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Thread ID: 70004	2006-06-19 04:40:00	Using a PSU for other appliances	Feelers (10034)	Press F1

Post ID	Timestamp	Content	User
464368	2006-06-19 04:40:00	Hi there, I have a large car blower fan (the one that blows the air around inside the car) that is 12V obviously, but I dont know how many watts it is and I need it for a project of mine. I have another similar car fan - that is140W, so the fan I want to use is probably around there, I dont really know. Anyway - I want to power the fan with a PSU from a computer. It needs about 10 amps??? which is within some power packs ability. I have had the fan going on an old 4 amp 12v pack, and it doesnt want to go anymore, despite the power pack still working normally in the computer when the fan isnt in. I tried it on a 8amp pack, and it cuts out straight away. Can you tell me why this is happening? Im geussing its because its drawing too much current, but that doesnt explain why it woks on the 4 amp unit sometimes. I want this to be a standalone unit - so I will be wiring green14 to black so I can just use a switch instead of the mother board. What can I do to make this fan go? Thanks guys.	Feelers (10034)
464369	2006-06-19 08:05:00	You will probably need about 12-14 amps, 140W@12volts is just over 11.5 amps The 8 amp supply will cut out because it is grossly overloaded, and the 4 amp is probably too cheap and stupid to know it is overloaded and the output volts could be sagging quite low, which may be all that is stopping it from going up in a cloud of heifer dust. Find a higher rated supply, or parallel 12 volt outputs from a supply until you get the current capacity you need. Might be simpler if you just gave up smoking in your room.:D Cheers Billy 8-{)	Billy T (70)
464370	2006-06-20 02:42:00	I'd look at a different approach. You've got a car blower ... so you want a 13.8V supply (not a 12V one). How about a cheap fan heater? Some are quite small, yet move a lot of air. You could cut the heater wires if you don't want hot air.	Graham L (2)
464371	2006-06-20 05:56:00	I need quite a lot of flow- 300 to 500 cubic feet per minute, rougly equalls about 11 cubic meters per minute . . . . which is just under 200L of air per second . If I did my maths correctly . :D Thanks for the parallel wiring idea - definately gonna give that a go, unless I can find a cheap 12Amp power supply - anyone in Christchurch got one lying around haha? I dont know about the 13 . 8V thing - It would only adjust the fans speed anyway, but it seems that cars are rated to supply between 11V and 13 . 8V, so I dont think it's a problem . All car battery chargers are 12V too . I'm gonna hook up a vairable resistor to be able to alter the fan speed too . If you guys wanna know what its for . . . its for a Laminar Flow Hood . . orchideenvermehrung . at/cgi-local/framebreaker/reload . pl?english/lfh/index . htm" target="_blank">www . orchideenvermehrung . at Someone on another forum said this about parallel wiring the computer power supplys - Unless the power supplies are made to work together, they won't load share . If one is even 1/10 of a volt higher than the other, it will try to hog all the load and burn up, then the other one will . Is this the case? I dont want anything unstable/unsafe . Has anyone tried this?	Feelers (10034)
464372	2006-06-20 06:58:00	Unless the power supplies are made to work together, they won't load share . If one is even 1/10 of a volt higher than the other, it will try to hog all the load and burn up, then the other one will . No worries, the load will drag them down to equal voltages because it bridges the outputs . With a piece of wire across the two outputs there's no show of a voltage differential appearing . Similarly the 13 . 8 volt thing . You might need more volts for full flow if the fan output is marginally rated, but otherwise it will be just fine on 12 volts . I think Graham was just having a senior moment there . :D Cheers Billy 8-{)	Billy T (70)
464373	2006-06-20 08:48:00	Sweet - Thanks for that! :D	Feelers (10034)
464374	2006-06-21 02:58:00	12 volt batteries are not 12V. Anything made to work in a "12V" car is rated to run on 13.8V nominal. A battery at 12V is well down. If a motor needs a certain wattage, it will pull proportionately more current. Car battery chargers are not 12V. If they were they wouldn't be any use. The cheap ones give 17V or more, and are better described as battery boilers, if left unattended. Good chargers are voltage limited at 13.8. A car alternator is regulated to give 14.5V maximum. Paralleling two supplies might work, as long as the regulation sensing is on the 5V supply as is usual, and there's not too much difference in the settings.	Graham L (2)
464375	2006-06-21 03:47:00	12 volt batteries are not 12V. No, they are 12.6 volts (2.1 volts per cell fully charged) after you let the float-voltage from charging dissipate. Don't make it difficult for the poor guy Graham, he just wants to run a fan and probably doesn't give a rats what the terminal voltage is, provided he gets enough cfm at the other end. Likewise for the paralleled PSUs or paralleled 12 volt outputs from one single supply, it still doesn't really matter how they sense the reference for regulation purposes, the whole shebang will be tied together and the load will flatten any differences. He might even get 12.6 volts.:D This is neither rocket science nor a super-critical application so don't sow too many doubts. A cheap battery charger would probably do as well, provided it has the current capacity. Cheers Billy 8-{)	Billy T (70)
464376	2006-06-21 04:02:00	Yeah I looked into car battery chargers- the most I could find was 12V at 6amps. I'm after a cheap solution - and considering old PSU's are pretty much free, this seems like a good way of doing things. Its true - all I'm looking for is a car fan that goes nuts when I turn it on, that isnt a fire hazard :D The 4 amp supply that I rigged it up to made it go nuts, so I think its gonna be sweet with a couple on there. Also what happens when I have an 8 and a 4amp supply in parallel? Is the total available 12Amps? Or is it the average of 12/2 ie 6Amps, which would burn out the first unit? Sorry I dont know too much about this stuff! Thanks for your help.	Feelers (10034)
464377	2006-06-21 05:37:00	. . . No, they are 12 . 6 volts (2 . 1 volts per cell fully charged) after you let the float-voltage from charging dissipate . Which takes a day or so . You never see that except on the way down . The car battery is not meant to supply loads for any length of time; that's the alternator's job . 13 . 8 is considered "normal voltage" for any equipment designed for a car . The service manual for the UHF sets I'm looking at at the moment says to feed them 13 . 8V, the standard supply voltage . A motor driving a fan will require a certain amount of power to turn that fan . If the volts are lower, say 12, it will pull more current than it would at 13 . 8V . I tried to keep it simple: I suggested a fan which plugs into 230V ac . That would remove the complexity . I agree the parallel suplies will "probably" work . But I wouldn't guarantee it, without making some measurements first . They are each trying hard to maintain constant output at its own (unloaded) 5V . The 12 V output is determined by the relative turns ratios of the sections . Connecting Billy's wire between them will certainly show a single voltage, but that's no test . That's the voltage of the one with the highest voltage . They are effectively isolated by their rectifiers . In use, the highest voltage one will take the load, and if that's too much current it will shutdown . Then the lower voltage one will have a go . And shut down . That's assuming they have decent overload sensing . If it's measuring the input current, which is likely, the fact that the 5V is usually the gruntiest part means the 12V could be grossly overloaded when it is taking all the input energy . Smoke could happen . Finding a suitable variable resistor for speed control might be a problem . That alone could cost more than a mains driven blower . ;)	Graham L (2)
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