[SOLVED] What is volt/amps in power supply?

[sxk1277]

I'm so confused. I'm looking to game Apex Legends on ultra graphics with OC RX 580 with i7 4770, 2 4 gb ram, 2 monitor & 2 hard drive. I know about minimum wattage, continuous wattage and 80+ efficiency rating. But the 12v rail and amps confuse the <Mod Edit> out of me. What does 12v mean? Why is the number 12 and not 20 volt? How does it relate to wattage? What do amps mean in relation to that? Are there power supply with multiple rails or is that gimmick? Do i need multiple rails? If its just 1 rail, how do I know if it will go to GPU or if it is going to CPU?

Btw, how do I find out if RP-500-PCAR is compatible with okwvt8 mobo?

 

[jonnyguru]

PC power supplies typically output multiple voltages.

+12V. +3.3V and +5V. There's also a -12V and really old PSUs had a -5V, but they're not needed anymore.

There's also "standby power" that puts out +5V even when the PC is "off" (this is called "+5V Stand By).

People tend to focus on the +12V output of the PSU because that's what 90% of PCs made in the last decade actually use.

The reason why wattage alone on the PSU isn't an important number is because you can have a PSU with a high total wattage capability, with very little of that power available where you need it: On the +12V rail. It's like having a car with 300 horsepower, but 200 of that horsepower is only used to turn the air conditioner compressor.

The Cooler Master Extreme RP-500-PCAR is a prime example of what kind of power supply NOT to by. It's about 15 years old and wasn't a very good power supply 15 years ago. It's absolute garbage, and here's why:

• No power factor correction (read: https://www.fuseco.com.au/power-fac.../articles/the-nee-for-power-factor-correction )
• It has horrible efficiency which means it's driving up your power bill and producing more heat than a more efficient power supply because what's not converted to DC is wasted as heat.
• It's loud. Because it produces more heat, the fan has to spin faster to exhaust that heat to keep the PSU operating at a safe temperature.
• It has a sleeve bearing fan. These fans do not tend to last long because there is no actual "bearing". Just a bushing that the fan spins on.
• 500W is it's peak outuput rating. Meaning it can only output 500W for a few milliseconds before exploding. The real continuous rating of that PSU is more along the lines of 430W or so.

And here's the part that addresses your original questions specifically:

* Despite being a "500W" PSU, it only has 360W on the +12V rail. These days, you can get that kind of performance out of a quality 400W power supply.

And, yes.. The PSU has multiple +12V rails. But it's not really two separate +12V sources inside the PSU. You have one +12V source capable of outputting up to 360W. They take that and split it into two circuits, each with a limitation of delivering no more than 16A each.

And by "0KWVT8 mobo" I think you mean the motherboard out of a Dell XPS 8700, right? That motherboard seems to be a standard ATX12V motherboard, with a standard 24-pin and a standard 4-pin for additional CPU power. So if you bought an ATX12V power supply for it, it should work.

Not all Dell owners are so lucky. Many of their motherboards, particularly Optiplex models, require a proprietary power supply. They have only 8-pins to the motherboard instead of 24-pins. And often they will have +12V on standby instead of +5V. But XPS models tend to be pretty standard stuff. Just don't power it with that garbage power supply unless you want to have tons of problems down the road.

 

[jay32267]

There are a lot of complicated answers for some of these but I'll do my best to keep it brief.

The 12 volts is the voltage...or "potential". It's the potential to deliver current through a resistor.

It is 12 and not 20...probably for multiple reasons.
12V is a pretty standard voltage and it has become that way over the years....like 5 volts has...and 24 volts has (in industry)....and also....the devices that the 12V rail powers are designed to run on 12V due to the components used.

Certain components like fans work better with high voltages.
Integrated chips work better with lower voltages.
This is why you have multiple voltages in a PSU.

Voltage doesn't relate to wattage at all.....UNTIL you add either current or resistance.

They are related like this.

Voltage=Current x Resistance
or
Voltage = Power/Current

There are PSUs with multiple rails.....it's not a gimmick.

You don't need multiple rails.

If it's just one rail....it's very easy.
The 12V rail goes to all the 12V pins in all the 12V plugs coming from the supply.
Same for the 5V and 3.3V if they are one rail each.

....and I'm not sure regarding the compatibility.

 

[sxk1277]

I wouldn't be buying it though. Its already lying around the house unopened. Is it worth trying it out or just need to buy a new power supply? If the power supply only has 1 12volt rail capable of dealing out 19 amps (RX 580 requirement), how do I know this is the one that will be going into GPU and not CPU?

And I'm worried about cooling. I just bought the RX 580 so I've no experience with it. Would 2 fans be enough? the way my case is designed, the 2 fans of RX580 aim toward the bottom of case and are also lined next to it. The problem is, the bottom of case has no holes for the heat to escape. Should I keep my case open? Thanks.

 

[jonnyguru]

I wouldn't be buying it though. Its already lying around the house unopened. Is it worth trying it out or just need to buy a new power supply?

If it's that old and never been used, the value of the caps have probably changed and the PSU won't function properly anyway. Shelf life for a good cap without a charge is only two years: https://passive-components.eu/get-the-lowdown-on-shelf-life-and-storage-of-capacitors/

And that Cooler Master has some pretty cheap caps, so I doubt they'd even last that long.

Of course, if you know what you're doing, you can "reform" capacitors: https://www.qsl.net/g3oou/reform.html

But, like I said, you have to know what you're doing. ;-)

I'd chuck the PSU.

If the power supply only has 1 12volt rail capable of dealing out 19 amps (RX 580 requirement), how do I know this is the one that will be going into GPU and not CPU?

Good question. IF the graphics card actually needs 19A, then that PSU isn't going to work because the rail is limited to 16A. 16A < 19A.

But honestly, an RX580 doesn't need 228W for itself (12V X 19A = 228W). Most RX580's don't even have PCIe power connectors and the slot can only deliver 75W (that's 6.25A on the +12V rail). Even if you had a card with an additional power connector (like the MSI Armor), you're not going to be able to deliver 228W to the card on a good day. I think what they're suggesting is that you need a power supply with at least 19A on the +12V rail FOR THE WHOLE SYSTEM.

And I'm worried about cooling. I just bought the RX 580 so I've no experience with it. Would 2 fans be enough? the way my case is designed, the 2 fans of RX580 aim toward the bottom of case and are also lined next to it. The problem is, the bottom of case has no holes for the heat to escape. Should I keep my case open? Thanks.

Don't leave your case open.

Which Dell XPS do you have and what RX 580 do you have? These are important details.

You're assuming the fans on the card are blowing down, but I bet you dollars to donuts that they're sucking air INTO the GPU. And as long as you have about an inch between the card and the bottom of the case, the air is going to find it's way from the front of the case to the card. But you should also have some case fans to move air around the case... which goes back to the question of "Which Dell XPS do you have?" You only listed your motherboard.

 

[sxk1277]

I've XPS 8700.

 

[jonnyguru]

I've XPS 8700.

Ok. I still don't know what RX 580 you have... but the PCIe slot is not the bottom most slot, so it's not really "right up against the bottom of the case". That Dell's case layout is actually quite normal.

The only thing it's missing is a front intake fan. That Dell only has a rear exhaust.

 

[sxk1277]

Sorry, I've got Sapphire. So, will the cooling be enough?

 

[jonnyguru]

Sorry, I've got Sapphire. So, will the cooling be enough?

Ok. That card sucks in from the fans and blows out the back. You're fine.