How much power surge is there at startup? + other questions

Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

Several power-related questions:

Do ball-bearing fans and disk drives draw a lot of extra power when they
start up after the system is turned on? If so, how much, and what
effect does this have on proper sizing of the power supply? How long
does the extra power requirement last?

I recall that some disk drives that I've used in the past have some sort
of deferred startup: they wouldn't start until the first command
arrived, or something like that. The idea was to draw less power at the
instant the machine is turned on. Is this still done? The drives I
bought came without instructions, just in an anti-static pack.

And another related question: does putting a fan under load (by having
a filter in front of it, for example) increase power consumption as well
as slowing the fan down?

Finally, how does the power requirement of a PC interrelate with the
voltages provided by the PSU? Do these voltages decline even when load
is well below the nameplate capacity of the PSU, or do they drop only
when the load approaches the limit, or what? According to my
calculations, my power supply is loaded at barely a third of its
nameplate capacity, but the 12V supply is still just a tad below 12V
most of the time.

--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly.
11 answers Last reply
More about power surge startup questions
  1. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    Mxsmanic wrote:
    > Several power-related questions:
    >
    > Do ball-bearing fans and disk drives draw a lot of extra power when they
    > start up after the system is turned on?

    Yes

    > If so, how much, and what
    > effect does this have on proper sizing of the power supply? How long
    > does the extra power requirement last?

    It's of relatively short duration and the surge capacity of the PSU is
    usually sufficient.


    > I recall that some disk drives that I've used in the past have some sort
    > of deferred startup: they wouldn't start until the first command
    > arrived, or something like that. The idea was to draw less power at the
    > instant the machine is turned on. Is this still done? The drives I
    > bought came without instructions, just in an anti-static pack.
    >
    > And another related question: does putting a fan under load (by having
    > a filter in front of it, for example) increase power consumption as well
    > as slowing the fan down?

    It depends on the fan's efficiency vs static pressure curve and how much
    static pressure is introduced by the filter. But unless you're installing a
    building air conditioning system, don't worry about it. Airflow is a
    bigger issue.


    > Finally, how does the power requirement of a PC interrelate with the
    > voltages provided by the PSU? Do these voltages decline even when load
    > is well below the nameplate capacity of the PSU,

    Not if it's working properly.

    > or do they drop only
    > when the load approaches the limit, or what?

    Might drop on overload, just before overload protection kicks in and turns
    it off.

    > According to my
    > calculations, my power supply is loaded at barely a third of its
    > nameplate capacity, but the 12V supply is still just a tad below 12V
    > most of the time.

    Everything has tolerances (there's no such thing as a "1 inch hole." [and
    even if there was you couldn't know because your measuring device has
    tolerances] It's 1 inch, plus or minus some tolerance, hole) and a 'tad',
    whatever that is, is fine as long as it's within the specified tolerances.
  2. Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

    On Fri, 11 Feb 2005 08:01:35 +0100, Mxsmanic
    <mxsmanic@hotmail.com> wrote:

    >Several power-related questions:
    >
    >Do ball-bearing fans and disk drives draw a lot of extra power when they
    >start up after the system is turned on?

    Fans don't usually make enough of a difference, relatively
    speaking it would take very many to matter. HDDs though do
    generally consume 2-3X as much 12V power.

    > If so, how much, and what
    >effect does this have on proper sizing of the power supply?

    Mostly it can take the drives longer to spin up, and with a
    modern system that could mean some aren't properly detected
    because everyone seems in a hurry to get past the POST
    enumeration and onto windows. If your PSU is sufficient
    for the everyday running of the system it should have enough
    reserve to spin up the drives too... though it's never a
    good idea to rapidly power-cycle a power supply on and off
    over and over and over.

    > How long
    >does the extra power requirement last?

    A few seconds, maybe 2, 3, 4...


    >
    >I recall that some disk drives that I've used in the past have some sort
    >of deferred startup: they wouldn't start until the first command
    >arrived, or something like that. The idea was to draw less power at the
    >instant the machine is turned on. Is this still done? The drives I
    >bought came without instructions, just in an anti-static pack.

    Tpical PC drives don't wait. Some SCSI can. It's not much
    of an issue unless you were trying to power too many drives
    from too small a PSU. ALways use a quality PSU befitting
    the parts it's powering, it is not the place to try to save
    an extra $20 (over cost of a lesser PSU).


    >
    >And another related question: does putting a fan under load (by having
    >a filter in front of it, for example) increase power consumption as well
    >as slowing the fan down?

