options if PSU too small for new CPU

[Franklin]

I have an SD-RAM motherboard with a 700 MHz Duron. I want to replace
the Duron with a faster processor (the mobo will take up to a 2400
MHz T'bred).

The existing PSU may turn out to be too small for a faster processor.
If this happens then can I run the FSB at 100 instead of 133 in order
to get the CPU to need less power? (I don't want to get a new PSU.)

Would the CPU's reduced power consumption be proportional to the
reduced FSB speed - would I save about 25% of power supplied to the
CPU if I went from 133 to 100?

Are there any other power saving options I might reasonably consider?

 

[Iain Dingsdale]

I have an SD-RAM motherboard with a 700 MHz Duron. I want to replace
the Duron with a faster processor (the mobo will take up to a 2400
MHz T'bred).

The existing PSU may turn out to be too small for a faster processor.
If this happens then can I run the FSB at 100 instead of 133 in order
to get the CPU to need less power? (I don't want to get a new PSU.)

Would the CPU's reduced power consumption be proportional to the
reduced FSB speed - would I save about 25% of power supplied to the
CPU if I went from 133 to 100?

Are there any other power saving options I might reasonably consider?

Telling us what sort of psu and the power rating of it would be a good
start!

 

[Wes Newell]

Are there any other power saving options I might reasonably consider?

If you lower the CPU speed you also need to lower vcore to achieve max
power savings. Just lowering the FSB won't save you near as much as
lowering vcore along with it. A 2400+ at 100Mhz FSB probably won't need
more than 1.4v. At least the 2100+ at 100MHz ran fine at 1.4v. That was as
low as the bios would go.

 

[Alex Fraser]

I have an SD-RAM motherboard with a 700 MHz Duron. I want to replace
the Duron with a faster processor (the mobo will take up to a 2400
MHz T'bred).

I assume you mean an Athlon XP 2400+, which IIRC is 2GHz.

The existing PSU may turn out to be too small for a faster processor.
If this happens then can I run the FSB at 100 instead of 133 in order
to get the CPU to need less power? (I don't want to get a new PSU.)

Yes, if the motherboard allows it.

Would the CPU's reduced power consumption be proportional to the
reduced FSB speed - would I save about 25% of power supplied to the
CPU if I went from 133 to 100?

Yes, roughly.

Are there any other power saving options I might reasonably consider?

You can try reducing the CPU core voltage if the motherboard allows it.
(Underclocked, it wouldn't surprise me if it would work at a low enough
voltage to consume _less_ power than the curren Duron.)

Unlocking the multiplier may be another option.

Alex

 

[jacoby]

There is approx 30 watt difference between the two CPUs. Expect about a 4
watt savings for every 1/10th in vcore reduction.

 

[CBFalconer]

.... snip ...

Would the CPU's reduced power consumption be proportional to the
reduced FSB speed - would I save about 25% of power supplied to
the CPU if I went from 133 to 100?

Are there any other power saving options I might reasonably consider?

More or less, yes. CMOS logic generally charges and discharges
lots of dinky little capacitors on each clock cycle. That
constitutes a charge per unit time, which is a current. Increasing
the unit of time decreases the current, and thus the power. The
other factor involved is the voltage to which things are charged.

 

[starfleet40]

if you are upgrading to a processor that has a higher voltage rated psu
then the only option is to upgrade your psu. period. cheaping out on
the psu when you're dropping good cash for a cpu upgrade is not wise.
many have tried to run a newer machine with a radeon 9800xt and a 230
watt psu and don't know why the machine is blue-screening and not
functioning properly?!? HELLO somebody!! Tell this guy what he needs to
hear not what he wants to hear. why would you even want to lose
performance by lowering your bus speed 25% to avoid getting a new psu
which you can find by the way if you shop around for about 50.00 for a
real GOOD one that will last and which you can use and re-use for later
projects and upgrades!
good luck with your decision.
visit me on the web at http://starfleetscommand.com
visit us on the web at http;//ezarlodge281.org

 

[Rob Morley]

if you are upgrading to a processor that has a higher voltage rated psu
then the only option is to upgrade your psu. period.

PSUs are specified by the current they can supply on each line, not
by voltage. The motherboard determines the voltage that's supplied
to the CPU.

cheaping out on
the psu when you're dropping good cash for a cpu upgrade is not wise.
many have tried to run a newer machine with a radeon 9800xt and a 230
watt psu and don't know why the machine is blue-screening and not
functioning properly?!?

