What happens if PSU is too small?

[TakoTako]

Are there any long-term ill effects from using a PSU that's too small that aren't immediately apparent? Or will the computer simply fail to start (or crash when overloaded) if there's insufficient power?

I recently got an HIS Radeon 5750 1GB IceQ card, and I figured I'd test it out in my system (330W PSU) and see if it worked - so far, no problems! Boots fine, games fine, shuts down fine. I have a 380W PSU I was prepared to put in the system, but if the 330 is fine I'll just keep using it.

System Specs:
SeaSonic S12-330W PSU
Asus M2N-SLI Deluxe
Athlon 64 X2 6000+ (Windsor 125W)
4 x 1GB DDR2
2 x 500GB Samsung SpinPoint 7200RPM HDD
Sunbeam Rheobus 4-channel fan controller
4 x 140mm low RPM fans
3 x 120mm low RPM fans
GF8600GTS recently replaced with Radeon 5750

 

[Harvey]

An insufficient power supply will probably not cause any catastrophic hardware failure. The most likely things that could happen would be that the machine locks up or reboots, or the power supply just shuts down. Most supplies are short circuit protected to prevent damage under excess current draw,

Your video card is only one factor. You also have to consider the power requirements of your CPU and peripherals.

This power supply calculator on Newegg will give you some ideas. If they don't include all of your particular components on their list, search google for "computer power supply calculator" for some others.

Hope that helps.

 

[TakoTako]

Thanks for the reply - if I have surprise shutdowns or reboots, I'll swap in the 380W PSU.

Outer Vision has a very nice PSU calculator:
http://extreme.outervision.com/psucalculator.jsp

According to that I'm within tolerance using the 330W PSU; I just wondered what problems might arise from using an under-powered PSU - don't want any nasty surprises!

 

[mpilchfamily]

Also with the lack of power can come HDD curruption. But not even that 350W PSU is going to be insufficient. Sure the PSU calc says you need 312W. But then we start up the old argument of what the calc is looking at versus what you will really need. Some say its right on. Others say that is your base line and you need to go 10% or more higher. I like to be on the safe side and go higher. Persoanly i'd go with a 400W to 450W name brand PSU. If its not a good quality name brand PSU you'll want to give yourself some more head room then that to be safe. BUt thats just me. Other PSU Gurus around here can tell you better then i could.

BTW how old are the PSUs in question? What brand are they and did they come with a case?

If they came with a case then they definatly won't support your system even if they where 500W PSUs. Most PSU bundled with cases are older models and are from the low end of some generic brand supplier. These often have less power going to the 12V rails where it needed then newer and name brand PSUs do.

One last thing. AMD recomends the use of a 450W PSU with this card.
http://www.amd.com/us/products/desk...s/ati-radeon-hd-5750-system-requirements.aspx

 

[jvroig]

If they came with a case then they definatly won't support your system even if they where 500W PSUs. Most PSU bundled with cases are older models and are from the low end of some generic brand supplier. These often have less power going to the 12V rails where it needed then newer and name brand PSUs do.

He wrote down his PSU, It's a Seasonic S12. I don't think it's a generic crappy PSU, and it's definitely not a generic brand supplier.

Are there any long-term ill effects from using a PSU that's too small that aren't immediately apparent? Or will the computer simply fail to start (or crash when overloaded) if there's insufficient power?

Will simply lock up or reboot during times of heavy load like while playing a game.

I would say the 5750 is fine for your PSU. It only has a maximum board power of 86 watts. What's more power hungry is your CPU (to me, anyway), as its TDP (supposedly also the maximum power draw when running applications) is a bit higher (125W). Combined, that's still just a little over 200W (let's peg it at 215W, with buffer). HDDs should take about 2 - 8W each. Let's peg it at 10W. You have two, so that's 20W to add to your previous total, giving us 235W.

Not sure about the motherboard and misc components on it, maybe 30W. We're up to 265W.

You have 7 fans. You used to have an 8600GTS (same card as mine). You are now getting a card that seems to be cooler and more power efficient. I only have one case fan for my 8600GTS and Athlon X2 7750. I have no idea why you would need seven. Whatever your reason, 120mm fans consume about 1.25W each, maybe 1.5W. 140mm fans might consume a bit more, or maybe the same. Let's all peg them at 2W each anyway, so we have 14W. From 265W, we now get up to 279W, so let's round it off to 280W.

You look fine to me, especially since it's a Seasonic PSU.

(However, if you overclock your processor, that raises its TDP. So with the 5750. How much extra power gets to be used depends on how much overclocking we are talking about.)

In summary, I think you'll be just fine. If your 330W PSU was a generic, no-name PSU, I'd tell you otherwise, but it's a Seasonic one. If you do wish to swap the 380W PSU in, I hope it's also a Seasonic one instead of a generic no-name brand, otherwise swapping your 330W out for a 380W would actually be a "downgrade".

 

[Red Squirrel]

A PSU wattage is nothing but a rating. You can probably run one past it's rating for a while, maybe even very long, or maybe it will fail right away. Really depends on lot of things like the quality of the PSU. The parts are designed and tested to handle NN amount, but it does not mean they wont work past that amount, unless they purposely stop it with maybe a fuse or something.

In most cases it will probably die prematurely. It's best to run a PSU below it's rating. Same with UPSes, transformers and other equipment, it's best to oversize then to be dead on.

 

[jvroig]

A PSU wattage is nothing but a rating.

That "rating" is how much DC power they can output based on design. Good PSUs are underrated, so they may have a bit more juice beyond the rated capacity. Crap PSUs (like cheap, generic ones) are overrated, sometimes even by 100%, so a crap 500W PSU may only actually output 250W max.

For good brands like Seasonic, therefore, the rating is not unimportant.

 

[TakoTako]

I got the SeaSonic PSU because they have a reputation for quiet, reliable, and efficient PSUs. (The 380W I have in another system is an Antec EarthWatts).

I have 7 fans because... because I can? I've got a Cooler Master CM690 case, which has 6 fan mounts (the 7th fan is on my stupidly gigantic Cooler Master GeminII CPU heatsink). Also, summers get really hot, so it's nice to be able to crank up all the fans when necessary. They're all low RPM, and the four 140mm fans are on a fan controller, so I can run them even slower (and quieter!).

MSI's "manual" for their fanless 8600GTS is just a quick-start guide with little info, so I wasn't even aware of the power requirements for it - I figured I was probably increasing my power draw jumping from the old, weakly 8600GTS to a nice new 5750 (though I did choose the 5750 in part because of its low power requirements).

Anyway, nice to know that insufficient power should be easily detectable by boot failures or random shutdowns, rather than surprise fried components or fires. The system has been pretty reliable for 2 years with the 8600GTS and I haven't had any problems so far with the 5750 (even under extended gaming).

 

[BushLin]

I've seen a couple of power supplies simply refuse to start once it's rated wattage was exceeded, removing a couple of drives allowed it to run again.

Would be nice if they all did that, rather than have an unstable system without necessarily giving an indication of why.

 

[ehume]

This power supply calculator on Newegg will give you some ideas. If they don't include all of your particular components on their list, search google for "computer power supply calculator" for some others.

Interesting. That calculator says I need 384W, with only 4GB RAM (it doesn't go higher). OTOH, the Antec calculator (re-badged Outer Vision) says I need only 239W. Go figure.

 

[Puffnstuff]

If you really want to know what happens to a ps that is not up to the task go to johnnyguru and read a couple of articles on the matter.