ATX Power Supplies - What do I REALLY need?

So I'm building my pendant which houses the PC bits and monitor.

Long story short - I am unhappy with the ATX power supply that came in the PC I cannibalized.
( Dell Optiplex 745 with a standard sized 350W ATX power supply. )

What do I REALLY NEED for power supply for this thing? 350W? 300W? 200W? Ho do I find out?
I'd like to be able to get the smallest one possible that is reasonably priced.

Using a SSD SATA drive and the large processor fan that came with the box. Likely not use a permanent CD/DVD.

Mesa 5i25 if that matters.

Can I use one of those pico power supplies? ( 200W )

Thanks.

i'd go for a 350 or 400
put the largest one in you can, it will be more stable and run cooler and as the encoders or whatever is hung on the machine 5v rail is supplied from the pc
although you should not see any difference . the reason here is stability and long running

Dave,

To clarify - it will power the PC and only the PC. I have DIN rail power supplies for everything else. ( 5V, 12V, & 24V ) Basically, just the motherboard, memory, a fan, the processor, and SSD

Does that change anything? Thanks.

You know... What prevents me from simply wiring the appropriate wires from each DIN rail power supply????? What is the down side to that?

Poor form, I know, but I'll reply to myself in the hopes that it helps someone else later.

Nothing. It won't hurt to run the PC from a DIN rail power supply, but I forgot about the 3.3V needed as well. By the time I buy another power supply or do some magic to drop the voltage down on one to fit the 3.3V, it's just simply easier to have an "all-in-one" power supply at your beck and call.

I found some estimating calculators online ( my Goole-Fu is strong today... ) but it's not 100% clear to me that I can trust them. If I did things correctly, it appears that I could very well get by with a 200W power supply but that a 250W one is recommended for margin of error.

I _could_ buy one of the "pico" power supplies usually intended for mini ITX type motherboards, but they typically top out at 200W so it would be a gamble....
In the 250W range, there are some nicely sized boards that are intended more for automotive and marine applications, but they're pricey at ~ $75-$85 USD. For that money, I could just buy a mini ITX board and pico power supply...

I think I will likely look for a "slim-line" 250W "traditional" power supply and call it a day.

I have bought a picopsu 120w some years ago for using it in a mini itx atom server.
I was not stable as a real psu.
HDDs reported smart errors etc when the load was close to the limits.
The probems was only to HDDs the rest system was fine.
If you plan to use ssd which not have power peaks it should be ok.

vre wrote:

I have bought a picopsu 120w some years ago for using it in a mini itx atom server.
I was not stable as a real psu.
HDDs reported smart errors etc when the load was close to the limits.
The probems was only to HDDs the rest system was fine.
If you plan to use ssd which not have power peaks it should be ok.

I would LOVE it if that were the case, here. But my understanding is that the ATOM motherboards and processors use much less power than their ATX counterparts. This is/was a Dell Optiplex 745. The problem is that I can't seem to find a definitive answer on this, and don't really feel like gambling on such a trivial thing to have it come back and bite me in the buttockal unit.

andypugh wrote:

I have been running a D510 atom board on an 80W PicoPSU for years. But that system has no fans and uses a DOM SSD instead of a HDD so is very low power.

Andy,

Yes, if I had my "druthers", that's what I would be running. But it was hard to justify doing right this minute with all of this kit laying about, here. I _might_ have dodged a bullet, as I might be able to indeed use this power supply if I get creative with its location and use some extension cables. I ordered them this morning. LCD should be here tomorrow, so I might be able to have a PC/Pendant running this weekend. Would really like to button this thing up...

If you really lack space for a normal-sized ATX PSU I suggest that you remove the PCB from it's housing and mount it separately. Often that gains you a few centimeters in height (but little in footprint).

Make sure it gets some airflow; ATX-PSU manufacturers skimp on metal and use airflow to make up for it. If your total enclosure has enough surface area and is made of metal you might get away with only circulating air inside. Not sucking in dirty shop air is a huge plus.

A warning though: those PSU's work with voltages up to 400V, these stay there for a while after powering down the PSU, and the heatsinks are often not at a safe voltage either.