That's probably because the GPU draws a considerably higher current,
not because the CPU uses a few more Watts. The current requirement
on each line must be met - it's not enough to say e.g. a 500W PSU
will be sufficient, or a 350W PSU will be insufficient.

HELLO somebody!! Tell this guy what he needs to
hear not what he wants to hear. why would you even want to lose
performance by lowering your bus speed 25% to avoid getting a new psu

Underclocking is useful for low-noise machines, or using a CPU with a
higher FSB on a mobo that won't run at that speed.

which you can find by the way if you shop around for about 50.00 for a
real GOOD one that will last and which you can use and re-use for later
projects and upgrades!

I doubt you'd get much quality for 50.00 - that's a number. If
you're talking about £50.00 I'd still be doubtful.

good luck with your decision.
visit me on the web at http://starfleetscommand.com
visit us on the web at http;//ezarlodge281.org
CBFalconer wrote:

<snip>
Learn about the proper use of a signature separator, and how not to
top-post.

 

[Franklin]

There is approx 30 watt difference between the two CPUs. Expect
about a 4 watt savings for every 1/10th in vcore reduction.

Forgive me, but I am not entirely clear on your what you are saying.

When you say a 4 watt savings for every 1/10th in vcore reduction,
are you saying that for every 1/10th of a volt which vcore is reduced
by then there will be approx a 4W saving?

 

[CBFalconer]

*** Rude and foolish top-posting fixed ***

if you are upgrading to a processor that has a higher voltage rated
psu then the only option is to upgrade your psu. period. cheaping
out on the psu when you're dropping good cash for a cpu upgrade is

If you are changing supply voltages you are probably destroying
something. Components expect to operate at certain rated supply
voltages.

not wise. many have tried to run a newer machine with a radeon
9800xt and a 230 watt psu and don't know why the machine is
blue-screening and not functioning properly?!? HELLO somebody!!

I only heard a question about how to control power consumption.
The only one mentioning 9800xt and 230 watts is you. Try to get
out a little more.

Tell this guy what he needs to hear not what he wants to hear. why
would you even want to lose performance by lowering your bus speed
25% to avoid getting a new psu which you can find by the way if you
shop around for about 50.00 for a real GOOD one that will last and
which you can use and re-use for later projects and upgrades!

Maybe he wants to run from a battery and extend life. Don't make
so many silly assumptions. Try to learn some elementary
netiquette, including how to post and snip properly. Also crack
some elementary books on electricity, the care and feeding of.
Some people actually know what they are talking about and what they
don't know, and I suspect this applies to Franklin.

 

[kony]

Forgive me, but I am not entirely clear on your what you are saying.

When you say a 4 watt savings for every 1/10th in vcore reduction,
are you saying that for every 1/10th of a volt which vcore is reduced
by then there will be approx a 4W saving?

wattage is proportional to the square of the voltage and the
frequency. To calculate this use C as a constant for the
CPU to be determined (calculated) based on the manufacturer
specs (which can be seen per each CPU from their respective
manufacturer datasheets, the following is a fictional CPU
for example purposes only:

[ frequency * (voltage)² ] / C = Wattage

As an example, if a particular CPU is spec'd to run at 1.5V
and 2.0 GHz (2000 MHz), which radiates 100W heat, (uses 100W
power), then from above equation we have:

[2000 * (1.5)² ] / C = 100

Solved for C,
C = 45

Now taking same CPU where C= 45, but running it at 1.3V and
1600MHz-

[ 1600 * (1.3)²] / 45 = Wattage

Wattage = 60.1

 

[kony]

I have an SD-RAM motherboard with a 700 MHz Duron. I want to replace
the Duron with a faster processor (the mobo will take up to a 2400
MHz T'bred).

The existing PSU may turn out to be too small for a faster processor.
If this happens then can I run the FSB at 100 instead of 133 in order
to get the CPU to need less power? (I don't want to get a new PSU.)

Would the CPU's reduced power consumption be proportional to the
reduced FSB speed - would I save about 25% of power supplied to the
CPU if I went from 133 to 100?

Are there any other power saving options I might reasonably consider?

IMO, it is a false economy to underclock the CPU for the
sole benefit of reusing your current PSU. Moreso than many
devices, power supplies do eventually fail, especially when
ran near their max output, which seems to be what you're
trying to accomplish. Otherwise, to get about same power
consumption from a newer CPU, you'd not be getting THAT much
more performance.

