Buying a PSU

  • Thread starter Thread starter Tomaz Cedilnik
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Tomaz Cedilnik

I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment,
doesn't it?
How do I find out what wattage I need?

Tom
 
Tomaz Cedilnik said:
I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment, doesn't
it?
How do I find out what wattage I need?

Tom
Click to expand...

There are various wattage calculators online. But any Athlon XP system
should be happy with less than 400W. In looking for a power supply, name
brand is most important, but also make sure it has the right connectors for
your mainboard. For example, an Athlon XP system probably has a 20-pin
power connector. SOME newer power supplies have a 24 pin power connector.
Most are "20 + 4" (modular connector, 4 pins slide off if not needed), so
those would work, regardless. Looking at dabs in particular, the only
decent brands they carry are tagan and enermax. I'd recommend tagan, and
something around ~400W. -Dave

BTW, be careful using the wattage calculators. They are good at adding up
watts your system will probably use. But your power supply should only be
outputing a percentage of maximum rated wattage at any given moment.
Example: online calculator says your system uses 150W. That does NOT mean
you buy a 150W power supply. Not only would your system run unstable, but
the power supply would fail quickly, as it is running near maximum output,
constantly. More realistic would be a 450W supply, so that the system will
draw a maximum of about 33% of what the power supply can handle. But I've
had systems much more powerful than yours running off 380W! Name brand
helps. Many no-name 600W power supplies wouldn't perform as well as a good
380W supply.
 
Tomaz said:
I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment,
doesn't it?
How do I find out what wattage I need?

Tom
Click to expand...

You calculate the power required, after a fashion.

To do that requires some info, like what motherboard (since some use +5V
and some use +12V for the processor), what processor, what video card.
Those are major consumers. Disk drives and optical drives have a power
rating available for them (although the optical drive figure is usually
harder to believe).

This post contains a table of numbers for Athlon that I got from qdi.nl
a while ago. This info is also available in raw form, from various AMD
datasheets. Your processor is somewhere between 55W and 62W.

http://groups.google.ca/group/alt.comp.periphs.mainboard.asus/msg/373addb10b12b54f?dmode=source

For Intel P4, you use processorfinder.intel.com for power info, and for
Athlon64, you use www.amdcompare.com for power.

For video cards, Xbitlabs has measured a number of them. Current cards
use a lot of 12V, while older cards have a mix of voltages. Old AGP
cards might use 3.3V @ 6A, for example, as part of their power input,
and less of the 12V power. My old 9800Pro uses 5V @ 5.5A or so, for
example. Power numbers can range from 20W total at idle, to 180W for the
most power hungry cards. With even more power hungry cards coming
in Q2 2007.

A 12V rail rating of 15A, is a typical value for a small system. But
really, you try to calculate it as best as you can, to avoid buying
things like 700+ watt supplies. The computer only draws the power
it needs, so a 700W supply would be "mostly idle" and not used
to its fullest extent.

In terms of the power supplies theoretical lifespan, the components
inside will have a thermal aspect to their lives. Capacitors dry out
in less time, under high heat. Some of the modern supplies, use a
slightly more efficient switching architecture, and are in the "80+"
percent efficiency class. Such a supply, will create less heat internally
than an older supply with unstated efficiency (68%). You pay a lot
more for the supply, and maybe if you use it long enough, it pays off
on the electric bill. The efficiency is not a constant number, and usually
the efficiency value printed on the label, is when the thing has a
significant load on it.

Once you've decided on the power requirements (worked out a 12V @ 15A
plus 50W more for the 3.3V/5V rails), you go shopping. If a web site
has reviews, you look for reports of DOA or Dead On Arrival. That will
give some idea as to how good or bad it is. Also, there are web sites
that review power supplies, by testing them with a load tester, opening
them up and examining them for careful construction and so on. One
such site is jonnyguru.com .

