[ILUG] UPS for home use?

[Conor Daly]

-----Original Message-----
From: Kenn Humborg <kenn at linux.ie>
To: Conor Daly <conor.daly at met.ie>
Cc: ilug at linux.ie <ilug at linux.ie>
Date: 30 June 2000 10:06
Subject: Re: [ILUG] UPS for home use?


>On Fri, Jun 30, 2000 at 08:58:47AM +0100, Conor Daly wrote:
>>
>> Easy enough to do for yourself...
>
>Don't try this at home kids...


Oh, I know...

>
>> 1.  Get a 12v Truck battery (Wait until the truck owner is not with the
>> vehicle.  It's hard to run with a 20Kilo box sloshing HCL all over the
>> place!)
>> 2.  Hook up 12v to the 12v output of the PSU and set up a potentiometer
or
>> other voltage changing device to provide the 5v output.
>> 3.  Put a diode on the incoming 12v line to prevent feedback (I think the
>> diodes in the PSU rectifier will prevent any feedback through the trafo).
>> 4.  Hook up a charger to the battery.
>
>This isn't going to work and will probably fry something.
>
>Are you aware of the current requirements of a modern motherboard?  IIRC
>the PC's PSU's 5V output is rated at 20 amps.  Using a potential divider
>to provide this kind of supply has two _big_ problems:
>
>  1.  You'll need seriously large, low-valued power resistors which
>      are going to get seriously _hot_ so will need to be heatsinked
>      (and maybe a fan too).
>
>  2.  You get _no_ regulation.  That 5V supply will be jumping all over
>      the place as the load from the motherboard (and disks, video cards,
>      etc) changes.


Probably need a proper *solid state* (rather than switching) regulator all
right.
>
>Never mind the fact that the battery voltage will probably jump to
>over 14V when you hook up the charger.  I don't have a hard disk handy
>here, but chances are the 12V supply to it must be within +/-10%.


Regulator would handle that too.
>
>And your diode in the 12V line?  That'll need to be at least a 5A
>diode (10A if you want a sensible safety margin).  And it's going
>dissipate a few watts and will get fairly hot.


Yeah, I haven't engineered that yet.  :0)
>
>And the 'diodes in the PSU rectifier'?  That's a switch mode power
>supply in your PC, not a simple transformer, 4 diodes and capacitor.
>
>> If you wanted to get fancy, you could scavange an old PSU for the
four-pole
>> connectors and wire them up.  Then, you could hook up and unhook just by
>> using a disk power connector in the (any) case.
>
>I hope you're not talking about doing that live?  Those 4-pole connectors
>are so unbelievably _not_ designed to be hot plugged.  Insertion force is
>way too high.  No guarantee that ground will mate first.  Contacts will
>bounce multiple times during insertion, causing horrible fluctuations on
>the power lines, which will probably trip out your PSU.


ok OK!!!

I'm wrong!

Right?

:-)
>
>> No software warnings though!
>>
>> If you want a portable solution, you can get small sealed lead-acid 12v
>> batteries with enough capacity for a reasonable time.  Some might be
small
>> enough to keep inside a tower case!
>
>If you want a portable solution, get a small UPS and bolt it to your
>case.  If you go the DIY route, you'll end up re-inventing the UPS
>anyway.
>
>Later,
>Kenn
>

It's only a first guess off the top of my head.  Probably should have
included the health warning though.  It's all from my Motor Mechanic days
where things work in a voltage range of about 9 - 15 volts and use up to
80amp battery drains at times!

Only thing is,  You can add capacity to your homemade UPS just by adding
extra lead-acid batteries in the chain (in parallel of course!) but you'd
have to work out a solution to prevent the batteries from mutual drainage
due to differing internal resistances.

---
Conor Daly
-------------------------------------
General Forecast Division
Met Eireann
Glasnevin Hill
Dublin 9.

ph +353 1 8064255
fax +353 1 8064275

conor.daly at met.ie
-------------------------------------