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PICList Thread
'Brownout circuit'
1997\08\21@095442 by lilel

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Gregg Wrote,


> I saw your post today about your success with using the PNP
> transistor-based brownout circuit from Microchip.  I haven't used a
> brownout cicuit in my new design, and though I haven't seen any
> problems, I need to play it safe.
>
> I'm running an LF84, from 3 AAA cells.  The cells are expected to
> run down to about 3 volts during use, but there;s nothing to prevent
> a careless user from running them to zero... hence my concern for a
> brownout cicuit.
>
> I was hoping you could share some of your experiences in this area
> ... my main concern with the PNP circuit is the constant battery
> drain from transistor's bleeder path.  Since I'm completely battery
> powered, that could be a real killer.  Did you find a clever way to
> reduce the drain?


Unfortunately my application was in a 120 volt appliance where I can
waste megawatts if I want to.  <slight exaggeration>  I also need to
be very low cost.  I doubt the PNP transistor trick is the right
thing for your application, unless you're building toys.


Also, what is the danger in a brownout?  In my application, a stuck
program could mean a heating element turns on, resulting in a melted
appliance and a mad customer.  In a toy, who cares?  In a cell phone,
you might care.  I doubt a battery powered application has enough
power available to go into a self-destuctive mode, maybe it just
wouldn't work right, and the customer would say "Hmm - low battery".


-- Lawrence Lile

Download AutoCad blocks for electrical drafting at:
http://members.sockets.net/~llile/index.htm

1997\08\22@133840 by Martin McCormick

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       I don't think it matters whether it is a PIC or some other
controller.  The Motorola 68HC11 definitely will corrupt it's Eprom
in a brownout and there have been several cases of wireless telephones
using an unknown type of processor that have started dialing preprogrammed
numbers such as 911 or equivalent numbers when their batteries began to fade.

       We used to have a large RS-232-based data communications network
on campus consisting of several nodes, many terminal servers, and muxes.
The nodes were built around the Motorola 68000 and each had a battery soldered
to the main board that held the node's configuration.  One day, we took the
power hit from Hell.  The mains voltage dropped to about half what it normally
should be and stayed there for a sickening two or three minutes.  I remember
standing in the room containing the serial data net and hearing a sound like
a million crickets as switch-mode power supplies tried unsuccessfully to
sustain oscillation and their frequencies dropped in to the audible range.
When I hit the emergency shut-down button which is supposed
to kill all power to the room, absolutely nothing happen.  The
voltage was too low to operate the relay.  GRRR

       When the power came back up to full value, some of the nodes came
up.  Others seemed to, but pieces of their configuration were changed.  The
batteries did an excellent job of holding the mangled configurations so
the only fix was to physically go all over campus and manually short out
the two reset pins that were available on each board to do a cold reset.

       I believe it was two weeks before we found all the pranks that the
bronout had plaid on us.

       Basically, one should assume that this might happen regardless of
the source of power.  I would think it very important to have some secondary
method for preventing dangerous conditions such as would happen if heaters,
motors, or transmitters came on at the wrong time or refused to shut off.
Remember that this is not a programming issue.  The system is no longer under
the control of the original program, but is totally randomized.  Anything
can happen.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

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