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Need to learn about power supplies


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#1 StartingOver

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Posted 05 May 2011 - 03:08 PM

My son just bought a computer (semi-custom build from Directron) using a friend's advice. They seemd to have done OK but, research was not their strong point. I don't have all of the specs but, he received a mid-tower ATX case and power supply. I don't know what motherboard he has but, it's running an AMD Quad 4 processor. OS is Windows 7 Home. The problem I noticed when he set it up was the power suppy. It is a 480 watt PSU. I have not read further on the label for amp information. The wattage made me wonder if the demands of the grapics card were being met. So, I went shopping (translate: researching).

I soon found out that I knew nothing, zero, nadda about PSU's!! And I've learned little that I understand after trying to read through several sites on the web. Here's what I think I know. Using wattage as a reliable criteria to make a decision is nearly useless. I've read that it's how the voltage is distributed (over my head), the amperage to each "rail" (over my head, again). And the list of information I've unsuccessfully tried to digest goes on.

So, with that said, here is what I do know. I know the graphics card installed in his system is a Radeon HD 6850. It has 1GB GDDR5 on-board. The system requirements on the Radeon box state: "500 watt or greater power supply with one 75W 6-pin PCI Express power conectors recommended. (600W and two 6-pin connectors recommended for AMD CrossfireX technology in dual mode). I know that he does not have whatever Radeon is talking about in the parenthesis.

What is CrossFireX & SLI and so on and so on? I know his system is an ATX form factor (whatever form factor means). Now I find out that there is ATX 2.0 and ATX 2.2 and......

Finally, I meet a guy that builds computers & has a few minutes to talk. I asked him what "rails" were & how can I tell what a power supply has (or is doing or whatever). He begins to explain that I would have to buy a "digital" power supply" to have "rails". I glazed over at "digital power supply".

HELP! TEACH ME!!! I'm lost. I'm looking for a good PSU for my son's new computer (maybe 2 months old) in the $75.00 to $175.00 range. He really wants to stay under $125.00 drive-out price (tax, title and license LOL). How do I tell if a power supply is digital? What am I looking for to determine what "rails" it has and what they should be supplying?

Thanks in advance!
StartingOver


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#2 Platypus

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Posted 05 May 2011 - 04:40 PM

He begins to explain that I would have to buy a "digital" power supply" to have "rails".

Ignore this, it's wrong. The "rails" which all PSU's have are the multiple connections available for each voltage the PSU supplies. The important ones are +12 Volt, +5 Volt and +3.3 Volt.

To know the specification of your current PSU you will need to read off the label - if you post all the ratings written on the label in Volts and Amps (eg +12V 20A) or a clear image of the label, we'll be able to check if it's OK. 480W appears to be marginal, unless the PSU is conservatively rated, which most aren't.

To get your system specifications you could Publish a Snapshot using Speccy. That would also help to confirm if you can get by with the existing PSU.

Edited by Platypus, 05 May 2011 - 04:46 PM.

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#3 dpunisher

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Posted 06 May 2011 - 06:59 AM

First rule, all current desktop computer power supplies are "digital".

On modern systems the rail you need to be concerned about is the "12V+". This is the "rail" that supplies the CPU, as well as the videocard, your 2 biggest power users. Many cheap/outdated power supplies have big watt ratings on their 3.3/5V rails (which inflates their advertised wattage on the box), which is absolutely useless now. Your better power supplies will deliver 90+% of their total wattage on the 12V+ rail.

Figure a 6870 pulls ~120-130 watts loaded, your CPU from 95-125W loaded, motherboard/memory/hard drives another 25 watts (worst case), 280 watts total (worst case).

Technically a power supply (PSU) that supplied at least 24 amps on the 12V+ rail should be enough to power your system, but "technically" isn't the real world. PSUs work best/most efficiently at around 50% load. Good ones (80+ gold/platinum) hit the 90% efficiency mark. If you can buy a PSU that runs at 50-60% of it's rating supplying that 280 watts to your system, then you are near the sweet spot. A 550-650 watt PSU fits that bill. That being said, a real quality 480 watt PSU with a ~36 amp rail will work fine for your current setup.

Crossfire/SLI means using 2 or more videocards in the same system.

ATX is the form factor. It refers to the standardized layout/size of the motherboard/case in your system.

I just looked at your location, I am near CC myself. I remember going out at night in the shallows with a Coleman lantern and gigging flounder. Good times.

I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)

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#4 MrBruce1959

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Posted 06 May 2011 - 09:00 AM

Startingover I can feel your stress and your willingness to learn...learn...learn.

Okay here's the beef.

I am going to post a picture below that shows the guts found inside a basic power supply, so here goes.

Attached File  PSU.jpg   151.32KB   4 downloads

You can click that image above and it will somewhat blow up a bit for clarity if you wish.

Any how, if you examine that image above and you take notice of the silver colored long metal things (there is two of them and those are called heat sinks) and try to look behind the large black cylinders and to the left of the yellow transformer, you will notice a slight view of what is called a bride rectifier (the black colored thing attached to the heat sink, a series of diodes that take an AC current from each branch off of the down-step transformer (That's the yellow colored square thing in between the two silver things) those rectifiers also have built in voltage regulators, which regulate the current so it is at a constant flow at a constant rating, it does not fluctuate or deviate from the specified rating.