This is an example of a general purpose supply, suitable for Athlon or
Pentium P4 systems. This one has a total power rating of 350W. Usually,
what you'll notice, is if you SUM(V * I ) for the rails, the numbers
don't add up. The idea here, is the supply will most likely be heavily
loaded on only one output rail, and the other rails are loafing along.
You can draw any set of V * I values, up to the limits on the label,
as long as the total does not exceed 350W. If your motherboard used
the 5V rail for the processor power source, a typical value would be
5V @ 20A. If the motherboard used the 12V rail, then a typical value
might be 12V @ 15A. (The presence of a 2x2 square power plug for the
processor, tells you it is using +12V for sure.) Those two V * I
numbers are not equal to one another, because the processors are
from different families, and there are other loads to include in the
power numbers besides the processor. But for an older system, this
supply, with 32A on 5V, and 26A on +12V, would power most of them.
It is about $56 USD. The second link here, is the customer review
section, where you search for DOA information.

+3.3V@32A, +5V@32A, +12V@26A, -5V@1A, -12V@1A, [email protected]
ENERMAX EG365P-VE FMA 1.3 ATX 350W Power Supply 90V~135V or 180V~265V
http://www.newegg.com/Product/Product.asp?Item=N82E16817103455
http://www.newegg.com/Product/CustratingReview.asp?item=N82E16817103455

Usually, if you find a supply for $20 or whatever the cheapest is
in your market, half the customer reviews will complain about
early failures. And if you cannot find a quality web site review
for the many cheap brands that exist, DOA reports is all you can go on.

For example, this power supply is advertised as "480W", and it
costs $12!, but the DOA reports tell you it is crap. Not good
value if it killed $1000 worth of computer hardware.

http://www.newegg.com/Product/CustratingReview.asp?item=N82E16817170014

HTH,
Paul
 
Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).
Click to expand...

Because so many computer assemblers do not even know how electricity
works, then power supply manufacturers have discovered how to sell a
power supply cheaper AND reap a higher profit. The naive will select
a power supply only on money and watts. Those inferior supplies leave
a glaring symptom. A long list of functions that power supplies had
to perform even 30 years ago is not provided. Those cheaper power
supplies just forget to provide those numerical specs.

Why provide those numbers when so many have no idea what those
functions are. However a decent supply can provide those facts - has
nothing to hide. These examples will also be included:
Specification compliance: ATX 2.03 & ATX12V v1.1
Acoustics noise 25.8dBA typical at 70w, 30cm
Short circuit protection on all outputs
Over voltage protection
Over power protection
100% hi-pot test
100% burn in, high temperature cycled on/off
PFC harmonics compliance: EN61000-3-2 + A1 + A2
EMI/RFI compliance: CE, CISPR22 & FCC part 15 class B
Safety compliance: VDE, TUV, D, N, S, Fi, UL, C-UL & CB
Hold up time, full load: 16ms. typical
Efficiency; 100-120VAC and full range: >65%
Dielectric withstand, input to frame/ground: 1800VAC, 1sec.
Dielectric withstand, input to output: 1800VAC, 1sec.
Ripple/noise: 1%
MTBF, full load @ 25°C amb.: >100k hrs

What happens when essential functions are missing? Short together
all power supply outputs. No damage occurs if a power supply has
those essential functions. Medium wave radio sits right next to a
computer with no interference if power supply has an essential
function. List of what is missing in cheaper supplies includes the
ability to damage motherboard and disk if an internal supply component
fails. No power supply with essential functions will damage
motherboard, et al. But then many computer assemblers have said
otherwise.

Price does not mean it is a better supply. But a very low price is
synonymous with 'missing essential functions'. No long list of
numerical specs? Then find a responsible manufacturer.

Meanwhile a supply rated at 500 watts may have same power as a more
responsible supply rated at 350 watts. They did not lie. They just
changed what was measured - leaving you to figure it out. However
watts is only a ballpark numbers - insufficient to select a supply.
You are more concerned with maximum possible current for each of 3.3,
5, and 12 volt outputs. Again, those selling only to naive computer
assemblers hope you don't learn about currents for each voltage. Good
luck hunting for responsible manufacturers. The first symptom of
inferior products - they don't want to provide that long list of
numerical specs. Otherwise you might discover missing essential
functions.
 
Tomaz Cedilnik said:
I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment, doesn't
it?
How do I find out what wattage I need?

