     If you really want to regulate the oven`s supply voltage, my National 
>>> Voltage Regulator handbook shows that the LM317T will supply over 2 Amps, 
>>> with an input/output differential of between 5, and 12.5 Volts. A single one 
>>> of these should do the job OK.
>>> Cheers!,.................................Don Collie jnr.
>> Never rely on typical specs always use the minimum spec which is 1.5A
>> not quite enough.
> You want design-margin. Some of that toll will be in less than optimum heating,
> some will be in less heating in the first place (compared to upper limit) and
> for a power regulating aspect, headroom allows better regulations.
> In one design we had to parallel the regulators since the regulator the
> designer put in just barely was able to regulate the CPU core voltage.
> It worked, but at just rebooted at some vauge point an the memory tests.
> What actually happend was that as soon as it started to actually do anything,
> the regulator was running at its limit and output voltage dropped as the
> current was rising and the voltage supervision pulled the RESET.
> That's what you get from reading the typical reading on the CPU current and
> match that with the maximum rating of the power regulator. A no margin design.
> That designer had a few more flaws which was creeping around in that design,
> but let's not bring that can of worms open here. :)
> The 5A LM338 will be just fine. Infact, you can pull 8A out of it under
> certain conditions.
> Cheers,
> Magnus

Hej Magnus

The LM338 thermal design is also much easier (it has a much lower
junction to case thermal resistance than an LM317) especially if the
circuit is intended to operate over wide temperature (0 -40C or more)
and mains voltage ranges (+-10% or more).

The alleged problem with the high short circuit current is easily solved
by using diodes with adequate current ratings in conjunction with a fuse
to protect the transformer if it isnt rated to produce an 8A dc output.

The startup current of the load (rubidium standard) may also vary with
temperature and /or input voltage.
Either  find the manufacturers specifications or allow adequate margins.

Worst case design is desirable even for one off circuits especially if
the circuit is published.
When the design is  publicly available one is in effect transferring the
production run problems associated with a marginal design to many
individuals rather than a single factory or production line.


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