That current is then passed through a series of filters which are those large round cylinders which are called electrolytic capacitors to filter out the AC ripple, this leaves us with a pure DC signal at the output.

Now going back to the transformer again, the transformer has a series of copper wire which is coiled around a ferrite core and a frame. The input is a pure AC sine wave of around 115 to 120 Volts AC (house current) AC stands for Alternating Current, it alternates in a manner so that neither is just Positive or Negative, it can be either. The job of a Power supply is to do two things, first, it drops the current to a safe usable level, second it filters out the harmful AC which is unstable and vary noisy.

As the AC is passed through a down-step transformer, there are various stages where it is tapped into, these taps intercept the AC current at different levels of the coil where it picks up the different voltages, those are fed into a diode network called a bridge rectifier, it is through this network you obtain the AC voltages of 3.3 5.0 and 12 volts, The diodes are put in such a network that they form a square, a square with four corners, if you look at the square, each point of the square can be labeled, commonly the symbol ~ is used for AC, then + and - are produced where the diodes block part of the AC sine-wave.
So basically, as you go around the corners of the square you would have ~ + ~ -

The + is the POSITIVE phase and the - is the NEGATIVE phase.

This is done for each network, 3.3, 5.0 and 12.0 Volts.

The result still has AC present, so a series of large electrolytic capacitors are put into the circuit between the + and -
Because these devices are polarized and they store a current like a battery, when the pulse attempts to go negative, the capacitor discharges a positive pulse into the output, when the pulse attempts to go positive, the capacitor injects a negative pulse into the output.

This result removes the AC ripple by creating a solid wave form rather than an alternating one, POSITIVE (RED) remains POSITIVE, NEGATIVE (BLACK) remains NEGATIVE.

If you are a bit lost here, think of an AC wave form as being like the letter "S" laying on its side.

Such as this image below.

Attached File  sinewave.jpg   5KB   4 downloads

An Ac sine wave has a ripple, where as DC runs a straight line of the POSITIVE pulse above the center line and a straight line of NEGATIVE below the line, the figure "S" is removed.

In computer systems, the term "RAIL" is used to describe the (Transformer, rectifier, electrolytic capacitor) network I spoke of above. If you have ever seen the rectifier which is mostly hidden out of view in the image above, you'd understand why the term "rail" is used.

I explained it as basic as I possibly can, as there is much more circuitry involved in a power supply than what I mentioned here, such as resister networks and such, but going into that will only draw this post on and on.

As for digital, digital is basically a state of it's either on or it's off.

Yeah I know confusing isn't it?

Basically IC chips are based on logic, their state involves being either Neutral or grounded to create the OFF status.
When the state goes HIGH or POSITIVE its state is said to be ON.

IC chips are now being incorporated into modern day power supplies, because they produce a much cleaner DC signal than the older bridge rectifier types.

As far as answering this question you asked: What is CrossFireX & SLI?

This is a new technology that allows two video cards to be put into a motherboard that has two PCI-E X16 slots side by side, this allows running dual monitors, one on each slot and card.

Check this image below.

Attached File  images1.jpg   10.07KB   3 downloads

As you can see from my image above there are two video cards placed side by side those are Nvidia brand SLI cards to which as you can see have a circuit board connecting them together at the top as well as both being in a PCI-Express slot on the motherboard.

Sorry for the long lecture, but it is my hope that I have demystified your curiosity a bit in the process.

Bruce.

Edited by MrBruce1959, 06 May 2011 - 03:25 PM.

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#5 dpunisher

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Posted 06 May 2011 - 11:55 AM

To elaborate on the above, here is a good tutorial on PSUs: http://www.hardwaresecrets.com/article/327

I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)

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#6 StartingOver

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Posted 06 May 2011 - 01:18 PM

WOW!! This is exactly what I wal looking for! I can't thank everyone enough. To Platypus & dpunisher, your responses are what my son will read and use. Thank you. To Bruce, your response fits me prefectly. I'm a lot more ... uh ... retentive, to be polite, than my son (or most anyone else I know for that matter!). I'll be digging into you post to learn. It will not only help me answer his questions but, I love getting that deep into stuff like this.

I think my son is dead set on getting a new PSU with a higher "rating" anyway. Now I can show him what to shop for so that he gets the best bang for his buck.

Sorry for the delayed response. It's been crazy around here lately. Oh yeah, and to dpunisher, I've spent more than my share of time taking the jetty boat out of Port A & spending the day on the jetty catching specs & reds. My brother-in-law (well, ex brother-in-law now) is the outdoor reporter for the CC Caller Times. He's a great guy. He lives on Mustang Island and gets paid to hunt, fish and write about it. I hate him! LOL If you're ever in the Houston/Galveston area, I know a great wade fishing spot near San Luis Pass, especially during the flounder run in October & November. Just PM me & we'll get a line wet! May the wind always be at your back, the sun in your face and your fishing rod bent!

Thanks again to everyone. I'm sure I'll be back with questions.
StartingOver
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