Tom
Click to expand...

its the AMPS you have to look at not the WATTS. In particular the AMPS on
the 12volt rail - remember PSUs supply 12v 5v and 3.3v? well 12v supplies
the CPU, graphics card, HDDs etc ie most of hardware so if you have a fast
graphics card and multiple HDDs you need something along the lines of 32amp
or more on the 12v line. expect to pay £40+ for a decent PSU - you can pay
less but dont be suprised if it dies in 12 months and takes the mobo with
it.
 
Tomaz Cedilnik said:
I'm a bit confused. Looking on dabs.co.uk they cost anything
between 15 and 150 pounds (ok, the top end are those that output
700+, but even exactly 500 can cost anything between 20 and 110
pounds).

Seems there is much more to a PSU that the wattage...
Click to expand...

The wattage rating is deceiving. My Antec TruPower II 380 is
better and more powerful than some so-called 500 W power supplies.

Good luck.
 
There are various wattage calculators online. But any Athlon XP system
should be happy with less than 400W.
Click to expand...

Beware that with current generation PSU that can sometimes
be backwards, that if the Athlon XP based system draws 5V
current for the CPU vcore supply, it will be more cost
effective to buy an older generation PSU because it will
need a higher wattage current gen. PSU to get a high 3V+5V
rating as some needed.



In looking for a power supply, name
brand is most important, but also make sure it has the right connectors for
your mainboard. For example, an Athlon XP system probably has a 20-pin
power connector. SOME newer power supplies have a 24 pin power connector.
Most are "20 + 4" (modular connector, 4 pins slide off if not needed), so
those would work, regardless. Looking at dabs in particular, the only
decent brands they carry are tagan and enermax. I'd recommend tagan, and
something around ~400W. -Dave

BTW, be careful using the wattage calculators. They are good at adding up
watts your system will probably use. But your power supply should only be
outputing a percentage of maximum rated wattage at any given moment.
Example: online calculator says your system uses 150W. That does NOT mean
you buy a 150W power supply. Not only would your system run unstable, but
the power supply would fail quickly, as it is running near maximum output,
constantly. More realistic would be a 450W supply, so that the system will
draw a maximum of about 33% of what the power supply can handle. But I've
had systems much more powerful than yours running off 380W! Name brand
helps. Many no-name 600W power supplies wouldn't perform as well as a good
380W supply.
Click to expand...


A max of 33% is somewhat overkill, generally if the PSU's
most heavily loaded rail is under 75% utilized you're ok...
providing the PSU was accurately rated.
 
I'm a bit confused. Looking on dabs.co.uk they cost anything between 15
and 150 pounds (ok, the top end are those that output 700+, but even
exactly 500 can cost anything between 20 and 110 pounds).

Seems there is much more to a PSU that the wattage... What do I need to
look for in a PSU so it will work with my machine? I've got an Athlon XP
2000+ (1.67 GHz).

About wattage - it only draws as much power from mains as needed
considering the components that have to be powered at that moment,
doesn't it?
How do I find out what wattage I need?

Tom
Click to expand...

List all major parts in your system, including motherboard
make, model, and hopefully a link to the product page or at
least a (link to a) larger picture of it (need not be a
picture you took, only of same thing).

With this we can approximate a ballpark figure with at least
some margin. From the age of your CPU there were many
motherboards still using 5V power for CPU subcircuit so it
could make a significant difference what motherboard you
have, towards which PSU spec is the most cost effective,
then from there further narrowing down potential candidates.
Then there is the issue of whether there is any chance you
might want to reuse the PSU on your next system rebuild, if
that is the case then you may need a fairly high wattage
"IF" it currently uses 5V rail for CPU subcircuit, as it
would later need 12V rail for that, so a fairly high(er)
total wattage unit would be necessary to cover both
scenarios.

Avoid the lowest cost PSUs which tend to be quite overrated,
generally most people can use a median priced unit unless
their system was particularly well endowed, but some
prettied-up generics also sell for more than they're worth,
name brands can help but first you'll need to consider the
specific parts in the system.
 
kony said:
List all major parts in your system, including motherboard
make, model, and hopefully a link to the product page or at
least a (link to a) larger picture of it (need not be a
picture you took, only of same thing).

