Will5455
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computer power supply

Fri Jan 18, 2019 5:51 pm

I want to connect a rasspberry pi to a computer power supply does a computer power supply work by supplying demanded amps or just give it all the juice at once?
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rpdom
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Re: computer power supply

Fri Jan 18, 2019 5:58 pm

All computer power supplies should have a fixed voltage (the most important bit) and a maximum amount of Amps they can supply if required. The system they are supplying defines the Amperage level.

Take a car battery for example. One might be able to provide 600 Amps for a short time to start the engine, but it can't force 600 Amps into the car's electrical system all the time because the car will only take what it needs.

Bench test power supplies can be set to provide a fixed Amperage, but in that mode they will vary the voltage to provide the requested Amps. But that is not the sort of power supply you are using.

Heater
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Re: computer power supply

Fri Jan 18, 2019 6:31 pm

As noted PC, and other power supplies will provide whatever fixed voltage they are, the amount of current depends on the load, so a Pi will only draw the current it needs.

But a couple of things to take care of:

1) It is possible a PC supply will automatically shut down if the current demand is not big enough. A Pi might not be enough load to keep int running. I have never done this and it may vary according to supply so I don't know for sure.

2) PC power supplies can supply huge amounts of current, enough to melt cables and start fires if you mess up an short circuit something. I would put a fuse in the line. Take care.

fruitoftheloom
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Re: computer power supply

Fri Jan 18, 2019 7:02 pm

Will5455 wrote:
Fri Jan 18, 2019 5:51 pm
I want to connect a rasspberry pi to a computer power supply does a computer power supply work by supplying demanded amps or just give it all the juice at once?

Yes a standard ATX PSU has a 5V output:

https://www.instructables.com/id/How-to ... hout-a-PC/

The Raspberry Pi SBC will take the Amps it requires and if power via the microUSB it has protection.
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wh7qq
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Re: computer power supply

Fri Jan 18, 2019 7:18 pm

The fuse may still be a good idea in the event of "misadventures" between the computer supply and the RPi.

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thagrol
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Re: computer power supply

Fri Jan 18, 2019 11:03 pm

I've run various Pi from an ATX PSu without problems (so far).

However, it's not quite as straightforward as you might think:
  • If there's not enough load the PSU may not shutdown, instead it may just not regulate voltages correctly.
  • As has been said there is an awful lot of current available in the event of a short. The PSU will likely protect itself but not before your Pi, etc has been fried
  • The power switch on the PSU does not control the main outputs. The only output present when it's on is the 5v standbay feed (which is enough to run some models of Pi)
  • The behaviour you see when pushing the power button on a PC's case isn't really controlled by the PSU. Transition from off to on is handled by hardware on the motherboard, from on to off by a combination of the OS and hardware on the motherboard

Bottom line?
Will it work? Yes
Will it push 20 amps or more into the Pi? Only if the Pi "wants" 20 amps
Is it safe? Mostly, usual caveats and warnings apply

You should read up on the ATX PSU spec, espeically things like pinouts and control signals. A google search for something like "use ATX psu as bench supply" might also be helpful.
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HawaiianPi
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Re: computer power supply

Sat Jan 19, 2019 4:46 am

Will5455 wrote:
Fri Jan 18, 2019 5:51 pm
I want to connect a rasspberry pi to a computer power supply does a computer power supply work by supplying demanded amps or just give it all the juice at once?
Curious as to why you'd want to use a computer power supply on a Pi?
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Imperf3kt
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Re: computer power supply

Sat Jan 19, 2019 10:15 am

HawaiianPi wrote:
Sat Jan 19, 2019 4:46 am
Will5455 wrote:
Fri Jan 18, 2019 5:51 pm
I want to connect a rasspberry pi to a computer power supply does a computer power supply work by supplying demanded amps or just give it all the juice at once?
Curious as to why you'd want to use a computer power supply on a Pi?

sledgehammerflyswatter.jpg
You'd probably be surprised to know that most of the power supplies I've seen only supply about 3 to 5 amps on the 5v line

Hardly sledgehammer vs flyswatter.
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thagrol
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Re: computer power supply

Sat Jan 19, 2019 1:40 pm

HawaiianPi wrote:
Sat Jan 19, 2019 4:46 am
Curious as to why you'd want to use a computer power supply on a Pi?
I can't speak for the OP, but my use cases have included running an internaly mounted auxilary allways on LCD and enhanced WoL support (adding wake on wifi, shutdown via WoL packet, ...).