    Not significantly relative to the load on that power rail.
    Fans really don't use much power at all, a dozen of (typical
    size & speed fans) is less load than a single HDD spinning
    up. Percentagewise it might be possible the fan uses a tiny
    bit more power, honestly I don't know and don't think it
    matters enough to find out. Fans don't actually draw the
    entire amperage stamped on their label more often than not.

    >
    >Finally, how does the power requirement of a PC interrelate with the
    >voltages provided by the PSU? Do these voltages decline even when load
    >is well below the nameplate capacity of the PSU, or do they drop only
    >when the load approaches the limit, or what?

    They drop the voltage relative to the capacity of that power
    rail which is determined by a voltage monitored by the PSU.
    Typically that is at least the 5V rail and often a weighted
    measurement of the 12V rail too. In other words, you should
    be able to add parts and expect the voltage to remain within
    an acceptable range until the unit is overloaded OR until
    there is a drastic imbalance of power draw. For example if
    you had a high-end workstation power supply that has 25A of
    12V capacity but only 35A of 5V, and you use a motherboard
    that draws a lot of 5V power but had no HDDs connected, you
    might find the power supply shuts off due to sensing a 12V
    rail that's drifted too high. The rail might not be over
    the 10% official spec per 12V rail but some PSU are even
    tighter than 10%.


    >According to my
    >calculations, my power supply is loaded at barely a third of its
    >nameplate capacity, but the 12V supply is still just a tad below 12V
    >most of the time.

    Did you measure this with a multimeter or by the
    motherboard sensors? MOtherboard sensors are notoriously
    inaccurate, if the PSU is outputting 12.0V (exactly) the
    sensors will typically register lower due to trace
    resistance. Sometimes a lot lower, maybe even 11.6V if
    overclocking. You really need to use a multimeter at the
    connector attached to the load (which in this case is the
    motherboard ATX or 4-pin connectors for 12V).
  3. Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

    In article <ppeq0118g6khlgnsh8gnoiluf6dl1brj24@4ax.com>,
    kony <spam@spam.com> wrote:
    >On Fri, 11 Feb 2005 08:01:35 +0100, Mxsmanic
    ><mxsmanic@hotmail.com> wrote:
    >
    >>Several power-related questions:
    >>
    >>Do ball-bearing fans and disk drives draw a lot of extra power when they
    >>start up after the system is turned on?
    >
    >Fans don't usually make enough of a difference, relatively
    >speaking it would take very many to matter. HDDs though do
    >generally consume 2-3X as much 12V power.
    >
    >> If so, how much, and what
    >>effect does this have on proper sizing of the power supply?
    >
    >Mostly it can take the drives longer to spin up, and with a
    >modern system that could mean some aren't properly detected
    >because everyone seems in a hurry to get past the POST
    >enumeration and onto windows. If your PSU is sufficient
    >for the everyday running of the system it should have enough
    >reserve to spin up the drives too... though it's never a
    >good idea to rapidly power-cycle a power supply on and off
    >over and over and over.
    >
    >> How long
    >>does the extra power requirement last?
    >
    >A few seconds, maybe 2, 3, 4...
    >

    I'd guess that charging all the capacitors on the mobo is
    a big part of the power-on surge.


