Your 12V/300mA wall-wart might catch on fire when it is overpowered with a 12V/600mA fan.

Naw, most of them have nonreplacable fuses. That would pop and that would be that.

 

hi all
wall warts sounds like something my cellar is suffering from, i think we call them plug in power supply?
you computer psu could be a bit fiddly there should be a sticker on the case with outputs volts etc
if all else fails -a car battery charger -10 a penny at local car boots a bit overkill but youve always got spare capacity if you need to add
you will be connecting in paralell red to red- black to black

 

You can get IC regulators from the local Radio Shack using a 7812 and a couple of caps. Kinda overkill, but it will work. I suspect you'll need two of them.

Ok, I'm determined to get this spare laptop power supply running and powering up my (2) 12v fans. I have a couple 7812's on hand but are you sure that I can wire 2 of them up? I think they support 1A and my fans together would eat up 1.2A. Power supply is putting out 19 volts. The 7812 would bring that down to 12v but now it's a question of amperage.

Out of curiosity, couldn't I just limit the current to the fans with a 2 watt resistor using a single 7812? Not sure if that affects fan speed but these fans are just for show.

 

This may be somewhat foolish but if you take a look at the joule thief circuit over at www.overunity.com it may be of some use in this case I am not entirely sure it will work but it just may if crafted properly worth a shot.

 

Using more than a 1A with a 7812 is probably not a good idea. You don't need any resistor with the regulator, but you could use a resistor instead of a regulator since the laptop power supply is regulated. The resistor for one fan (600ma) would need to disappate 4.2 watts (hot!) and be around 11.67Ω. A regulator disappates as much wattage, but doesn't require any finiky adjustments. You would have to heat sink the regulator, and if you went the resistor route you could use 16 or so ½W resistors (per fan).

A wall wart rated for 12VDC doesn't have these issues.

A joule thief circuit isn't called for in this in any way. A joule thief typically boosts voltage, this needs a simple regulation or voltage drop. It is already getting overcomplicated as is for such a simple job.

 

Ok, since this should be simple and I'm still struggling, can you tell me what you would do to power up these 2 FANs and some LED's? Is the right answer here finding a specific power supply that outputs 12V @ 1.2A? If that's the case, I'll need a different power supply for everything I wanna try putting together.

My options for power is an unregulated wall wart as stated in my original post and the laptop power supply.

 

Thing is, you're trying to make do with parts. Sometimes that works, sometimes it doesn't. I can advise, but I have no control over this.

I've done the project you describe exactly, I ordered regulated wall warts to power the 2 fans. Don't even remember who I ordered it from, I shopped online and got the lowest price I could (around $5 at the time). These fans were used to keep the rest of a hot plate cool, so people could rest their hands next to 200°C comfortably.

There is another possibility that won't dissipate the watts, but from what you've said I don't think your at that level, and it involve more parts. That is PWM, you make a PWM oscillator using a 555 and a power transistor, then adjust the PWM output to provide the voltage you need. You want to see how something like this looks I have a work in progress for my 555 Projects in my blog here at AAC.

The multiple resistor idea strikes me as a good solution for your problem, because you can spread the resistors around to dissipate the heat.

 

bill, correct me if im wrong, neggetz seems to think he needs to limit/control the amps ,
as long as his power supply is capableof supplying the load, amps are not an issue,
d

 

bill
need your help http://forum.allaboutcircuits.com/showthread.php?t=28322

 

bill, correct me if im wrong, neggetz seems to think he needs to limit/control the amps ,
as long as his power supply is capableof supplying the load, amps are not an issue,
d

Possibly, the fans are rated for 12V, this is the important spec. Meet this and the 600ma falls into place. 4 Watts of heat (per fan) isn't extreme, but it is rather toasty. It will raise a blister on your finger if you touch it and wattage is all in one part. This wattage is going to be there if it is a chippie or a resistor.

 

Ok, so how about this. What if I use an adjustable voltage regulator with heatsink (LM317) which I think can handle 1.5A max. I think that would solve the amperage problem. I could use say a 240ohm resistor for R1 and say around a 2K resistor on R2 to dial in around 12v.

 

Make sure the heat sinks don't get too hot. It will work, but if the LM317 overheats it will shut down (so will the other regulators). You will need a 10µF and 0.1µF cap on the output of the LM317 (per data sheet).

 

Ok, but I guess the only thing I'm worried about now is going over the 1.5A limit. If the fans start to fail, will they draw more current or is the 600mA listed in the fan specs a maximum taking into account the fan stalling? You said "other regulators", does that mean I could potentially wire up 2 regulators and double the supported amperage?

 

That's what I meant. Redundancy is good. Redundancy is good.

 

Bill, I have a hard time following you some time You say redundancy but wouldn't one shut down after the other? I wouldn't even know how to wire up 2 of them for redundancy. Will having 2 increase the amount of amps or would the 2nd one be there in case the first one shuts down?

 

Output of the regulator doesn't necessarily have an effect on the input. You could have one fan running, the other not. I assume you know not to connect the two outputs together.