If you wish to get the maximum out of your current PSU _AND_
your motherboard has multiplier adjustment capability,
choose a mobile Athlon XP, which has lower voltage
requirements, and then consider reducing the multiplier
moreso than the FSB speed. Your board MUST support
muliplier changes to use one though, otherwise it will be
stuck at 6X default multiplier unless you used hacks to burn
bridges, solder leads or wire-wrap pins to get some other
multiplier. Frankly I suggest you just buy another PSU.

Since you have such an old board, it's most likely using 5V
power for CPU. A fairly good budget range PSU for higher 5V
current would be a Thermaltake 420W, Newegg frequently has
them for $36-40 on sale.
http://www.newegg.com/app/viewproductdesc.asp?description=17-153-006&DEPA=0

 

[General Schvantzkoph]

I have an SD-RAM motherboard with a 700 MHz Duron. I want to replace
the Duron with a faster processor (the mobo will take up to a 2400 MHz
T'bred).

The existing PSU may turn out to be too small for a faster processor. If
this happens then can I run the FSB at 100 instead of 133 in order to
get the CPU to need less power? (I don't want to get a new PSU.)

Would the CPU's reduced power consumption be proportional to the reduced
FSB speed - would I save about 25% of power supplied to the CPU if I
went from 133 to 100?

Are there any other power saving options I might reasonably consider?

Bite the bullet and get an adequate power supply. The last place you
should cut corners is on the power supply. A marginal power supply can
give you all sorts of flaky hard to diagnose problems. A power supply
failure can also take out your entire system. If you can't afford a decent
power supply now then put off your upgrade until you can. If I were you
I would add 75W to the size of your current power supply and use that as a
minimum value for a new one. I bet you'll find that it's not all that
expensive.

 

[jacoby]

Yes. The 2400 draws max 41 amps.

Forgive me, but I am not entirely clear on your what you are saying.

When you say a 4 watt savings for every 1/10th in vcore reduction,
are you saying that for every 1/10th of a volt which vcore is reduced
by then there will be approx a 4W saving?

 

[Mescalito]

PSUs are specified by the current they can supply on each line, not
by voltage. The motherboard determines the voltage that's supplied
to the CPU.


That's probably because the GPU draws a considerably higher current,
not because the CPU uses a few more Watts. The current requirement
on each line must be met - it's not enough to say e.g. a 500W PSU
will be sufficient, or a 350W PSU will be insufficient.


Underclocking is useful for low-noise machines, or using a CPU with a
higher FSB on a mobo that won't run at that speed.


I doubt you'd get much quality for 50.00 - that's a number. If
you're talking about £50.00 I'd still be doubtful.

<snip>
Learn about the proper use of a signature separator, and how not to
top-post.

A touch arrogant Rob.

 

[Alex Fraser]

[top-posting fixed]

Yes. The 2400 draws max 41 amps.

But if the voltage is changed, the current will change too, by a similar
factor. If you assume the same factor (a close approximation, AFAIK), you
get the result kony gave: power proportional to square of voltage (all else
being constant).

Alex

 

[jacoby]

But if the voltage is changed, the current will change too, by a similar
factor. If you assume the same factor (a close approximation, AFAIK), you
get the result kony gave: power proportional to square of voltage (all else
being constant).

Alex

This is true. So a 0.25 reduction in volts will not exactly equate to a 10
watt power reduction. I was merely pointing out that there is only a 30 watt
difference in the two cpus and a only slight reduction in power consumption
may be obtained by lowering voltage, thus lowering the difference between
the two cpus. As Kony stated your mb most likely feeds the cpu from the
3.3/5 volt rail. If your mysterious psu has been stable running the Duron,
the question is weather or not the additional 25 to 30 watt increase will
render instability issues. How many watts does your current psu supply to
the combined 3.3/5 volt rail?

 

[Rob Morley]

A touch arrogant Rob.

Just a touch?

 

[Alex Fraser]

This is true. So a 0.25 reduction in volts will not exactly equate to a
10 watt power reduction.

Nor will it necessarily equate to anything like a 10W power reduction. It's
not linear; it depends what voltage you are reducing it from.

I think 10W is a significant underestimate for a reduction of 0.25V from the
default core voltage of the CPU in question (an XP 2400+).

I was merely pointing out that there is only a 30 watt difference in the
two cpus and a only slight reduction in power consumption may be obtained
by lowering voltage, thus lowering the difference between the two cpus.

I was merely pointing out that your original 4W per 0.1V figure was
apparently based on a false premise (that the current does not change), and
as a consequence liable to be wrong.

Alex

 

[jacoby]

ok, fine. I do not have time to delve into such trivial matters. Either get
the new cpu and roll the dice your current psu will be sufficient or get a
new psu or do nothing. Jeewiz. I am out!