    >
    >
    >>
    >>I recall that some disk drives that I've used in the past have some sort
    >>of deferred startup: they wouldn't start until the first command
    >>arrived, or something like that. The idea was to draw less power at the
    >>instant the machine is turned on. Is this still done? The drives I
    >>bought came without instructions, just in an anti-static pack.
    >
    >Tpical PC drives don't wait. Some SCSI can. It's not much
    >of an issue unless you were trying to power too many drives
    >from too small a PSU. ALways use a quality PSU befitting
    >the parts it's powering, it is not the place to try to save
    >an extra $20 (over cost of a lesser PSU).
    >
    >
    >>
    >>And another related question: does putting a fan under load (by having
    >>a filter in front of it, for example) increase power consumption as well
    >>as slowing the fan down?
    >
    >Not significantly relative to the load on that power rail.
    >Fans really don't use much power at all, a dozen of (typical
    >size & speed fans) is less load than a single HDD spinning
    >up. Percentagewise it might be possible the fan uses a tiny
    >bit more power, honestly I don't know and don't think it
    >matters enough to find out. Fans don't actually draw the
    >entire amperage stamped on their label more often than not.
    >
    >>
    >>Finally, how does the power requirement of a PC interrelate with the
    >>voltages provided by the PSU? Do these voltages decline even when load
    >>is well below the nameplate capacity of the PSU, or do they drop only
    >>when the load approaches the limit, or what?
    >
    >They drop the voltage relative to the capacity of that power
    >rail which is determined by a voltage monitored by the PSU.
    >Typically that is at least the 5V rail and often a weighted
    >measurement of the 12V rail too. In other words, you should
    >be able to add parts and expect the voltage to remain within
    >an acceptable range until the unit is overloaded OR until
    >there is a drastic imbalance of power draw. For example if
    >you had a high-end workstation power supply that has 25A of
    >12V capacity but only 35A of 5V, and you use a motherboard
    >that draws a lot of 5V power but had no HDDs connected, you
    >might find the power supply shuts off due to sensing a 12V
    >rail that's drifted too high. The rail might not be over
    >the 10% official spec per 12V rail but some PSU are even
    >tighter than 10%.
    >
    >
    >>According to my
    >>calculations, my power supply is loaded at barely a third of its
    >>nameplate capacity, but the 12V supply is still just a tad below 12V
    >>most of the time.
    >
    >Did you measure this with a multimeter or by the
    >motherboard sensors? MOtherboard sensors are notoriously
    >inaccurate, if the PSU is outputting 12.0V (exactly) the
    >sensors will typically register lower due to trace
    >resistance. Sometimes a lot lower, maybe even 11.6V if
    >overclocking. You really need to use a multimeter at the
    >connector attached to the load (which in this case is the
    >motherboard ATX or 4-pin connectors for 12V).
    >


    --

    a d y k e s @ p a n i x . c o m

    Don't blame me. I voted for Gore.
  4. Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

    kony writes:

    > Did you measure this with a multimeter or by the
    > motherboard sensors?

    All I have is the motherboard sensors, unfortunately.

    > MOtherboard sensors are notoriously
    > inaccurate, if the PSU is outputting 12.0V (exactly) the
    > sensors will typically register lower due to trace
    > resistance. Sometimes a lot lower, maybe even 11.6V if
    > overclocking.

    Well, that's reassuring, in a way, since it means that the PSU is
    probably at 12V even if it doesn't show.

    --
    Transpose hotmail and mxsmanic in my e-mail address to reach me directly.
  5. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    As an aside, power surge at startup re draw from the utility-co
    is actually quite high for SMPS. An input draw 30A or even 60A
    is not uncommon even for very small PSUs (120-150W), and the
    really big SMPS can draw over 200A surge at turn on.

    One reason why you see the electricity meter kick sharply for a
    barely perceptible fraction of a second, then slow down again.
    Power devices present a "real load" & "reactive load", if you
    are a big company you get billed for both loads.

    The only time the input surge at startup matters (mainly from
    the big primary side electrolytic capacitors are charging) is if
    you are using a battery &/or DC-to-DC convertor (Mini-ITX).

    A lot of the Mini-ITX DC-to-DC convertor boards still have a
    lot of problems with bigger boards, bigger loads & such like.
    Particularly an issue with home/DIY in-car PC solutions.

    So if planning a "mini-itx car PC", watch the surge issue, large
    devices can cause the DC-to-DC convertor boards to shutdown.
    Not fail, simply shutdown & require reset or load lightening. I
    also wonder if the capacitors are a bit undersized on some.

    For home PCs, it's an issue in choosing UPS - yes you may only
    want a few minutes to shutdown, but the UPS must be sized ok.
    --
    Dorothy Bradbury
    www.dorothybradbury.co.uk for quiet Panaflo fans
  6. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    A power supply that draws that much power on starting has
    serious and unacceptable design problems. Even in the 1950s,
    TVs had inrush current limiters so that no major current was
    present on startup.

    Been looking at this question with an oscilloscope scope for
    a long time. Have yet to find an electronic appliance that
    draws, during powerup, anything much more than steady state
    current.

    Incandescent bulbs: now there is a device that does have an
    inrush current. Typically we design on the assumption that
    incandescent bulb will draw as much as 8 times its steady
    state current for a very minimal time. But electronics -
    properly designed - must have inrush current limiters and not
    have a large startup current.