I'm also planning to use an ATX psu when I get around to building the cluster I've been planning for some time.
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drgeoff
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Re: computer power supply

Sat Jan 19, 2019 3:18 pm

Imperf3kt wrote:
Sat Jan 19, 2019 10:15 am
You'd probably be surprised to know that most of the power supplies I've seen only supply about 3 to 5 amps on the 5v line
I would be surprised given that the weedy, more than 10 year old one that I'm looking at as I type this is labelled:
+5v 15A
+12v 3A
-12v 0.5A
+3.3v 12A
+5vSB 1.5A

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thagrol
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Re: computer power supply

Sat Jan 19, 2019 5:10 pm

drgeoff wrote:
Sat Jan 19, 2019 3:18 pm
Imperf3kt wrote:
Sat Jan 19, 2019 10:15 am
You'd probably be surprised to know that most of the power supplies I've seen only supply about 3 to 5 amps on the 5v line
I would be surprised given that the weedy, more than 10 year old one that I'm looking at as I type this is labelled:
+5v 15A
+12v 3A
-12v 0.5A
+3.3v 12A
+5vSB 1.5A
Yeah second that. the 850W PSU in this ere ATX PC can push 33A on +5v and 28A on each of it's two 12v rails. Oh and 30A on +3.3v

A quick search of a few online retailers suggest that even the cheap 300W PSUs will have at leat 12A available on the +5v line.
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Heater
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Re: computer power supply

Sat Jan 19, 2019 5:39 pm

Exactly, never mind how surprised Imperf3kt is.

PC supplies can deliver 70 or 100 or whatever watts. Enough to cause significant damage and start fires when things go wrong.

Heck, I have had smoke coming out of a very hot SD card in a Pi just using a regular wall wart. Plenty enough to cause ignition in the wrong circumstances.

As always, take care, check the specifications, use fuses and other protection measures as appropriate.

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rpdom
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Re: computer power supply

Sat Jan 19, 2019 5:49 pm

thagrol wrote:
Sat Jan 19, 2019 5:10 pm
drgeoff wrote:
Sat Jan 19, 2019 3:18 pm
Imperf3kt wrote:
Sat Jan 19, 2019 10:15 am
You'd probably be surprised to know that most of the power supplies I've seen only supply about 3 to 5 amps on the 5v line
I would be surprised given that the weedy, more than 10 year old one that I'm looking at as I type this is labelled:
+5v 15A
+12v 3A
-12v 0.5A
+3.3v 12A
+5vSB 1.5A
Yeah second that. the 850W PSU in this ere ATX PC can push 33A on +5v and 28A on each of it's two 12v rails. Oh and 30A on +3.3v
Agreed. The tiddly 275W Dell one I replaced for my local Off licence when their "till" broke (yes, I did get paid in beer) claims 18A on +5V and 17A on +12V. That one is so old it has a manual 220V/115V switch on it :o I think it dates from 2003.

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Imperf3kt
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Re: computer power supply

Sat Jan 19, 2019 10:09 pm

Hmm. Several people seem to disagree with me so I guess either I've misread the labels or I'm looking at the crappiest power supplies I've come across xD

Anyway, a fuse is a good idea, but make sure it is a fast blow kind.
Recently had one save me and my Pi when I accidentally (I'm assuming because I don't actually know what I did) short circuited a lipo while soldering it to my portable Pi power system.
Safe to say, all electronic equipment came out unharmed, but I had to replace the fuse.
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markkuk
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Re: computer power supply

Sat Jan 19, 2019 10:56 pm

Imperf3kt wrote:
Sat Jan 19, 2019 10:09 pm
Hmm. Several people seem to disagree with me so I guess either I've misread the labels or I'm looking at the crappiest power supplies I've come across xD
I guess you were looking at the spec for the 5V Standby line, which is always powered on even if the main outputs are off. 5VSB is typically 2-3 Amps.

Will5455
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Re: computer power supply

Sat Jan 19, 2019 11:14 pm

drgeoff wrote:
Sat Jan 19, 2019 3:18 pm
Imperf3kt wrote:
Sat Jan 19, 2019 10:15 am
You'd probably be surprised to know that most of the power supplies I've seen only supply about 3 to 5 amps on the 5v line
I would be surprised given that the weedy, more than 10 year old one that I'm looking at as I type this is labelled:
+5v 15A
+12v 3A
-12v 0.5A
+3.3v 12A
+5vSB 1.5A
just so we are clear that is about the era of mine
and my use case is a computer nas running several hard drives some sata some molex.
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HawaiianPi
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Re: computer power supply

Sun Jan 20, 2019 1:22 pm

The smallest ATX compatible power supply I have, both in wattage and physical size, is a small-form-factor (SFF) 320W that outputs 15A on the 5V line (+3.3V at 15A, +5V at 15A, +12V at 16A, -12V at 0.6A, +5VSB at 2.0A < standby line). I built a mini-ITX gaming system for my friend Kristine awhile back with an impressively small 450W PSU that seemed to actually deliver. She still has that system (and still loves it).
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Re: computer power supply

Sun Jan 20, 2019 3:52 pm

Note that fuses have resistance (otherwise they would not be able to heat up, and burn through). Make sure you use one that does not impair the voltage the PI receives, or it will do exactly the same as a "bad USB cable with wires that are too thin", and becomes a problem all in itself.
The fuse should be just a tiny fraction of an ohm, and it will drop the same tiny fraction of a volt, for each ampere that is running through it, and the PI can only tolerate 0.25 V drop. meaning the resistance of the fuse should be (far less) than 0.1 Ohm.

