actual power consumed by a power supply

[darren]

Hello all,

I have a question that's been bugging me for a long time and I can't seem
to formulate a proper query on Google to get at an answer.

I'm a total newb at electrical issues so please bear with me.

I'd like to take an old power supply I have that's rated at 200W and just
use it to run some 12V DC fans (to cool an aquarium). Now here's the
question (and it has actually been bugging me longer than I've had the
aquarium issue wrt whether I'm always drawing 350W on my computer when I
leave it on constantly or whether an idle system -- idle but not hibernating
or whatever -- draws as much current as a busy one).

So.... when you plug in a power supply/computer, are all the rated watts
automatically drawn, or is it a matter of whether there is hardware installed
that extracts the wattage on an "as needed basis"?

IOW, am I going to be drawing 200W to run a couple of measley fans?

Thanks,

Darren
--
merge ahead.

d r e a l t l x s e d t c m
a r n t i t e y t r x x

then change every 'x' to an 'o'

 

[Timothy Daniels]

I'd like to take an old power supply I have that's rated at 200W
and just use it to run some 12V DC fans (to cool an aquarium).
Now here's the question (and it has actually been bugging me
longer than I've had the aquarium issue wrt whether I'm always
drawing 350W on my computer when I leave it on constantly
or whether an idle system -- idle but not hibernating
or whatever -- draws as much current as a busy one).

So.... when you plug in a power supply/computer, are all the
rated watts automatically drawn, or is it a matter of whether
there is hardware installed that extracts the wattage on an
"as needed basis"?

IOW, am I going to be drawing 200W to run a couple of
measley fans?

No. The fans will "draw" what they need - actually they
will "allow" the necessary power to be pumped to it from
the power supply - and the power supply won't be
supplying 200 watts. The power supply, as an ideal,
supplies the necessary DC voltages, and the devices it
powers allow the current they need to pass through
them. The rating "200 watts" means that the power
supply stops acting ideally when it supplies more power
than 200 watts total and one or more voltages may drop
to values less than specified.


*TimDaniels*

 

[darren]

No. The fans will "draw" what they need - actually they
will "allow" the necessary power to be pumped to it from
the power supply - and the power supply won't be
supplying 200 watts. The power supply, as an ideal,
supplies the necessary DC voltages, and the devices it
powers allow the current they need to pass through
them. The rating "200 watts" means that the power
supply stops acting ideally when it supplies more power
than 200 watts total and one or more voltages may drop
to values less than specified.

Thank you for clearing up something I've been wondering about
for a long time.





--
merge ahead.

d r e a l t l x s e d t c m
a r n t i t e y t r x x

then change every 'x' to an 'o'

 

[user]

Hello all,

I have a question that's been bugging me for a long time and I can't seem
to formulate a proper query on Google to get at an answer.

I'm a total newb at electrical issues so please bear with me.

I'd like to take an old power supply I have that's rated at 200W and just
use it to run some 12V DC fans (to cool an aquarium). Now here's the
question (and it has actually been bugging me longer than I've had the
aquarium issue wrt whether I'm always drawing 350W on my computer when I
leave it on constantly or whether an idle system -- idle but not hibernating
or whatever -- draws as much current as a busy one).

So.... when you plug in a power supply/computer, are all the rated watts
automatically drawn, or is it a matter of whether there is hardware installed
that extracts the wattage on an "as needed basis"?

IOW, am I going to be drawing 200W to run a couple of measley fans?

Thanks,

Darren

Since you have at this point received messages explaining the way power
consumption works, you might want to consider using a wall wart power
supply instead of a computer power supply. The chances are your fans
will consume so few milliamps that such a power supply will suffice.

Ken

 

[Ralph Mowery]

I'm a total newb at electrical issues so please bear with me.

I'd like to take an old power supply I have that's rated at 200W and just
use it to run some 12V DC fans (to cool an aquarium). Now here's the
question (and it has actually been bugging me longer than I've had the
aquarium issue wrt whether I'm always drawing 350W on my computer when I
leave it on constantly or whether an idle system -- idle but not hibernating
or whatever -- draws as much current as a busy one).

So.... when you plug in a power supply/computer, are all the rated watts
automatically drawn, or is it a matter of whether there is hardware installed
that extracts the wattage on an "as needed basis"?

IOW, am I going to be drawing 200W to run a couple of measley fans?

Most computer supplies will need to have a small load on them to work. It
is usually about 10 % of the rated power. In your case of a 200 watt supply
you will need to draw about 20 to 30 watts all the time from it. An old
hard drive will usually draw enough just to run the motor from the 4 pin
plug.

The supply will draw only what the load needs from the AC line plus whatever
the efficency of the supply is. Supose your fan load is actually 50 watts.
The supply will draw 50 watts from the AC line plus a small ammount. Say
the supply is 80% efficient. Then it will draw around 65 watts from the AC
line.

I am sure that you found out that the AT supplies usually only need to be
turned on but you must jumper a couple of pins in the ATX supplies to run
the supplies.

 

[Marvin E. Kurtti]

Hello all,

I have a question that's been bugging me for a long time and I can't seem
to formulate a proper query on Google to get at an answer.

I'm a total newb at electrical issues so please bear with me.

I'd like to take an old power supply I have that's rated at 200W and just
use it to run some 12V DC fans (to cool an aquarium). Now here's the
question (and it has actually been bugging me longer than I've had the
aquarium issue wrt whether I'm always drawing 350W on my computer when I
leave it on constantly or whether an idle system -- idle but not hibernating
or whatever -- draws as much current as a busy one).

