Mains adapter for 12V fans - back to school!
 

I want to set up a rack of 3 small fans (as a gadget to put under a radiator
on a cold day). I've got 3 Sharkoon 1000 PC fans, 12v 1.44 Watt each. I want
to power the rack from a normal plug-in mains adapter. I've got a 12V DC /
800mA adapter available but I'm not quite sure if this will be OK.

So delving way way back to my schooldays I think I dimly remember that:

W = VA so as W=1.44 and V=12, current will be 0.12 or 120mA for each fan.
V = iR so if V = 12 and i = 0.12 then R is 200, so resistance of each fan is
200 Ohm.

So I'm thinking I'd be OK to wire up the 3 fans in parallel, current 3 x 120
= 360mA, resistance 66 Ohm?

Also, I don't know if the 12/800mA adapter I've got is regulated, or if this
matters. It has a symbol on it like a segmented orange (a bit like
'Panaorama' logo) and also a square within a square if that helps.

Thanks,
Skeggy.

Mains adapter for 12V fans - back to school!
 

Skeggy wrote:

I want to set up a rack of 3 small fans (as a gadget to put under a radiator
on a cold day). I've got 3 Sharkoon 1000 PC fans, 12v 1.44 Watt each. I want
to power the rack from a normal plug-in mains adapter. I've got a 12V DC /
800mA adapter available but I'm not quite sure if this will be OK.

So delving way way back to my schooldays I think I dimly remember that:

W = VA so as W=1.44 and V=12, current will be 0.12 or 120mA for each fan.
V = iR so if V = 12 and i = 0.12 then R is 200, so resistance of each fan is
200 Ohm.

So I'm thinking I'd be OK to wire up the 3 fans in parallel, current 3 x 120
= 360mA, resistance 66 Ohm?

Also, I don't know if the 12/800mA adapter I've got is regulated, or if this
matters. It has a symbol on it like a segmented orange (a bit like
'Panaorama' logo) and also a square within a square if that helps.

Thanks,
Skeggy.

I doubt you'll need all three. Last time I used this trick I used one
pc fan, 0.2-something amps, and it was plenty. A simple bafflebox can
quieten it if needed.


NT

Mains adapter for 12V fans - back to school!
 

In article 457c195f.0@entanet,
Skeggy wrote:
I want to set up a rack of 3 small fans (as a gadget to put under a
radiator on a cold day). I've got 3 Sharkoon 1000 PC fans, 12v 1.44 Watt
each. I want to power the rack from a normal plug-in mains adapter. I've
got a 12V DC / 800mA adapter available but I'm not quite sure if this
will be OK.

So delving way way back to my schooldays I think I dimly remember that:

W = VA so as W=1.44 and V=12, current will be 0.12 or 120mA for each
fan. V = iR so if V = 12 and i = 0.12 then R is 200, so resistance of
each fan is 200 Ohm.

So I'm thinking I'd be OK to wire up the 3 fans in parallel, current 3 x
120 = 360mA, resistance 66 Ohm?

You ain't half complicating things. ;-) Watts is V x A so 12 x 0.8, in the
case of your adaptor, gives 9.6 watts maximum output.

3 x 1.44 for the fans gives 4.32 watts so OK.


Also, I don't know if the 12/800mA adapter I've got is regulated, or if
this matters. It has a symbol on it like a segmented orange (a bit like
'Panaorama' logo) and also a square within a square if that helps.

With the load being approx half the maximum of the supply you're fine
either way. (An unregulated supply delivers rather more volts than its
nominal if very lightly loaded)

--
*There's no place like www.home.com *

Dave Plowman London SW
To e-mail, change noise into sound.

Mains adapter for 12V fans - back to school!
 

Thanks all for your reassuring feedback. I've assembled my rack and it now
works - no smoke!.

