mITX newbie seeks case & PSU efficiency advice

[James W]

I'm new to mini-ITX, but will be building a system with Gigabyte's new H55 mini-ITX board when it becomes available. My other requirements are a slimline optical drive and space for two 2.5" drives (the second to leave room for my dreams of one day buying a solid-state hard drive for the system partition).

However, I'm completely at a loss to know what case to use. I'd like it to be as small as possible given those specs, but even more important is that it should have a highly efficient power supply. I need no more than 125W, and it should have an 80plus rating as an absolute minimum - I believe there's an 80plus Gold rating, but nothing seems to have it. My aim is the lowest possible idle power - inspired in this by a good article on Tom's Hardware about building an i5-based system with 25W idle power.

I saw the thread from earlier this month "Mini ITX Computer Spec Help", and was interested in the Antec ISK300-150 mentioned there - it looks to be about the right size for me, but 300W is more than I need and crucially hardwaresecrets says that its power efficiency is lousy, always below 80%.

I'd be grateful for any ideas, because the case specs I've looked at have scant information on power efficiency.

[simonw]

You could still get the Antec case, and pair it up with a 120/150W PicoPSU (which is 80+) and matching AC adapter.

[James W]

You could still get the Antec case, and pair it up with a 120/150W PicoPSU (which is 80+) and matching AC adapter.

Great, thanks - the things I don't know...

Now that I have my eyes set on a picoPSU, I see that there are very few (if any) cases that are designed to be used with one, i.e. that have no space for an internal PSU. Even the Antec case uses up a large amount of space just for the DC/DC part of its power supply. Is it possible to go even smaller?

[McPhee]

http://forums.hexus.net/small-form-f...-itx-case.html

It's not possible to go smaller with mITX than the M350 Buy a second HDD bracket and a very low profile CPU cooler and you're good to go! The one at the top of this page might fit; http://www.mini-itx.com/store/?c=9 not certain though.

[peterb]

Choose your PSU with care. The pico PSU's use an external 12 V adaptor, and these operate most efficiently when they are operated at or near maximum load - if you have a PSU that is higher rtated than you need, the efficiency will be lower. On the other hand, that leaves little room for expansion.

As an example, I have a mitx system with a 120W pico psu. With no load, the adaptor takes about 15Watts and zero efficiency. Loaded up and the power consumtion in total is about 40 - with the system taking about 30 - so 75% efficient. Had I used a lower rated PSU I would probably had better efficiency (and I have used it with a 60W adaptor from an old LCD display, and that is marginally more efficient)

[James W]

Thanks, that's useful advice about taking care over the external AC/DC adapter. I notice that the US energystar site has a long list of qualifying external power adapters. This includes data on what they call "Average Active Efficiency 230V @ 50Hz (%)". Looking at the specs for this (Eligibility Criteria (Version 2.0), effective 1 Nov 2008), this is worked out in the following way:

Calculate the model’s single average Active Mode efficiency for each test voltage by testing at 100%, 75%, 50%, and 25% of rated current output and then computing the simple arithmetic average of these four values...

Looking at the 12V-output supplies that are rated as 70W or over, they all have 87-91% "Average Active Efficiency". This doesn't tell us what the individual values were at the four tested loads - they could be only 75% efficient at 25% load, with high 90s% efficiency for higher loads, for instance - but this seems to be about the best data I can find.

The list is US-centric but some of those adapters may be available in the UK. Indeed, one of the best is the FSP150-AHBN1 150W adapter, which has 90.4% average efficiency and is available from LinITX.