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

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.

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

On Sun, 20 Feb 2005 14:46:52 GMT, "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.

Many use a power resistor and/or thermistor to limit this
somewhat. Similar strategy might be employed where absent
in a DIY supply.

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.

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.

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.

=20
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/...ZM300BAPS.html

ZALMAN USA ZM300BAPS - 300W Noiseless Power Supply

INRUSH CURRENT LIMIT (@ Cold start at 25 degrees C)- 115 VAC: 60A=20
230 VAC: 90A

http://www.plasmatvbuyingguide.com/p...-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/produc...ucts_id=3D1426

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

Inrush Current............15A@115VAC or 30A@230VAC cold start at 25=B0C
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.=20
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=3D1206.pdf

And here w_tom goes into his trademark pompous ass routine.


Dorothy Bradbury wrote:
=20
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.