Power Supplys help

[nymph4]

When making a computer and getting a Powe Supply I know off hand you should just get the most power you can.

But if you get a power supply and it has say 5 Taps for Hards type devices and 1.  Tap for the CPU and a 15 Pin Tap for the motherboard and all of the Taps are use and connected to a device.

The Power Supply should be ok because if it chould not supply that mutch power it would not give you that many taps RIGHT???

[ChrisXPPro]

IMO the number of taps - lead-outs, is no guide to power availablity at all.

I have had 250W PSU's in past with loads of available connectors ..... more even than might be found on a 600W even.

Much will depend on the per-unit requirements within the hardware config - for instance you could have a very ''hungry'' graphics card, even if rest was modest.

Assess best you can your power requirements and get the best rated PSU you can afford ... assuming it has enough connectors ... but even so you can daisy chain safely with adaptors if you have a generous wattage.

[nymph4]

I do understand what you are telling me but how do the Power Supplys work?

What I meen is say a Motherboard take 5 AMPS  and you have two Hard Drives both are are what 2 AMPS.

Well what I ment is every tap on the Power Supply is for say Hard Drives and drives like that those kinds of taps only will supply 2 AMPS.

So if all the taps are going to be connected to a device then every tap will be giving out 2 AMPS just say.

So you want to make shure the total WATAGE can give that many AMPS at one time to those taps.

I do get that so when getting a Power Supply should I look at first the Watage and then see how mutch power it give to every tap. And if every tap togather comes out lower then the Wattage of the Power Supply I will be ok with it???

[ChrisXPPro]

Most computer power supplies are generating voltage outputs thru a switching system - as against ''conventional'' transformers with multiple transformer winding taps and rectifiers.  They generate very accurate voltages with stabilization.

To work out your wattages - multiply the volts of each item by the current ..... so if 12V @ 5 amps for one mobo input then that is 60W.  If 5V @ 5 amps then that'd be 25 Watts.. and so on.

The taps do not ''give out'' 2A ..... they provide a voltage and the current used needs to be be cumulatively (the total of everything) within the capacity of the power supply unit.

Just remember - amps x volts = wattage ... so try and go thru all your hardware re it's voltage requirements and current drain - then do the math and total everything up.

The PSU as I am trying to explain - does not ''give'' current to an item so much as supply a voltage - within which a certain current will be needed.

Voltage is (water analogy) the ''pressure'' - which supplies a volume flow (amps) - providing the power unit has enough capacity to supply the total current required by the whole system.

If I remember right the PSU will be delivering 12V and +/-5v feeds.  Maybe a 3v too - but I forget without checking.

[nymph4]

Thanks for the help I now know that the Power Supply just supplies the Voltage not AMPS.

And just ad up all my AMPS that all my devices will be taking and Times it but the Voltage and that will give the Wattage I will need.

But can you tell me what a Rail is I see a lot of things like 12 Volt Rail

[ChrisXPPro]

Slight correction - the PSU supplies both volts and amps ...... but the voltage is the ''pressure'' to deliver the amps the unit is capable of producing (its total power rating in watts - amps x volts.)

Yes check current requirement for each item but - times that by the voltage of the device.

A ''Rail" is a term used to desribe a voltage supply line .. so a 12v 'rail' is a section of wiring or circuitry concerned with that specific voltage - likewise a 5v 'rail' is concerned with supplying that voltage to relevant devices.

Example - taking a set of figures from a hard drive I have to hand - it is rated 5v at 0.65A and 12v at 0.9A.  So - this represents in power terms ...... 5 x 0.65 = 3.25W and 12 x 0.9 = 10.8W.  Add these and we have total 14.5W.  One of the more modest power requirements within a system.

In contrast however, things like graphics cards in particular can require much more power.  Single cards might be around 25W to 50W but some higher end cards can go up to over 200W.  You may have to do some search on your graphics card to find out its power needs.  There is some useful info - here ....

http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354

[nymph4]

Thank you for your help so far.