So.... when you plug in a power supply/computer, are all the rated watts
automatically drawn, or is it a matter of whether there is hardware installed
that extracts the wattage on an "as needed basis"?

IOW, am I going to be drawing 200W to run a couple of measley fans?

Thanks,

Darren

Get yourself a meter called "KILLAWATT"for around $35. Do googol.
Plug the n\meter into an outlet, plug your computers AC cord into the
meter. push a few buttons and you can see line
voltage,amps,watts,va,hz,pf,kwh and hours.

marv

 

[ric]

So.... when you plug in a power supply/computer, are all the rated watts
automatically drawn, or is it a matter of whether there is hardware installed
that extracts the wattage on an "as needed basis"?

IOW, am I going to be drawing 200W to run a couple of measley fans?

No. Power put out by a PSU is determined by power required by the connected
components.

But, for your application, why not use AC powered fans powered directly
from your AC line? They come in all sizes, including 80mm and 92mm.

 

[Duddley DooRight]

A computer power supply can best be described as a collection of power
supplies. It supplies +-12V, +-5V, +-3.3V. If a power supply claims to be
able to deliver 300 watts, that is only by adding all of these lines
together. A couple of places on the Internet have attempted to test power
supplies and some will not even reach their combined totals before they blow
up. If one individual line on one of these power supplys fails, then the
entrire power supply fails. Not only do the periferals use power like the
drives and the fans, and the lights, but the motherboard components like
Memory and CPU also use power. A good video card uses almost as much power
as a CPU.

 

[DaveW]

You only draw as many watts as the attached equipment needs.

 

[darren]

Wow. Thanks everyone for all the info. Lots to consider.

Ken:

Since you have at this point received messages explaining the way power
consumption works, you might want to consider using a wall wart power
supply instead of a computer power supply. The chances are your fans
will consume so few milliamps that such a power supply will suffice.

I do already have a couple of fans wired up using old 12v adapters that
I have lying around from other stuff that I don't use anymore. I'm only
doing 1 fan per adapter because I had heard that it's not really a good
idea to daisychain fans. I don't remember the explanation, but it seemed
like something that a lot of folks do but that isn't really a good idea.
So to be on the safe side I decided to stick with 1 adapter per fan. Now
my powerstrips are getting a bit crowded! That's when the power supply
idea began to seem more desirable.

Marv:

Get yourself a meter called "KILLAWATT"for around $35. Do googol.
Plug the n\meter into an outlet, plug your computers AC cord into the
meter. push a few buttons and you can see line
voltage,amps,watts,va,hz,pf,kwh and hours.

Checked it out. Very cool. I guess it's a layman's amp probe? i.e. does
all the calculations for you and breaks things down on a cost basis? The
pic I saw only had an input for a typical 120V plug but at the same time they
discussed the cost savings using an air conditioner and a refrigerator as
examples which makes me think that there must be some way to test 230V app-
liances? I'd really like to plug my 12 year old wall-mount AC into it and
see what it has to say. At any rate thanks for the recommendation, I can see
this thing being handy for LOTS of stuff.

Ralph:

I am sure that you found out that the AT supplies usually only need to be
turned on but you must jumper a couple of pins in the ATX supplies to run
the supplies.

Yeah, the only extra supply I have around at this point are ATs -- it'll be
a while before any of my ATXs will be ready for reclamation. Thanks for the
info. I had no idea there were even *any* jumpers in/on an ATX supply -- I
guess there would have to be since it relies on the MB to start it up?

Ric:

But, for your application, why not use AC powered fans powered directly
from your AC line? They come in all sizes, including 80mm and 92mm.

Yeah, I know about those. I do actually have a couple on order just to check
them out. What I've heard so far is that they don't really seem to put out
as much flow and they're allegedly a bit noisier than a DC equivalent. We'll
see. Plus, since I've already got some old parts lying around, I thought this
would be a good way to get some more use out of them.


Again, thanks everyone for all the useful information!!

Darren

 

[Ralph Mowery]

I do already have a couple of fans wired up using old 12v adapters that

I have lying around from other stuff that I don't use anymore. I'm only
doing 1 fan per adapter because I had heard that it's not really a good
idea to daisychain fans. I don't remember the explanation, but it seemed
like something that a lot of folks do but that isn't really a good idea.
So to be on the safe side I decided to stick with 1 adapter per fan. Now
my powerstrips are getting a bit crowded! That's when the power supply
idea began to seem more desirable.

I don't see what problem that could cause as there is just one point in a
computer supply where all the wires terminate for each voltage. I would
wire all the fans to one adapter that is big enough to carry the current.

 

[darren]

big enough to carry the current.

Maybe this is from where the warning originated: not knowing that not
all 12V adapters are created equal -- I went shopping yesterday in a
electronic surplus store that usually has a couple boxes full, and I
had a heck of a time finding one that would carry 500mA. There were
tons of 100 and 200's.

I don't see what problem that could cause as there is just one point in a
computer supply where all the wires terminate for each voltage. I would
wire all the fans to one adapter that is big enough to carry the current.

--
merge ahead.

d r e a l t l x s e d t c m
a r n t i t e y t r x x

then change every 'x' to an 'o'

 

[Timothy Daniels]

Maybe this is from where the warning originated: not knowing that not
all 12V adapters are created equal -- I went shopping yesterday in a
electronic surplus store that usually has a couple boxes full, and I
had a heck of a time finding one that would carry 500mA. There were
tons of 100 and 200's.

Try Radio Shack's online store:
http://www.radioshack.com/category.asp?catalog_name=CTLG&category_name=CTLG_009_001_001_001&Page=1

They're called "AC-to-DC power adapters".


*TimDaniels*