Power requirements / PSU recommendations for mini ITX system

[malfunction]

Hi folks,

Hope some of you can point me in the right direction here. I'm building a (general purpose) mini-itx system and as I can't cope with the idea of using any of the via CPU options I am intending to build this:

1 x Hoojum Cubit 3
1 x Socket 370 mobo (intel 815 - Freetech P6F135)
1 x P3 1.2GHz (Tualatin, 1.5V)
1 x 512MB PC133
1 x 3.5" 320GB HDD (WD - 7,200 RPM)
1 x slim line DVD burner (Panasonic UJ-845)

With a low power 80mm fan (as came with my case). For now I'll be using the onboard GFX but may upgrade to a PCI based GFX card at a later date (to free up RAM and gain DVI output).

The $64,000 question is:

What PSU should I be shooting at?

I know that the CPU's TDP (typical *thermal* output as I understand it?) is 29.9W so presume the actual draw will be around this figure (+/- 10%?). I may underclock and undervolt the chip (e.g. to 9 x 100MHz FSB at whatever voltage it will go down to) to get both the temps and power requirements lower. I had a few Tualatins back in the good old days and it's not uncommon to be able to run them at 900MHz at around 1.0V (for what it's worth)

Western Digital list the max power dissipation of the drive I'm intending to use as 8.6W.

So I already have 38.5W without the rest and I'm not sure what I should add for:

The motherboard (P6F135), 10W?
RAM (1 x 512MB PC133), 5W?
DVD drive (Panasonic UJ845), ?
A future graphics upgrade (whatever is available as a PCI card, reliable and low power - this will never be a games machine) - is 20W acheivable for a low spec card?

I may be able to find something for power draw on the label of the DVD drive when I'm next at home and will do the same thing for the fan.

So far I'm guessing at 90W required (max, not typical)...?

I'm also not sure what sort of safe value I can assume in terms of efficiency for the external (AC --> DC) PSU and the internal DC --> DC converter. Is it safe to assume 80-85% for AC --> DC supply and something higher - say 90-95%? - for the DC --> DC?

If so then I'd need 100W DC --> DC and 125W AC --> DC?

Free (virtual) beer / cookies / karma for anyone that can help

[jaganath]

This thread from the Power Supplies forum is very relevant IMO for this decision:

PicoPSU @ CES 2006

Your guesstimates for the various components' power draw seem accurate enough. DVD drive playing back a DVD is probably 5W, a little more for initial spin-up.

I await my free beer and cookies with great anticipation!

[malfunction]

This thread from the Power Supplies forum is very relevant IMO for this decision:

PicoPSU @ CES 2006

Your guesstimates for the various components' power draw seem accurate enough. DVD drive playing back a DVD is probably 5W, a little more for initial spin-up.

I await my free beer and cookies with great anticipation!

Your beer and cookies are in the post. I wrote both "Fragile" and "Urgent" on the box though so they may never arrive

Thanks for the link - I'm suprised quite how low the power draw is for the various (non ITX) systems posted in the thread. Various articles @ anandtech would seem to indicate higher values (though I appreciate they could be wrong - and the systems in question are usually higher spec than those listed). I appreciate that the max theoretical draw for all components isn't likely to occur at the same time but I figure it's a good figure to aim for...

[malfunction]

For those that are reading this and haven't read the following link:

http://www.silentpcreview.com/article28-page4.html

Very useful - makes me feel a lot happier about my requirements.

[Tibors]

Have you seen this thread about the efficiency of a brick+DC-DC PSU

[malfunction]

Have you seen this thread about the efficiency of a brick+DC-DC PSU

I have now. Thank you. So ~80% effeciency at 50W+... Think I now have an idea what I need. Thinking of swapping the cubit 3 for something else though - the price of ~120W PSUs (AC to DC and DC to DC kits) is almost as much as a complete casetronic / travla case that includes 120W PSU!

[Aris]

Have you seen this thread about the efficiency of a brick+DC-DC PSU

I have now. Thank you. So ~80% effeciency at 50W+... Think I now have an idea what I need. Thinking of swapping the cubit 3 for something else though - the price of ~120W PSUs (AC to DC and DC to DC kits) is almost as much as a complete casetronic / travla case that includes 120W PSU!

just to let you know, the 120w casetronic/travla psu is not passive. theirs a small ~40mm fan in the external brick.

also, you can buy just the psu from the case without the case if you want here:

http://www.logicsupply.com/product_info ... cts_id/254

[frostedflakes]

80w would probably be adequate, but 120w units seem to be more common, so I'd just get whatever you can find cheaper.

Keep in mind that power supplies are not rated for their maximum AC load, but rather maximum DC load. So when power supply shopping, you don't have to factor in efficiency when choosing size. An 80w power supply can handle 80w DC (100w AC assuming 75% efficiency), a 120w power supply can handle 120w DC (150w AC assuming 75% efficiency), etc.

[malfunction]

Thanks folks. Just ordered the mobo (in a cheap bundle on ebay - mobo + CPU + RAM for $129). Going to wait until it arrives before I either order a PSU for the case I have now or sell that and order a new case that includes a PSU (I still haven't decided which one I really want to do as yet).