Mains adapter for 12V fans - back to school!

[Skeggy]

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.

[Skeggy]

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.

[Skeggy]

Sorry to repeat myself, and I wasn't even using Deja News.

[Stuart]

All i recognise the square within a square whichI think means double insulated .

[Bob Minchin]

Skeggy, As your proposed load is nearly half the maximum capacity of the unit, it should not matter too much whether the PSU is regulated or not. The load will cause an unregulated supply to approach its nominal working voltage. Yes your recollection of Ohms law is correct. Bob

[Dave Liquorice]

3 fans at 1.44W each = 4.32W

Your adapter is only running at about half its capacity. No problem.

Regulation probably doesn't matter much in this instance, if the volts are low the fans won't go as quick.

Square within a square is "double insulated" or Class II equipment. There might be a symbol after the 12v 800mA section of the ratings just a line and broken line below it but all I can find out about that is that in means "DC". Not regulated/smoothed/full or half wave bridge etc

[meow2222]

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

[Dave Plowman (News)]

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.

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)

[Skeggy]

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

[The Natural Philosopher]

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]

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.

[meow2222]

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

[dennis]

?

[Skeggy]

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.

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.

[Skeggy]

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.

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.

[Skeggy]

Just thought I'd belatedly report how this went. N.B. measurements are fairly rough...

Using 5 (five!) akasa 2500 80mm PC fans (£5 each) set in a line in an 82cm raised MDF box with the gaps between fans blocked off.. Running at 12V increased the temperature drop accross the radiaor by about one third (approx - as measured by digital probe thermometer wedged gainst pipe/radiator using pipe insulation). IE it effectively turned the 1000mm radiator into a ~1300mm one. The room temperature went up in line with this (about 2.5 degr), Running at lower voltages appeared to have a roughly proportionate effect - eg about 15% improvement at 6V.

I didn't get to do any measurements with just 2x2500 fans, But I have the impression this would have raised the temp by more than 2/5 of the above one third but still some way short of the 33%,

I also ended up with a 3 x 1000 RPM box, which seemed to raise the output silently by about 10%.

I tried blowing the rad with 10" deskfan at various angles to the radiator. The rad was a double, with the inner faces and convection fins blocked off by the radiator sides and the back blocked off by the brackets. For this reason I wasn't suprised to find this had little effect, maybe 10%, the shelf fitted a few inches over the top might not have helped.

So I ended up with the small fax box in use all the time under RAD2 (room has 2 rads) and the 5-fan box on demand, including 20 mins after CH is switched off to extract the last dregs of warmth.

One thing I would like to have tried is something like a thin mains tower fan on it's side blowing air up through the radiator from the bottom - I think this might work a treat!.

Skeggy (on Googlr as server has lost my post).

[Skeggy]

Thats a shame, I meant the OP to go as a reply against my earlier topic, basically about trying to use a box containing a row of 12V PC fans to get more heat out of a radiator to get the room remp up a bit.