[time-nuts] Power Back-up

Philip Pemberton lists at philpem.me.uk
Sun Aug 9 23:40:44 UTC 2009


J. Forster wrote:
> When considering a battery backup you really have two ways to go:
> 
> Battery -->  Inverter -->  Load
> Battery ---> Load
> 
> The first option is generally easy to implement, but a lot of the battery
> capacity goes up as heat in the inverter. As others have pointed out,
> inverters are not very efficient or electrically quiet, and they sop up
> quite a bit of power w/o any load at all. For this reason, the second
> option is better, IMO.

In a lot of cases, you'd be stepping up from the 12V, 24V or 48V the 
batteries provide, then the PSU in whatever you're running would be 
stepping that back down to 5V, 12V or a mix of voltages.

I've built a (relatively) low-power PC server out of a Mini-ITX 
motherboard (Jetway NC92-230), two hard drives (a Seagate 'cuda 7200.10 
and a 7200.11, both 500GB), 2GB of RAM... and a 120W PicoPSU. The 
PicoPSU plugs straight into the ATX socket, takes 12V in, then converts 
it to the +5V, +12V, -5V and -12V the motherboard needs.

Efficiency is supposed to be around 96%, but I haven't done any 
measurements. As a bonus, it doesn't need any form of active cooling 
(read: cooling fans) to keep it cool, so the whole machine is pretty 
quiet. Well, aside from the Seagate Barracuda hard drives, which seem to 
be designed to make as much noise as possible.

My plan is to get a couple of lead-acid batteries and rig up a 
battery-backed AC-to-DC supply to run the server, external hard drive, 
DSL modem and WiFi access point. That's still on the drawing board, 
though -- the power to this menagerie has only been interrupted once 
since it was installed, and that was down to a lightbulb blowing and 
tripping the RCD...

As for the desktop... I'll probably end up buying a Back-UPS or 
something along those lines. I figure I only need 5 minutes to hit 
"Shutdown ==> Suspend to disc", and the laptops will run for a couple of 
hours without AC power...

> There are even purpose built battery backup supplies available for running
> oscillators, receivers, and such. Sadly, the NiCds tend to be bad and are
> quite expensive to replace.

In truth, most of the NiCd and NiMH packs I've used have been borderline 
useless. The Varta Mempac memory backup cells are OK, but most of the AA 
or AAA "off the shelf" cell above about 1600mAh have been dire.

The Sanyo Eneloops are pretty nice though. A bit more like the alkaline 
AAs of old (as in, they don't go flat if you leave them for a week or 
two), but rechargeable.

>  You can use diodes
> to make an OR circuitto switch automatically between DC supplies when the
> line fails.

But watch out for the voltage drop, and use suitably sized diodes.

A silicon diode drops about 0.7V (though big rectifiers are usually 
closer to 1V). If you have a supply with 2A going through it, a diode 
dropping 0.7V will be dissipating:

   P = I * V
     = 2 * 0.7
which works out at 1.4 Watts

At this point, you'll probably want something a little bigger than a 
1N4001...

-- 
Phil.
lists at philpem.me.uk
http://www.philpem.me.uk/



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