Only thing is results are a bit disappoiunting so far (or perhaps my
expectations were to high). At first I tried 80mm 1000 rpm fans, very quiet
but didn't really blow much. Perhaps might add a couple of drgrees during
continuous use but I'm aiming to turn my 1000 x 700 stelrad K2 double
convector into a big fan heater for fast warmup on cold frosty days. I put a
shelf over the rad to stop the hot air going straight up to the ceiling.

Since hen I've tried 2 x 2500 rpm fans . You can feel a breeze rather than a
blow with your hand at the top of the radiator, helpful but not quite what I
wanted and a bit noisy even running on reduced voltage. I fear the answer is
probably more fans (or fit a 9W Zantec(sp?) Tornado fan. I'll probably end
up with two under-radiator boxes, a quiet 1000 one for continuous use if
needed and a noisy power-booster. Then all I've got to do is to put it on
'Dragon's Den' ;-).

Skeggy

Mains adapter for 12V fans - back to school!
 

Skeggy wrote:
Thanks all for your reassuring feedback. I've assembled my rack and it now
works - no smoke!.

Only thing is results are a bit disappoiunting so far (or perhaps my
expectations were to high). At first I tried 80mm 1000 rpm fans, very quiet
but didn't really blow much. Perhaps might add a couple of drgrees during
continuous use but I'm aiming to turn my 1000 x 700 stelrad K2 double
convector into a big fan heater for fast warmup on cold frosty days. I put a
shelf over the rad to stop the hot air going straight up to the ceiling.

Since hen I've tried 2 x 2500 rpm fans . You can feel a breeze rather than a
blow with your hand at the top of the radiator, helpful but not quite what I
wanted and a bit noisy even running on reduced voltage. I fear the answer is
probably more fans (or fit a 9W Zantec(sp?) Tornado fan. I'll probably end
up with two under-radiator boxes, a quiet 1000 one for continuous use if
needed and a noisy power-booster. Then all I've got to do is to put it on
'Dragon's Den' ;-).


Hah. computer fans are not there to blow cold air, just extract hot air,
and low noise is one of the design targets.

What YOU need is a nice model plane ducted fan, powered by a 500 watt
custom made motor doing 35000 RPM and capable of flying a 5 lib model
over 100mph..

Shouldn't set you back more than about 300 quid in all..plus whatever
you spend on soundproofing :-)

BTW ducted fans are about as inefficient as it gets at moving air
volumes..what you need is a VERY large fan turning VERY slowly.




Skeggy

Mains adapter for 12V fans - back to school!
 

I thought of using a 16" desk fan, but I couldn't fit it under the radiatio
:-). Hadn't thought of a model plane propellor though.

It occurred to me that PC case fans normally operate in 'pull' mode, drawing
air from the case. When you run your hand over one in operation the blow is
quite spread out (about 45 degr). I even tought about having the fan rack at
the top of the radiator to pull hot air from the top - but not for long!.
The PC fans aren't perfect but in he end you do get a breeze, if I can find
something better then good!.

Indicentally both the 1000 and 2500 rpm fans are rated at 1.4W, funny
enough.

Skeggy.

Mains adapter for 12V fans - back to school!
 

Skeggy wrote:

I thought of using a 16" desk fan, but I couldn't fit it under the radiatio
:-). Hadn't thought of a model plane propellor though.

It occurred to me that PC case fans normally operate in 'pull' mode, drawing
air from the case. When you run your hand over one in operation the blow is
quite spread out (about 45 degr). I even tought about having the fan rack at
the top of the radiator to pull hot air from the top - but not for long!.
The PC fans aren't perfect but in he end you do get a breeze, if I can find
something better then good!.

Indicentally both the 1000 and 2500 rpm fans are rated at 1.4W, funny
enough.

Skeggy.

Last time I did this trick, a single 4" fan added about 3C to the room.
If you're looking for fast warmup then you're after greatly increased
output instead of a bit of increase. A deskfan would be nearer the
mark. It would be blowing air across the front of the rad rather than
behind. Standard rads are far from ideal for high fanned output, and
the more you push them the less ideal they behave. However you can stll
boost them a fair amount if needed. For a desk fan you can either point
it at the front or at the side, where it would cause airflow back and
front.

