[ILUG] UPS for home use?

[Kenn Humborg]

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...

> 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.

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%.

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.

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.

> 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