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
>> 2. Hook up 12v to the 12v output of the PSU and set up a potentiometer
>> 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
>>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
>> 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
>> 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
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.
General Forecast Division
ph +353 1 8064255
fax +353 1 8064275
conor.daly at met.ie
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