Therefore I would NOT recommend using a fuse, unless you power through the GPIO's, and bypass the (poly)fuse that is already in the RPI. Otherwise you will essentially use two fuses in series, and generate twice the voltage drop.

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Imperf3kt
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Re: computer power supply

Sun Jan 20, 2019 8:18 pm

That's an unexpexted suggestion.

How much voltage drop are we talking about? Because on my system which has a 3A fast blow glass fuse in series with the polyfuse, I am measuring a drop of 0.02v at the GPIO 5v pin (5.23v instead of the 5.25v I am supplying)

I' ll happily trade 0.02v for additional safety.
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Will5455
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Re: computer power supply

Sun Jan 20, 2019 9:56 pm

but the polyfuse protects the pi right?
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Imperf3kt
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Re: computer power supply

Sun Jan 20, 2019 11:34 pm

Will5455 wrote:
Sun Jan 20, 2019 9:56 pm
but the polyfuse protects the pi right?
It isn't indestructible. It won't last forever. One trip could be enough to make the polyfuse more of a problem than a safety device.
Some users have reported the polyfuse never returns to its original resistance and as a result, causes more voltage drop after recovery.
It also may not be quick enough, some Pi engineers claim that while the polyfuse will trip in the event of an overvolt, if it doesn't trip fast enough, the 5v regulator will be damaged.
Adding an extra fuse helps mitigate that issue and is far easier / cheaper to replace.
Desoldering the polyfuse and soldering a new one in place, might also damage the new polyfuse (it works on heat after all) or may damage other components on the Pi.

Finally, replacing the polyfuse voids your warranty.
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davidcoton
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Re: computer power supply

Mon Jan 21, 2019 9:41 am

Imperf3kt wrote:
Sun Jan 20, 2019 11:34 pm
Will5455 wrote:
Sun Jan 20, 2019 9:56 pm
but the polyfuse protects the pi right?
It isn't indestructible. It won't last forever. One trip could be enough to make the polyfuse more of a problem than a safety device.
Some users have reported the polyfuse never returns to its original resistance and as a result, causes more voltage drop after recovery.
It also may not be quick enough, some Pi engineers claim that while the polyfuse will trip in the event of an overvolt, if it doesn't trip fast enough, the 5v regulator will be damaged.
Adding an extra fuse helps mitigate that issue and is far easier / cheaper to replace.
Desoldering the polyfuse and soldering a new one in place, might also damage the new polyfuse (it works on heat after all) or may damage other components on the Pi.

Finally, replacing the polyfuse voids your warranty.
Protection circuits can be effective even if sacrificial. Usually not the polyfuse, but the TVS diode that prevents overvoltage affecting the "guts" of the Pi, converting the fault to overcurrent, which switches the polyfuse to high resistance. In the process it can fail short circuit. Recovery involves removing or replacing the diode. In the former case, a repeat overvoltage will be fatal. Ask Heater.

While the polyfuse is not fast enough to save the diode, the diode is fast enough to save the Pi.

An external fuse serves a different purpose, it protects the wiring from the PSU from burning under fault (short circuit) conditions.

I only remember one example of polyfuse incomplete recovery reported here, and that was when the Pi was refrigerated. I don't know whether that is relevant, but it might well be.

Do you have any evidence that soldering can damage the polyfuse or other components? (It needs some incompetance to do that). After all, the Pi is made by soldering!
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Imperf3kt
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Re: computer power supply

Mon Jan 21, 2019 11:22 am

davidcoton wrote:
Mon Jan 21, 2019 9:41 am
Do you have any evidence that soldering can damage the polyfuse or other components? (It needs some incompetance to do that). After all, the Pi is made by soldering!
Certainly.
The msmf250/x, which is the polyfuse on the Pi, has a solderability rating. That rating is, according to the data sheet, J-STD-002, which can be found here:
http://www.ipc.org/TOC/J-STD-002D.pdf

According to the document, soldering should take no longer than five seconds.
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davidcoton
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Re: computer power supply

Mon Jan 21, 2019 1:24 pm

Imperf3kt wrote:
Mon Jan 21, 2019 11:22 am
The msmf250/x, which is the polyfuse on the Pi, has a solderability rating. That rating is, according to the data sheet, J-STD-002, which can be found here:
http://www.ipc.org/TOC/J-STD-002D.pdf

According to the document, soldering should take no longer than five seconds.
Thanks, that's interesting. Five seconds represents an easily achievable standard of hand soldering -- as long as nothing going wrong!
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Will5455
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Re: computer power supply

Mon Jan 21, 2019 4:48 pm

so just so we are clear what I am doing is connecting several hard drives to a raspberry pi probably a 3a+ with a powered usb hub and A adapter without a power plug and want it to be connected to the computer power supply so that my life is easier later.
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