To save me cracking out the breadboard. Does anybody know wether you can buy a mains adaptor/battery changeover device? I've got an ADSL modem that I would like to protect against power failure and it seems overkill to use a mains UPS when all that really needs protecting is the 9V DC feed into the modem.

Out of interest, what are you going to have in the house that doesn't need power, but needs to connect to the ADSL modem ???

Cheers

Cris.

The servers are upstairs on a mains UPS but the modem/access point is downstairs, moving the modem upstairs makes the Wifi flakey in the living room, it also serves as the switch for all my wired ethernet stuff in the living room. I refuse to buy 2 UPS for this.

The DC UPS might also be used to power several small camera units in the future.

Replace the co-ax we ran between upstairs and downstairs with Sat co-ax with the motor control wires in, you can then plug the PSU for the router in upstairs and feed the 9v down to the modem through the co-ax, therfore no need for a custom UPS, just 10m of co-ax

Cheers

Cris.

If your only reason for not buying another UPS is cost - there are some very cheap models available. eBuyer have a 600VA unit made by Trust for £30.99 inc VAT. Would also be more versatile if you have other items nearby that would benefit from a UPS, such as a set-top box or cordless phone.

As long as the voltage drop across the cable isn't too great. Also I'm not sure wether the holes we drilled would be big enough. Isn't that stuff a figure-of-8 construction?

I wouldn't have thought the voltage drop would have been that bad, if it is replace it with a 12v and watch the modem fry

The holes should be big enough, with a little bit of modification with a hammer! (NOT THE CO-AX ) Drag the new cable in on the back of a bent coat hanger (NOT A WOOD ONE ). Most of that type pf co-ax has the wires welded to the side of it, so you can flatten the cable slightly to get it through the hole without damaging the co-ax.

Cheers

Cris.

Had a Eureka moment in the car on the way home from work. I'll just PoE it. I've got the Cat5 run connecting the modem and the server. I'll just run power down the two spare pairs. Job done.

But seriously if anyone does know of a DC UPS I'd still be interested to hear it.

DC UPS - Some assembly required

That is essentially the system that my ISP is using to battery-back his wireless routers.

I'd hazard a guess that the modem might work off of 8.4V. The question is how much current does it consume? If it's under 300mA then there's a fair chance that you could make a poor man's DC UPS using a 8.4V RC battery and a trickle charger.

You could very likely just float an appropriate battery (8.4V would do fine) across the input. The wall wart would trickle charge the battery as well as run the modem.

In fact, give or take the odd protection diode, this is probably all that the the cordless phone backups do.

-Z

Surely all you need, and I speak from experience here is:
1. A rechargeable battery, as used in alarm systems(Maplins).
2.A Battery Charger.(Also Maplins)
2. A cheapo timer, to ensure that the battery is always charged, but not overcharged(1.99 at Poundstretcher).
3. A 2 pole mains relay, from the bins under the dark arches in Leeds @99p.
When the power goes off, one pole of the relay switches from the normal power unit plugged in to the mains, to the battery, the other switches out the charger, which is dead anyway (I'm not sure that that's necessary, but I just happened to have a scrap 2 pole relay).
I used to use such an appartus to safeguard my radio cassette from missing my favourite programmes 15 or 20 years ago.
At a guess, using a sophisticated experiment last night, the modem won't notice the momentary break of power.
Of course, you could incude a car battery connected to a trickle charger, I just used what I had knocking about at the time.

That's exactly what I was thinking.

Not too bad for my first go with the dremel.

It seems to work on short distances so I'm going to measure the voltage drop over the intended run tomorrow.

Yup, that looks like just the ticket.

I've been inspired by this thread to think seriously about setting up a similar system for my own home. I have two machines - my firewall and internal web server - that use Via EPIA motherboards and cases which take 12VDC and have external 12V-4.5A 'laptop'-style power supplies to supply it. These would be easy to hook up to a DC UPS - I'm pretty sure that the internal power supply boards in the cases take care of minor voltage and amperage fluctuations that you might see from a SLA battery.

My main thought was to add a set of solar panels to the mixture and to try and wean the system off drawing home power altogether. Pulling numbers out of the air, I want a decent three days' worth of supply from the battery with no mains and no solar power, and ideally the system should run completely from batteries drawing no power from the mains at all. This means installing a sizeable quantity of batteries and solar panels - the charging input total has to be over twice the draw per day, which is (off the top of my head and adding a bit of fudge factor, 12V*10A*24Hr = 2880WHr). The grand plan is for the system to also power the media centre PC, which would have its own modified square-wave inverter. This would easily add 4800WHr and therefore will be quite expensive, so it's in the distant future. But the roof has little shade, and here in semi-outback Australia we get plenty of sun...

My plan is to start small - just get a battery or two and arrange them so that the machines have a small battery back-up. Include a circuit that detects whether battery power is going to fail (lack of mains and voltage under e.g. 10V) and notifies the machines so that they can shut down gracefully. Continue to add batteries to extend the battery-uptime life. Gradually add solar panels as money permits (through a solar panel controller) to supplant the mains backup until the entire system is solar powered.

A couple of questions to the electrical boffins here:

• Can anyone point me in the direction of a simple circuit or instructions on how to make a detector that will send a signal to shut down the machines should the battery pack be too low on charge to keep going?
  
• Would there be any problem with simply plugging the battery directly into the machines? Can you see any problem with battery chargers or other devices that might play with the charge going into the batteries? Do I need some sort of output isolation or voltage regulation circuit?
  
• Jaycar has an integrated device that will take the input from a wide variety of solar panels and charge a wide variety of batteries. Can you see any problems with feeding this device 12V from a mains converter (e.g. the two I have already powering the machines) to simplify the system?
  
• Any other tips, tricks and traps? Anyone else built a hybrid system like this?
  

Thanks in advance,

Paul