From: jim@n5ial.mythical.com (Jim Graham) Subject: Re: how safe is minix filesystem in power failure? Date: Wed, 14 Apr 1993 14:52:14 GMT
In article <1993Apr13.042529.12851@monu6.cc.monash.edu.au>
ent811l@monu6.cc.monash.edu.au (Christopher Kuperman) writes:
>but it sounds like you got a real problem with this power thing...
uh huh... we're expecting more thunderstorms tonight (90% chance, they
say). of course, we all know what that means (Jim shuts his system down).
>I personally would do one of two things...
>
>a> Buy an Uninterruptable Power Supply (UPS) - not very cheap :(
way, way out of my budget.... believe me, that's the first thing I
thought of (all I'd need would be a very tiny one capable of keeping the
computer up for a few seconds). when I get a job again, that's high on
the list (along with more hard disk, more memory, etc.).
>b> Do what my friend did and get hold of 3 or 4 car batteries and make
> one...
well first off, use gell-cells instead of car batteries---they're much
safer (acid from the car batteries will destroy anything it touches)....
I'm not sure what this does to the cost, but there's no way I'd use car
batteries for any kind of indoor power supply. this is also why most
Amateur Radio ops I've talked to who want to go portable (e.g., hiking
trip, etc.) with their rigs carry gell-cells instead of car batteries. :-)
> you can do this in two ways.. but both require you to make
> some detection circuitry so it detects the power loss or power unstab-
> ility.
> 1. Buy a kit or a commercial voltage inverter, they are typically
> things that DIck Smith or the like sell for people to run their
> 240VAC applications from a car battery, and with 3 or 4 batt's
> you'll be cruising, but i dont recommend using these inverters for
> long lengthas of time coz they make a very crude sine wave and
> even sometimes only make a square wave with lots a harmonics.
> 2. make a real simple ``regulator'' (12VDC,5VDC etc..) and run
> the computer direct from the batteries when the power fails.
> Isnt it great the way computers run off 12V and 5V.
actually, the way UPSs normally work is more along the same lines as the
battery in a telephone central office (switching center). basically, the
entire phone system (including your telephone, provided it isn't one of
the ones, such as cordless phones, that wants external AC power) runs off
of the battery the whole time, whether external power is there or not.
the external power, when it exists, is used to keep the battery charged.
(in the case of the telco, there are *MASSIVE* generators to take over
the charging of the battery when power failure is sustained.)
UPS systems for computers (and other devices, too, of course) work the
same way. the computer/whatever runs off of the battery, and the AC power
keeps the battery charged.
this, btw, is why there isn't a hit when the power fails---the computer
never sees a change in its power source, because there isn't a change.
I've seen demos of some UPSs where you see a scope showing the output
voltage level at the time of the power failure, and it's perfectly clean,
w/ no fluctuation at all in voltage or phase....rock solid. same for
when the power came back---no change. of course, this was also a very
high-end UPS system, too...not sure what the cheaper ones would look like
in that same demo.
anyways, this is more than a bit off the subject of this group, but I'm
not sure where to re-direct followups on this one...especially since it'd
almost certainly be a group I have no access to. :-(
later,
--jim