I was just at the web site Asus.com and I was looking at a Power Supply A-50GA
and I would like to know how to read the Rattings?

It say Total Output Wattage 500 Watt  ok I get this this is how mutch Wattage the power Supply can give.

But look ware it says + 3 . 3 Volts   under that it says 30 Amps. Then next to that it says + 5 Volts and under that it say 28 Amps.

So am I to read this as ONE Tap will supply + 3 . 3 Volts at 30 Amps   and ONE Tap will supply + 5 Volts at 28 Amps??

[ChrisXPPro]

Yes you are right - there is a total power rating but each voltage supply will in effect have its own individual rating too (current available at a given voltage).  It's been a while since I looked hard at a PSU spec'.



 - this pic is data on an Antec 500W unit. 

If we do the math we'll see that the power seems to come out as ...... 85W + 204w + 204W + 204W + 9.6W + 12.5W + 76W ........ total - 795W!!!  Wait you say - that's more than 500W ... yes it is but this calculation is based on max amps on all voltage lines at same time ....... and quite possibly not all 12v lines in this case will be used - or, the total drain on each will not be at max.

The ideal with any PSU is to have enough ''reserve'' supply power such that nothing gets max'd out ..... altho most PSU's will permit short duration peaks .... but sustained excess draw will only lead to overheating and probable failure.

[nymph4]

Ok here is one I found at Tiger Direct.com ULT40064
 
See ware it say output voltage and under that it shows a whole list of Rattings.
+ 3.3 V under that it says 24 A

So I would read this as telling me that this is ONE Rail and it will give + 3.3 Volts at 24 Amps.

And the one right next to it says 5 V and under it says 28 Amps  so that would be another Rail that will give 5 Volts at 28 Amps.

Do I have this down  that every group is a Rail???

[ChrisXPPro]

Well yes - if you want to use the term ''rail''.

Look at it this way - the PSU itself is a unit but you could regard it equally as being several units within one - with each supply line (or rail if you wish) being a discrete entity... and the current ratings are MAX - absolute max ...... and so better to know if possible you will, under use - be well under that figure.

[nymph4]

Well what I think I am not understanding about the Power Supply Chart is that when I se +3.3 Volts and right under it it says 24 Amps or what ever I now know they go togather. But is it telling me that that is one Tap??

What I meen is is it telling me that the Power Supply will have one Power Connector coming out of it that will supply +3.3 Volts at whatever Amps it says??

[ChrisXPPro]

The image I showed above for an Antec 500W is showing each discrete lead-out option - so each one has its specific volt and current designation.  Go to the Antec site and browse thru - this should help.

As I said - it is as if each feed coming out of the main PSU was - in effect - a discrete PSU on its own!

Of course when need be - with adaptors - it is quite OK to ''daisy-chain" off a feed to expand it - providing  the power available is not exceeded.

[nymph4]

Ok I think I understand a little more then  did.

I am looking at specs for Asus a-45ga Power Supply and I just want to know if I am readding it right?

It starts off with

+3.3V
30A

Then it has
+5 V
28A

Now I think I understnd that it is telling me that inside there is going to be one Rail that will be suppling the +3.3 V and the +5 V RIGHT??

Just explane this part

[ChrisXPPro]

Yep - in fact there would usually be 3.3v, 5v and 12v rails .... each being ascribed a max amperage.  Often tho if you total up all the rails for power you'll find it seems to exceed the PSU quoted rating ...... thing is the current figures are absolute max - and ideally you'd not be drawing too close to max on any rail.

No PSU will suffer by being over rated!!  But a PSU pushed to the max and beyond is not so good!  So - power ''in hand'' is good - probably run cooler and last longer.

[nymph4]

Ok I think I need a little more help understanding Power Supplys?

I would like to go about this One Step at a Time.

First of if I am looking at a Power Supplys Output Voltage it tells me the Total Output Voltage like 540 Watts and so on I get this.

But when it says Output Voltage and give you a list like this what it JUST this telling me?

+12V               +12V 2                 -12V
18A                  18A                       1.A