With this we can approximate a ballpark figure with at least
some margin. From the age of your CPU there were many
motherboards still using 5V power for CPU subcircuit so it
could make a significant difference what motherboard you
have, towards which PSU spec is the most cost effective,
then from there further narrowing down potential candidates.
Then there is the issue of whether there is any chance you
might want to reuse the PSU on your next system rebuild, if
that is the case then you may need a fairly high wattage
"IF" it currently uses 5V rail for CPU subcircuit, as it
would later need 12V rail for that, so a fairly high(er)
total wattage unit would be necessary to cover both
scenarios.

Avoid the lowest cost PSUs which tend to be quite overrated,
generally most people can use a median priced unit unless
their system was particularly well endowed, but some
prettied-up generics also sell for more than they're worth,
name brands can help but first you'll need to consider the
specific parts in the system.
Click to expand...

Or you can measure it by using the wattmeter method I outlined in
another thread.

I suggest people build suitable test loads. You can use 12V
automotive bulbs for the 12V supply, but you may have problems
finding suitable 3.3v and 5V bulbs. Now make a breadboard with a
suitable number of bulb sockets wired in parallel.

When someone offers an XXX watt PSU, load up sufficient bulbs to
dissipate XXX watts, distributed between the various lines
according to the mfg spec for max load. Connect, turn the PS on,
and wait. I predict 90% of PSUs will self destruct in 30 min or
less. So do this test at the sellers location, before buying. If
it passes, buy it. :-)
 
Or you can measure it by using the wattmeter method I outlined in
another thread.

I suggest people build suitable test loads. You can use 12V
automotive bulbs for the 12V supply, but you may have problems
finding suitable 3.3v and 5V bulbs. Now make a breadboard with a
suitable number of bulb sockets wired in parallel.

When someone offers an XXX watt PSU, load up sufficient bulbs to
dissipate XXX watts, distributed between the various lines
according to the mfg spec for max load. Connect, turn the PS on,
and wait. I predict 90% of PSUs will self destruct in 30 min or
less. So do this test at the sellers location, before buying. If
it passes, buy it. :-)
Click to expand...

There would definitely be quite a few PSU that fail such a
test, particularly those which came free-with-case, but
there are a couple of other problems in particular.

Crossloading - Cheap designs tend to be updated less often,
it is unlikely to see a very poor PSU optimized for the high
12V current a modern system needs. A high 12V load and
lower 5V and the 5V feedback/tracking for regulation results
in the PSU shutting off, if the owner is lucky enough that
the PSU at least has reasonable overvoltage or current
shutdown feature.

Longer term stress - A unit might run for months, even a
couple of years before blowing (usually it's capacitors or
switching transistors), at which point even if the system
wasn't damaged, repair is both unfeasible and beyond the
scope of most owners, and either the better PSU that
should've been used all along is then bought, or another
poor one and the process repeats again but possibly putting
excessive ripple/wear on parts.

There's also a few now made with notibly worse quality
control than seen in most generics over the past several
years, making *some* kind of non-system load test quite
prudent, to see if it self destructs immediately upon
turn-on.
 
kony said:
There would definitely be quite a few PSU that fail such a
test, particularly those which came free-with-case, but
there are a couple of other problems in particular.
Click to expand...

The main point of the post was that automotive, or other, bulbs
make a relatively cheap load, and sockets allow you to adjust the
values. Did you ever price high wattage low ohms resistors?
 
The main point of the post was that automotive, or other, bulbs
make a relatively cheap load, and sockets allow you to adjust the
values. Did you ever price high wattage low ohms resistors?
Click to expand...

They can definitely get expensive (for what they are) from
the major electronics houses.

Over the years I've used a number of different *materials*
for the load, some rather crude like the spiral steel
binding on some pads of paper (varies, but typically about
0.2 Ohm per segment), nichrome wire from a toaster or hair
dryer, etc., and the bulbs. Bulbs work fine and if that was
what I had available for a static load, I might use them but
at present I don't have but a few spares that are saved for
cars they might be used on.

