[time-nuts] Thunderbolt monitor
didier at cox.net
Thu Jul 17 00:54:15 EDT 2008
My preference is usually to have a voltage regulator on the board where the
"expensive" electronics is, simply because I use adjustable bench supplies
for testing and I also own and use a large number of 9 to 12V wall warts
such that I have blown great many parts by inadvertently putting 9 or 12V
into a 5V gizmo. Also, most of my processor boards take advantage of the
built-in A/D converter, and a local stable 5V source is a good design
practice that avoids a number of problems down the road. So it's been my
practice to put a 5V regulator, needed or not :-) Now, the Silabs chips run
off 3V nominal and have their own, relatively stable 2.5V reference, so that
problem also mostly goes away.
In this case, since the group buy Thunderbolt comes with a multi-output
supply which includes 5V, we could use that directly I guess. It just had
not occurred to me to do that... My original "red box" Thunderbolt runs off
28V, so that one will definitely need its own switching regulator.
Regarding the RS-232, the monitor only needs to convert the RS-232 to 0-3V
logic, so 12V is not needed there either.
In any event, a separate supply would be a waste and is not necessary.
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Thomas A. Frank
> Sent: Wednesday, July 16, 2008 11:06 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Thunderbolt monitor
> On Jul 16, 2008, at 10:46 PM, Didier Juges wrote:
> > One issue with the Noritake display (common to all VFD devices) is
> > that it draws over 100mA of current at 5V. A 3 terminal linear
> > regulator running from 12V will need a reasonable heat sink to
> > operate. A better choice would be a small switching
> regulator such as
> > the LT1375 which I have used in a great many products. It's
> a small 8
> > pin device that requires only a handful of external components to
> > convert 12V to 5 with excellent efficiency (no heat sink
> required). I
> > may lay out the PWB for both types of regulators, depending on the
> > type of display you want to use.
> > Didier KO4BB
> Why would you run this external box off of 12 volts? Do you
> have portable use in mind?
> I ask because to my way of thinking, I would provide the
> display/ processor box with 5 volts straight out of a "wall
> wart"; I've probably got a dozen in that voltage range lying
> around (left over from ZIP drives).
> The only use for 12 volts in the display box would be for the
> RS232, and that can come from a MAX232 chip.
> Seems like a simpler approach to me. If I really wanted
> portable, the 12-5v unit would be external.
> Tom Frank KA2CDK
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