Shelf over top of rad works well at directing airflow, preventing hot
air all gathering up near the ceiling.

Are you housing your small pc fans? If not, much of the air goes out
one side of the fan and back in the other.

Is this for a normally off but we want to be able to heat it up quickly
room?


NT

Mains adapter for 12V fans - back to school!
 

Skeggy wrote:

I thought of using a 16" desk fan, but I couldn't fit it under the
radiatio
:-). Hadn't thought of a model plane propellor though.

http://www.maplin.co.uk/Module.aspx?...=fan&doy=12m12 ?

Mains adapter for 12V fans - back to school!
 

Are you housing your small pc fans? If not, much of the air goes out
one side of the fan and back in the other.

At present there is a narrow raised box with two rails inside near the top
along the long edges. The 2 fans at present just rest on the rails, ie open
to 2 sides, the fans ending up just under the rad. I was wondering if I
should fit a board with two fan sized holes and fit to these to close off
the open space - not sure if it would make much difference.

Is this for a normally off but we want to be able to heat it up quickly
room?

I now have 2 objectives in mind:

1) Room gets used quite a lot (even when rest of house has CH switched off),
but usage is unpredictable so helpful to warm up fast from he thermal store
on demand. Temporary noise (and perhaps dust) acceptable.

2) To keep temp up in very cold weather without resorting to electric
heater, so continuous use of fan box at times might also be good subject to
low noise and dust. Extracting more heat would also allow me to turn the CH
stat down at times and, all round, I think help system efficiency.by running
a bit cooler. Latest thought was one box with two switchable fan arrays -
one fast and one quiet!..

I wasn't keen on permanently keeping a big fan by the radiator, though ths
would no doubt work for case 1). A 3c rise seems about what I was getting
for an 80 mm 2500 rpm fan (3x1000rpm a bit less). Obviously there is a
limit to how fast the rad can give up heat to air outside and the law of
diminished returns apples. But there still seems to be a lot of heat hanging
around the radiator even with the 2 x 80mm 2500 fans so there is a way to go
with this, either with more powerful or more directional fans. Well,
hopefully!.

Skeggy.

Mains adapter for 12V fans - back to school!
 

Are you housing your small pc fans? If not, much of the air goes out
one side of the fan and back in the other.

At present there is a narrow raised box with two rails inside near the top
along the long edges. The 2 fans at present just rest on the rails, ie open
to 2 sides, the fans ending up just under the rad. I was wondering if I
should fit a board with two fan sized holes and fit to these to close off
the open space - not sure if it would make much difference.

Is this for a normally off but we want to be able to heat it up quickly
room?

I now have 2 objectives in mind:

1) Room gets used quite a lot (even when rest of house has CH switched off),
but usage is unpredictable so helpful to warm up fast from he thermal store
on demand. Temporary noise (and perhaps dust) acceptable.

2) To keep temp up in very cold weather without resorting to electric
heater, so continuous use of fan box at times might also be good subject to
low noise and dust. Extracting more heat would also allow me to turn the CH
stat down at times and, all round, I think help system efficiency.by running
a bit cooler. Latest thought was one box with two switchable fan arrays -
one fast and one quiet!..

I wasn't keen on permanently keeping a big fan by the radiator, though ths
would no doubt work for case 1). A 3c rise seems about what I was getting
for an 80 mm 2500 rpm fan (3x1000rpm a bit less). Obviously there is a
limit to how fast the rad can give up heat to air outside and the law of
diminished returns apples. But there still seems to be a lot of heat hanging
around the radiator even with the 2 x 80mm 2500 fans so there is a way to go
with this, either with more powerful or more directional fans. Well,
hopefully!.

Skeggy.