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----- Original Message ----- From: "Bruce Griffiths" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Friday, October 26, 2007 12:36 PM Subject: Re: [time-nuts] Parallel voltage regulators > ); SAEximRunCond expanded to false > Errors-To: [EMAIL PROTECTED] RETRY > > Magnus Danielson wrote: >> From: Bruce Griffiths <[EMAIL PROTECTED]> >> Subject: Re: [time-nuts] Parallel voltage regulators >> Date: Thu, 25 Oct 2007 17:42:51 +1300 >> Message-ID: <[EMAIL PROTECTED]> >> >> >>> 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. > > Bruce >I don`t think the higher current created when a fuse is used instead of >near-instantaneous current limiting is "alleged", but rather a real problem >that can cause damage further down the line. Fast acting current limiting >is preferable to all but the fastest fuses that are designed to protect >semiconductors. Current limiting plus thermal shutdown in the regulator will protect both load, and regulator [and resovior capacitor, and diodes, and transformer] Commonly available fuses won`t give much protection to the load - especially the delay types often necessary with large filter capacitors. A precision, proven, high performance, low noise regulator like the 723 using an external pass transistor [or preferably a darlington], to avoid chip heating, and a well bypassed reference would be a lovely solution. Cheers!,............................................Don C. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.