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I have done something like this by setting up a constant current source to
feed an R + Zener. This keeps the wattage to a reasonable value as the volts
go up.

The total parts amount to 2 low power zeners, a few Rs and a small (but
relatively high voltage) transistor.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: "James Paul" <jim@JPES.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, August 16, 2001 2:10 PM
Subject: Re: RES: [EE]: tiny power supply


> Paulo,
>
>  True, but it cannot withstand the 80 Volts in for any appeciable
>  time.
>
>                                            Regards,
>
>                                              Jim
>
> On Thu, 16 August 2001, Paulo Ricardo Pabst wrote:
>
> >
> > Hi,
> >
> > Another good idea is the MC78L05ACP from Motorola.
> > It a small +5Vdc regulator in a TO-92 package and can
> > source 100mA.
> >
> >
> > ------------------------------------------------
> > Paulo Ricardo Pabst.
> > R&D
> > Pollux - Industrial Machine Vision and Tracking.
> > Joinville - SC - Brazil
> > ------------------------------------------------
> >
> > |-----Mensagem original-----
> > |De: pic microcontroller discussion list
> > |[mailto:PICLIST@MITVMA.MIT.EDU]Em nome de Lyle Killough
> > |Enviada em: quinta-feira, 16 de agosto de 2001 14:06
> > |Para: PICLIST@MITVMA.MIT.EDU
> > |Assunto: [EE]: tiny power supply
> > |
> > |
> > |I have a power supply problem which I am hoping to get some good
> > |suggestions
> > |on..... (I have lots of my own bad ideas)
> > |
> > |I am designing a small PIC-based device for use in mobile hydraulic
> > |machinery.  There is VERY little room on the board - the
> > |board will be less than .85x.75 inches / 22x19 mm, and there will be a
3A
> > |high-side (solenoid) driver along with a PIC12CE673 and a linear
> > |Hall effect
> > |sensor in a TO-92ish package.  The board will be potted into a .25" /
6mm
> > |thick package.
> > |
> > |The device will be connected to anything from 10 to 26 VDC with a power
> > |supply requirement of 20 mA @ 5 VDC.  All of the hideous
automotive-type
> > |abuse will be present; e.g. reversed battery, load dump, battery
disconnect
> > |and voltage peaks.  In addition, one of our customers has specified a 2
> > |minute, 80 volt survival test.
> > |
> > |The obvious solution of a LM2931-5.0 with a 32V over-voltage shut-down
will
> > |not work.  Way too much heat for a little linear regulator.
> > |
> > |I have a working option which will allow 10 to 80+ volts in, 5 volts
out
> > |with about .25 volts of ripple.  This is really just a voltage
comparator
> > |maintaining a voltage on a capacitor with a Schottky diode in the
> > |hysteresis
> > |loop.  This is built with discrete transistors to allow the large
supply
> > |voltage range.
> > |
> > |I know very close to nothing about switching power supplies, but I
suspect
> > |that there is a buck regulator which will do this in a neat
> > |package.  I have
> > |looked in the obvious places, but I cannot find a device which
> > |will take the
> > |high input voltages in a small (low pin count) package.  I prefer to
not
> > |have an inductor in this supply, current demand is low, I wish to
minimize
> > |stray magnetic fields due to the Hall effect sensor, and there is so
little
> > |space.
> > |
> > |If any of you PICsters have any suggestions or cautions about how
> > |to achieve
> > |the above, I'd love to hear about them.
> > |
> > |Thanks!
> > |
> > |
> > |____________________________________________
> > |
> > |Lyle Killough      lyle@suregripcontrols.com
> > |____________________________________________
> > |
> > |           Sure Grip Controls Inc.
> > |             4 - 1413 McGill Road
> > |            Kamloops, BC, V2C 6K7
> > |                   Canada
> > |
> > |              phone 250-374-2278
> > |              fax   250-374-1099
> > |
> > |____________________________________________
> > |
> > |          www.suregripcontrols.com
> > |____________________________________________
> > |
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> > |
> > |
> >
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> jim@jpes.com
>
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