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05-15-2021, 03:43 PM
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#1
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Member
Registered: Jun 2018
Posts: 63
Rep: 
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Pico PSU - faulty?
Hi,
I have a strange fault with a Pico PSU (for a mini ITX system I'm building). The 12V rail is putting out about 5V - I guess this is a broken PSU? The thing is, I don't understand how this can happen on a pico psu - as I understand it, it just passes the 12V straight through from the external power brick (which is giving 12.32V).
To make things even more strange, the system actually runs with this crazy 12V voltage! It sits quite happily on the bios screen for hours, and even boots into Ubuntu on a USB stick, but hung somewhere through installing. (That was before I found out about the voltage, now I don't want to plug it back in before I work out what's wrong.)
Unloaded, the +/-5V rails are showing +/-5.07V (both with a multimeter and in the bios) - and the +/-12V rails are also showing +/-5.07. But when it's plugged in and the system is running, the 5V lines are the same (give or take 0.01V), while the +-/12V lines both drop to about +/-4.9V.
What on earth is going on?? And instead of a new psu, can I fix it - can I just run the 12V from the external power brick straight to the 12V socket on the motherboard?
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05-16-2021, 01:44 AM
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#2
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LQ Addict
Registered: Dec 2013
Posts: 19,872
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Quote:
Originally Posted by Mantra
Pico PSU (for a mini ITX system I'm building)
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I recently had a problem with a mini-ITX's PSU - not sure it's the same "pico" thing, have a look.
The computer just stopped working, wouldn't boot, completely dead. I thought it was the motherboard, because (rudimentary) measurements of the PSU showed nothing wrong.
On a whim I put another PSU (normal, i.e. full size) and lo and behold, everything still works.
I see no difference in power consumption compared to the smaller one. Less noisy, too.
That's all I have, I wish I could help in a better way.
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05-16-2021, 06:23 AM
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#3
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Member
Registered: Jun 2018
Posts: 63
Original Poster
Rep: 
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Yeah power supplies can sometimes cause strange problems - my first thought was a corrupt USB stick. Before I saw it I would never have expected a system to boot with 5V on the 12V rail! My power supply is smaller than that one, it's basically just a circuit board on top of the ATX pin but it needs an external power brick too.
I tried fixing it just now, I soldered a wire across between the 12V input wire from the power brick, and the ATX 12V output pin. So far it's working, and I've just installed Ubuntu - now just have to see if telephone wire and solder gets too hot with a couple of amps going through it!
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05-16-2021, 07:50 AM
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#4
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LQ Guru
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, Slarm64 & Android
Posts: 17,270
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The thing that might give trouble heat wise is the enclosure and/or the pvc insulation.
I routinely used to use a single strand of multi core wire to bridge broken tracks and it functioned as a slow-blow 5A fuse. Alternatively, you may have heard of a water fuse, for R&D work. This again was a single strand in a bucket of water for cooling and had very good properties for inductive loads. A single strand took about 25A, two single strands in the same bucket were good for 50A.
Just keep it cooled or open. If it's heated by other stuff, it's a whole different ball game.
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06-03-2021, 05:02 PM
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#5
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Member
Registered: Jun 2018
Posts: 63
Original Poster
Rep: 
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Sorry, didn't get a notification of your reply otherwise I'd have replied earlier. I hadn't heard of a water fuse, good tip to know if I ever need that! My new system's been running for a couple of weeks now, and the wire hasn't melted yet so I think it's good
In fact the cooling on this setup seems to be *really* good. Sure, it's in a fairly open enclosure, and the chip TDP is only 10W, but at full load the cpu temp sits at around 60-65C - not bad for a totally fanless, silent system!
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06-04-2021, 07:00 AM
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#6
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LQ Guru
Registered: Jan 2006
Location: Ireland
Distribution: Slackware, Slarm64 & Android
Posts: 17,270
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Yeah, the water fuse has particularly attractive qualities for inductive loads. When it fuses, it gradually becomes a higher resistance, which prevents inductive spikes. That's because the water has the solder used in tinning and the copper in proximity and conducts, but these disperse. I'm sure am emptier bucket would reduce the fuse rating.
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