After hours of searching these forums and Google Groups I have just mastered how to power my OPUS when I take my CarPC out of the car and into the house - so I'm just letting everyone else who doesn't know, now know aswell. This is for those that use a DC-DC power supply but don't want to disconnect all the connectors inside their CarPC and reconnect them to a standard PSU as this is a big pain in the arse. This method uses a cheap standard ATX PSU to act as your car battery. This has been discussed before but I could never find a step-by-step method. So here it is.
Basically, buy a cheap standard ATX type PSU for around £5 ($10) from your local PC supplier shop. Make sure it's around 300Watt and can supply at least 15Amps at 12Volts on the 12V rail (it will have a sticker on the side of it saying the output Amps for the 12V rail). This PSU will now be your car battery when your in the house. You will obviously also need to supply mains power to this using a standard power lead.
As we have an ATX power supply and we're not using a motherboard we need to short a connection to make it turn on. The green wire (sometimes grey on Dell PCs) coming from the PSU should lead to the ATX motherboard connector (Pin 14). Short this with any black wire (there is one next to it) with a piece of wire to allow the PSU to turn on. Enure it does turn on (by listening to the fan rotate) before continuing. The method I used is here: http://modtown.co.uk/mt/article2.php?id=psumod
All the yellow wires coming out of the PSU are 12V - which is what we need (same as car battery). Cut all the yellow wires (just the yellow wires at the moment) at their connector ends leaving a bit of length to each wire. You should typically have 5 wires (maybe more). The yellow wires will typically be wired to all kinds of connectors. Don't worry, cut them all.
These yellow wires are the positive terminal of your house car battery.
Now do the same with any 4 of the black wires coming from the PSU (Obviously execpt the one you just used to short the green wire). HOWEVER, leave at least one molex connector with a red and the black wires intact. So one of your connectors will now have RED / BLACK/ BLACK / [cut off yellow] . This will be used later.
These black wires are the negative terminal of your house car battery.
Strip all the wires you just cut (yellow and black) and twist the bare metal ends neatly. Now take all the yellow wires ends EXCEPT ONE and twist them together to make one big wire. Do the same with ALL the loose cut black wires.
You should now have a bunch of yellow wires all twisted together, a bunch of blck all twisted together and a single yellow wire on its own. You should also have at least one molex connector with two connected black wires and a red wire going to it.
Connect the bunch of yellow wires to the positive inputs of the OPUS. I have the OPUS 120W which means I have two yellow input wires for positive. All these need to be connected up. How you connect these is up to you. You can simply twist them together and tape them or attach connectors. Twisting together and taping is cheaper but less convenient.
Do the same with the bunch of black wires and the negative input (2 black wires on my OPUS 120W).
STEP 7 (THE IMPORTANT ONE)
This is the part that finally got my setup working. After hours of searching I found that a small load has to connected to the 5V rail of the PSU in order for it to function properly. This is why we left a loose molex connector in STEP 4. Plug this loose molex into any device that runs on the 5V rail. I'm using an old CD-Drive. You could use an old floppy drive or other device instead (note: the floppy doesn't use a standard molex connector for power).
STEP 8 (ALSO IMPORTANT)
Now the remote feed has to be applied to your system. This is why we left the single yellow wire out in STEP 5. This is your remote feed. On my OPUS 120W, the remote feed is a red wire.
IMPORTANT: Turn on your PSU and WAIT (I mean it!!) for your OPUS to FLASH AT LEAST TWICE before connecting the remote feed up. As soon as you apply the remote feed, the PC should boot perfectly.
That's it. This is by no means a perfect solution - but it does work and it is cheap. The important things to remember is that you MUST have that old device connected to take a load from the 5V PSU rail and you MUST wait for 2 flashes from the OPUS. Of course, using the old atyle AT PSUs don't have these problems.
Hope this helps in some way!