    Many who just know without first getting educated will often
    assume the inrush current limiter is, instead, an MOV
    installed for transient protection. Clearly it could not be.
    They don't even know of a device that was standard 50 years
    ago. A current limiter example:
    http://www.alliedelec.com/catalog/pf.asp?FN=1206.pdf


    Dorothy Bradbury wrote:
    > As an aside, power surge at startup re draw from the utility-co
    > is actually quite high for SMPS. An input draw 30A or even 60A
    > is not uncommon even for very small PSUs (120-150W), and the
    > really big SMPS can draw over 200A surge at turn on.
    >
    > One reason why you see the electricity meter kick sharply for a
    > barely perceptible fraction of a second, then slow down again.
    > Power devices present a "real load" & "reactive load", if you
    > are a big company you get billed for both loads.
    >
    > The only time the input surge at startup matters (mainly from
    > the big primary side electrolytic capacitors are charging) is if
    > you are using a battery &/or DC-to-DC convertor (Mini-ITX).
    >
    > A lot of the Mini-ITX DC-to-DC convertor boards still have a
    > lot of problems with bigger boards, bigger loads & such like.
    > Particularly an issue with home/DIY in-car PC solutions.
    >
    > So if planning a "mini-itx car PC", watch the surge issue, large
    > devices can cause the DC-to-DC convertor boards to shutdown.
    > Not fail, simply shutdown & require reset or load lightening. I
    > also wonder if the capacitors are a bit undersized on some.
    >
    > For home PCs, it's an issue in choosing UPS - yes you may only
    > want a few minutes to shutdown, but the UPS must be sized ok.
  7. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    On Sun, 20 Feb 2005 14:46:52 GMT, "Dorothy Bradbury"
    <dorothy.bradbury@ntlworld.com> wrote:

    >As an aside, power surge at startup re draw from the utility-co
    >is actually quite high for SMPS. An input draw 30A or even 60A
    >is not uncommon even for very small PSUs (120-150W), and the
    >really big SMPS can draw over 200A surge at turn on.

    Many use a power resistor and/or thermistor to limit this
    somewhat. Similar strategy might be employed where absent
    in a DIY supply.
  8. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    > incandescent bulb will draw as much as 8 times its steady
    > state current for a very minimal time. But electronics -

    I peak (2 to 4 ms)= 14 times IRMS for a traditional 115 or 230V bulb and 20
    times for an Hallogen.
  9. Archived from groups: alt.comp.hardware.pc-homebuilt,alt.comp.hardware.homebuilt (More info?)

    w_tom wrote:
    > A power supply that draws that much power on starting has
    > serious and unacceptable design problems. Even in the 1950s,
    > TVs had inrush current limiters so that no major current was
    > present on startup.

    1950s TVs weren't using SM power supplies.

    >
    > Been looking at this question with an oscilloscope scope for
    > a long time. Have yet to find an electronic appliance that
    > draws, during powerup, anything much more than steady state
    > current.

    OK, so you're not good at it then.

    Just some random examples

    http://www.electronicsoutfitter.com/store/ZM300BAPS.html

    ZALMAN USA ZM300BAPS - 300W Noiseless Power Supply

    INRUSH CURRENT LIMIT (@ Cold start at 25 degrees C)- 115 VAC: 60A
    230 VAC: 90A

    http://www.plasmatvbuyingguide.com/plasmatv/gateway-plasma42.html

    Gateway 42" Plasma TV

    Input current 3.3. A
    Inrush current 60 A p-p/20 ms Max


    http://www.necvisualsystems.com/corpus/J/P/xv2930.pdf

    NEC XV29

    Input current 2 A at 120VAC / 60Hz
    Inrush current 50A

    http://www.mcmcomputers.co.uk/product_info.php?products_id=1426

    12V(4A) LCD Monitor Power Adaptor FSP048-10AV

    Inrush Current............15A@115VAC or 30A@230VAC cold start at 25°C
    Output Voltage............12 VDC
    Output Current............0A to 4A
    Efficiency................80% min. at full load


    > Incandescent bulbs: now there is a device that does have an
    > inrush current. Typically we design on the assumption that
    > incandescent bulb will draw as much as 8 times its steady
    > state current for a very minimal time.


    > But electronics -
    > properly designed - must have inrush current limiters and not
    > have a large startup current.

    Define "large."


    > Many who just know without first getting educated will often
    > assume the inrush current limiter is, instead, an MOV
    > installed for transient protection. Clearly it could not be.
    > They don't even know of a device that was standard 50 years
    > ago. A current limiter example:
    > http://www.alliedelec.com/catalog/pf.asp?FN=1206.pdf

    And here w_tom goes into his trademark pompous ass routine.


