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As far as I understand, computers don't always draw the same amount of power from the power supply all the time. There are times when hard drives are on standby and not using as much as when they're spinning and graphics cards save power when not fully utilized.
What happens when, for example, you have 100 hard drives installed in a desktop tower (or a server rack) with let's say 1000 watt PSU, and they're all on standby, and then suddenly some process accesses all of the hard drives and spins them up, drawing more power than PSU can give?
Is there some signal that hard drives send when they think they're not getting enough power? Or does each individual hardware piece ask the PSU if it can provide it X watts of power, and it may say "no, I don't have that available"? Does the motherboard decide if it can negotiate this power request and safely avoid sudden power loss and instant shutdown? Or is the standard protocol in this case to drop dead without trying to avoid this problem?
From my experience with my desktop and a few hard drives and a low power 350W PSU, it would instantly shut down if 5 hard drives were all trying to spin up at the same time. Nothing bad happened, fortunately, but I'd like to know if instant shutdown is an expected and planned reaction of hardware pieces, or just the motherboard (or PSU) freaking out and disabling everything unexpectedly.
To clarify my question: What I'm interested in is why the common result is system shutdown instead of a safe denial of power to the device which would overload the system? USB power management protects against such a scenario, so why doesn't SATA/Molex power cable management logic not have this (or if it does, why it so commonly fails)?
Update after seeing some answers: I'm really surprised there isn't some sort of power management logic built into PSUs like motherboards have for managing USB power distribution. That's what I got from the answers so far. If you know something that says otherwise, please share as an answer.
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@Ramhound fortunatly we have circuit breakers and fuses so in proper design it should not fry source.
– Maciej Piechotka – 2017-04-21T23:22:48.73010This is why in a typical build you want the power supply to be capable of delivering substantially more than the peak draw of all components. Not only does this protect from such events, but a power supply will also last longer when its normal load is a smaller percentage of its peak capacity. – music2myear – 2017-04-21T23:40:26.240
7FWIW, this is why high-end RAID controllers spin up disks on boot in batches and not all at once. – Jonas Schäfer – 2017-04-22T10:51:15.897
4@Ramhound That's not true. Many power supplies - in general, I'm not talking about desktop computer power supplies - will continue to provide a lower voltage, which may not be enough to operate the load, but won't necessarily damage anything. When this happens we say the load 'draws down' the supply. Other supplies have overcurrent sensing and will shut off, ideally with a beep or a visual indication. And of course, trying to use a device that's powered by rechargeable batteries when the batteries are low doesn't destroy them. – Jeanne Pindar – 2017-04-22T23:43:12.957
1Usually one of two things happens: either the voltage drops and your system malfunctions (generally, it will reboot unexpectedly) or your power supply dies (such as by blowing a fuse, or by exploding if it's really cheap). – user253751 – 2017-04-24T02:48:03.653
1I'm really surprised there isn't some sort of power management logic built into PSU Because they are pieces of crap that retail for $50, which means the real cost is about $15 in a pre-assembled PC. – Kaz – 2017-04-24T05:42:18.613
3That's just how electricity works - as you draw more power, the voltage drops, and the electronics in your computer mostly have a lower limit on voltage; if you draw enough power, they will simply stop working. Smarter PSUs (pretty much standard these days) will completely disconnect to prevent damage (e.g. from one component failing before another in a dangerous way). What would be the point of a PSU that randomly disconnects devices? The computer would be just as broken, possibly more so. And it would be more complicated - less reliable and efficient, more expensive than a bigger PSU. – Luaan – 2017-04-24T07:44:03.733
related question: https://superuser.com/questions/113113/why-are-brownouts-so-harmful
– Pieter B – 2017-04-24T12:51:24.077Mine blew itself up once... I installed a new GPU and didn't think about the PSU (I was 13 and didn't know better) and it simply burned itself out after about half an hour – Persistence – 2017-04-24T18:31:13.710
Is it possible to make a rails PSU that can draw all 10 amps the wall can give it? – Joshua – 2017-04-25T02:46:06.883
@Joshua Where do you get 10amps from? Modern PSU offers 20A+@5v and 30A+@12v. An increase in amps requires an increase in the diameter of the wiring, and better parts in general= more cost. – cybernard – 2017-04-25T14:03:19.067
@cybernard: My wall breaker is for 10 amps at 120 volts. – Joshua – 2017-04-25T15:14:23.653
@Joshua you must live in another country or a really old house or something 15A is a standard circuit break size. 15A is 1800w so buy a PSU to match yours. A 1000W or 1200W PSU will max your circuit assuming you connect enough devices to it. – cybernard – 2017-04-25T19:23:22.140