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----- Original Message ----- From: "Bruce Griffiths" <[EMAIL PROTECTED]> To: "Don Collie" <[EMAIL PROTECTED]>; "Discussion of precise time and frequency measurement" <[email protected]> Sent: Thursday, October 25, 2007 7:20 PM Subject: Re: [time-nuts] Parallel voltage regulators > Don Collie wrote: >> Bollocs, Bruce! If National say it will do it, you can bet that it will. >> An >> LM338K will do the job too, but in my opinion it`s overkill, and in the >> event of a short circuit on the output of the regulator the current for >> the >> LM338 will only be limited to [.......he gets the book......] 8 Amps >> [Typ], >> as against 2.2 Amp [Typ] for the LM317T. This would probably be too much >> for >> the transformer, rectifiers, and smoothing capacitor, effectively meaning >> that you would have no current limiting. If the input/output differential >> was kept in the range of 5 to 10 Volts, while the oven was stabilising, >> and >> the LM317 had an adequate heatsink, it would do the job nicely [and >> cheaper >> too!] Actually, it wouldn`t matter if the oven supply went unregulated >> while >> the temperature was stabilising, because you wouldn`t be using it for >> measurements during this time anyway - or is that a bit radical!? >> All the best!,..................................................Don. >> ----- Original Message ----- >> From: "Bruce Griffiths" <[EMAIL PROTECTED]> >> To: "Don Collie" <[EMAIL PROTECTED]>; "Discussion of precise time and >> frequency measurement" <[email protected]> >> Sent: Thursday, October 25, 2007 5:42 PM >> Subject: Re: [time-nuts] Parallel voltage regulators >> >> >> > Show me where that is actually guaranteed on the datasheet. > Only the inexperienced and the gullible fall into the trap of assuming > every regulator (or any other device) will meet its typical specs. > The designer of this particular regulator actually cautions against this > cavalier approach to design. > > If you worried about the transformer a simple fuse (resettable or > otherwise) will surely cure that problem. > > > Bruce 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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