    > Dorothy Bradbury wrote:
    >
    >>As an aside, power surge at startup re draw from the utility-co
    >>is actually quite high for SMPS. An input draw 30A or even 60A
    >>is not uncommon even for very small PSUs (120-150W), and the
    >>really big SMPS can draw over 200A surge at turn on.
    >>
    >>One reason why you see the electricity meter kick sharply for a
    >>barely perceptible fraction of a second, then slow down again.
    >>Power devices present a "real load" & "reactive load", if you
    >>are a big company you get billed for both loads.
    >>
    >>The only time the input surge at startup matters (mainly from
    >>the big primary side electrolytic capacitors are charging) is if
    >>you are using a battery &/or DC-to-DC convertor (Mini-ITX).
    >>
    >>A lot of the Mini-ITX DC-to-DC convertor boards still have a
    >>lot of problems with bigger boards, bigger loads & such like.
    >>Particularly an issue with home/DIY in-car PC solutions.
    >>
    >>So if planning a "mini-itx car PC", watch the surge issue, large
    >>devices can cause the DC-to-DC convertor boards to shutdown.
    >>Not fail, simply shutdown & require reset or load lightening. I
    >>also wonder if the capacitors are a bit undersized on some.
    >>
    >>For home PCs, it's an issue in choosing UPS - yes you may only
    >>want a few minutes to shutdown, but the UPS must be sized ok.
  10. Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

    Mxsmanic wrote:

    > Several power-related questions:
    >
    > Do ball-bearing fans and disk drives draw a lot of extra power when they
    > start up after the system is turned on? If so, how much, and what
    > effect does this have on proper sizing of the power supply? How long
    > does the extra power requirement last?
    >
    > I recall that some disk drives that I've used in the past have some sort
    > of deferred startup: they wouldn't start until the first command
    > arrived, or something like that. The idea was to draw less power at the
    > instant the machine is turned on. Is this still done? The drives I
    > bought came without instructions, just in an anti-static pack.
    >
    > And another related question: does putting a fan under load (by having
    > a filter in front of it, for example) increase power consumption as well
    > as slowing the fan down?
    >
    > Finally, how does the power requirement of a PC interrelate with the
    > voltages provided by the PSU? Do these voltages decline even when load
    > is well below the nameplate capacity of the PSU, or do they drop only
    > when the load approaches the limit, or what? According to my
    > calculations, my power supply is loaded at barely a third of its
    > nameplate capacity, but the 12V supply is still just a tad below 12V
    > most of the time.
    >
    When the computer starts up, every drive spins up at full speed. This
    is the normal procedure to test all the drives at start-up. So all the
    combined maximum wattages of each device produces the maximum load of
    the PC at startup. However, usually drives go into sleep or standby
    modes if they are not being used. So the maximum wattage is only needed
    at startup. However, this is when the OS is loading and the hard drive
    is very busy. This is when a low power event can cause damage to the files.
  11. Archived from groups: alt.comp.hardware.pc-homebuilt;,alt.comp.hardware.homebuilt (More info?)

    On Tue, 22 Feb 2005 21:30:12 -0600, Last Boy Scout
    <eggbtr@ezl.com> wrote:


    >When the computer starts up, every drive spins up at full speed. This
    >is the normal procedure to test all the drives at start-up. So all the
    >combined maximum wattages of each device produces the maximum load of
    >the PC at startup.

    No, the maximal wattage of the drives, only, is seen for ~
    2-4 seconds when the system is first powered on. No other
    devices are then at max powe usage unless you count the
    surge in charging the capacitors but this is not significant
    from a PSU capacity standpoint, so long as it starts up.

    >However, usually drives go into sleep or standby
    >modes if they are not being used. So the maximum wattage is only needed
    >at startup. However, this is when the OS is loading and the hard drive
    >is very busy. This is when a low power event can cause damage to the files.

    Maximum wattage for drives is not seen after the initial
    power-on, spin-up. Just because they're all spinning as OS
    loads doesn't means it's a particularly high load. In fact
    having a 2nd, 3rd drive spinning would be less load than
    having a couple go to sleep and spin down but have the
    system in a 100% CPU utilization state (due to modern CPU
    deriving power from 12V rail).

    Either way, it should never be an issue, the PSU should be
    sufficient to power either, any possible power situation the
    system can cause. This doesn't necessarily mean the max
    possible spec'd draw of ALL components simultaneously as
    that may not be possible, but rather the real max potential
    plus a fair margin. In a modern high-performance system
    this figure is still under 320W, but larger supply may be
    needed only because so much power is concentrated on one
    (usually 12V) rail.
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