I'm working on building a HTPC and have a burning question that I've been wondering about now for years. Why doesn't anyone build systems that take, or can take, an external power supply (beyond the mini-itx crowd). I realize that temperature really isn't much of an issue as it can vent the heat directly out of the case. However, providing external power can mean:
1) Less noise (there is no such thing as a "truly" silent fan) -- even if the reduction is tiny
2) Strategic placement of the power supply depending on the allowable cable length
3) Potential to use DC power for powering the PC directly from a battery or solar power which would, essentially, make the power supplies 100% efficient (or closer to 100% than an inverter would be)
4) Smaller case sizes suitable for HTPCs
And these are just four of the obvious benefits. I realize I'm a n00b here, but I honestly can't think of any serious downfalls other than, perhaps, having a huge "wall wart" that would likely just sit on the floor somewhere.
So why do we not see these being created? Beyond that, would it be possible to create a "modded" power supply and case that would take such a beast?
More simple to just buy a quiet, fanless PSU. You will have greater voltage drops on longer cables. Custom case. Custom cabling.
Ever wonder why wall warts are relatively small? It's because they are really only suitable for relatively low power outputs.
When you say batteries, I assume you mean something like a fairly heavy 12 volt automotive type battery. How are you going to generate all of the voltages (plus & minus 12 volts, plus & minus 5 volts, and 3.3 volts? The most efficient way is to use switching regulators. Where are you going to get them? You have two choices - buy or build. Buy is going to be expensive. They are limited production, specialty items. Can you build high efficiency switching regualtors? Then you have a packaging problem. You can build them into an HTPC case. That will simplify cabling, but they will take more volume than a regular PSU would.
None of these problems are insurmountable, but the reason you do not reasonably piced commercial solutions is that there is simply no market for them.
Or jump up to server hardware, where in some rack mount systems they run off DC directly from the building (the whole server room is DC and has to be wired that way) I guess they run cooler then having a bunch of AC to DC converters running and so can run either more CPU's or less AC(Air Conditioning in this case)
Edit: I should mention there is still an AC to DC conversion going on, its just not heating the servers and one large unit with large parts made extremely expensive is very unlikely to fail.
When you say batteries, I assume you mean something like a fairly heavy 12 volt automotive type battery. How are you going to generate all of the voltages (plus & minus 12 volts, plus & minus 5 volts, and 3.3 volts?.
12V to the required voltages would be a very simple setup if you are generating electricity. The most important thing to be certain of is that you can supply a stable 12VDC output. Remember dc->dc conversion is lossless the most efficient way of using power. You could split your 12V into the different voltages and wire that to a 24pin plug you plug into your motherboard. However there is a far easier way
I would recommend buying a carefully selected small form factor pc, for example this pc (), all you want is a computer that you plug in a 12V power supply to (for example it comes with a transformer that converts your ac voltage to 12VDC). This way if you supply the pc (from its own circuit breaker) with a 12V cable (ensure it will be 100% stable at 12VDC, the acceptable range is generally 11.5V-12.5V) you will easily have a pc that can run for free from the sun.
As a side note I would also use some Deep Cycle wet Flooded as backup supply for the pc. These batteries are cheaper as you need to check water levels and top up periodically and you can not smoke around them but you get the most bang for your buck. Alternatively if you can splash out on sealed batteries then by all means do so as they are maintenance free and you can install anywhere (out of direct sunlight). For one of these pcs you would only need one small solar panel to maintain the charge. If you look at the current rating of the transformer that supplies the pc it will have a max rating. You can use to calculate the size of the installation you will need to run your setup. For example as a standalone system the above pc (23W max load on ac) would need an absolute maximum of 560W solar panels and 463.83Ah worth of batteries would give 5 days autonomy (based on being on 24/7). 330W solar panels and 278.30Ah worth of batteries would give 3 days autonomy. Yet if the pc was only run 12hrs per day then you would be looking at 175W and 140Ah of batteries. At those requirements you could easily have a solar setup for under $1000. You can get 450Ah (2x6V cells) of batteries for $600NZD.
You could easily use this pc as a router, media server and media center as you would find it could easily be left on 24/7.
Good luck on your ventures, I will happily answer anymore questions.