Key to getting low cost power resistors is shopping around,
like the surplus electronics 'sites. A couple examples,

6 x 0.5Ohm, 15W $0.99
http://www.goldmine-elec-products.com/prodinfo.asp?number=G14375

10 x 1 Ohm, 20W $0.99
http://www.goldmine-elec-products.com/prodinfo.asp?number=G14510

While not as fancy as some of the power resistors with a
surrounding metal heatsink, those with the heatsink still
depend on being 'sunk to other metal, an addt'l cost and
time to implement. By using a greater number of the lower
cost resistors above, heat density is lowered, enough that
no further heatsink is required if only enough resistors are
used (though it can be a bit fiddly to have dozens of power
resistors lying out on a board, a proper heavy-duty circuit
board to hold them all is a bit better for repeated use).

Light bulbs are certainly another option, though they have
another issue that can make it more time consuming to
implement, that as the applied current goes up, the
resistance goes down, leaving one hoping the manufacturer's
wattage spec comes close enough to reality to estimate the
current per voltage.... at least with the automotive 12V
blubs, with a household 110V type it might be easier to just
take each in turn and measure the current through it.

Bulbs can get expensive too, a tail-light or turn signal
bulb may be cheap but the numbers required to arrive at the
rated capacity for a modern PSU, and sockets if you're not
willing to solder to them, can add to the expense.
Headlights are a little better, but the cheaper old type
with the housing and lens all-in-one are less common now and
make for quite a large setup on a bench, if you're testing a
PSU with a now-modest 18A current rating, it'll take ~ 5 @
50W. I imagine I could get some from a junk yard for
practically free, but I would like something a bit smaller
that could be placed next to a system for testing without
removing the PSU from the system, or if taking it to someone
else's system, something that looks a bit more professional
will give them less worries about what is being done.
 
CBFalconer said:
When someone offers an XXX watt PSU, load up sufficient bulbs to
dissipate XXX watts, distributed between the various lines
according to the mfg spec for max load. Connect, turn the PS on,
and wait. I predict 90% of PSUs will self destruct in 30 min or
less. So do this test at the sellers location, before buying. If
it passes, buy it. :-)
Click to expand...

How many sellers have you done this test at?

Bob
 
How many sellers have you done this test at?

Bob
Click to expand...

It is good point that one can't do the test at a store,
before purchase. The PSU could still be tested after
acquired, whether it be one bought or that came *free* with
a case, to determine that it meets minimal acceptible
standard for implementation in the intended system (if free)
or the labeled specs (if purchased separately).

One that came free with a case could make the return
unfeasible to the buyer due to the high return shipping cost
of an entire case, but if having the PSU included caused the
case to cost significantly more or if purchased separately
it would not be unreasonable to return it for a refund or
demand replacement.

The grey area here is that returning for a refund may not
be acceptible to the seller if the unit is damaged, it might
instead be replaced with same or have to be RMA'd to the
manufacturer and replaced with same (potentially poor)
quality or mislabeled PSU.

Many PSU that are poor will run for a few hours or longer
before self-destructing which is unfortunate as the longer
it lasted before failing, the greater the odds the testing
was terminated and it was integrated into a system. It can
be reasonable to pay a bit more for the name-brand for some
expectation of quality instead of trying to depend on
published specs or assuming it meets ATX guidelines for safe
shutdown.
 
kony said:
The PSU could still be tested after
acquired, whether it be one bought or that came *free* with
a case,
Click to expand...

Oh, I'd bet they figure in their cost of the PSU when pricing
the case. It's just one of many items in the case. Are you sure
that the case isn't *free* with the power supply? <g>
 
Oh, I'd bet they figure in their cost of the PSU when pricing
the case. It's just one of many items in the case. Are you sure
that the case isn't *free* with the power supply? <g>
Click to expand...


The only real index we have is whether same case is
available w/o PSU for cost comparison, or comparing to a
similar *enough* case. Many cases with junk PSU do seem to
make the PSU practically free in such a comparison, though
I'm sure many people would be as happy if the case were also
offered w/o for a lower cost since it can't really be free
to include. Whether such a case is any good or needs
modification just to have adequate airflow is a whole other
topic.
 
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