calling all electronics gurus....i really need your help!

[xero]

Okay guys....here's the deal:

I have two 4.72" fans i use to cool my amplifier....well, they're 12 volt DC fans, and currently i run them off a 12v lantern battery....but that gets expensive

so me and adul were chatting.....i have the fans wired to a switch, so i can turn them off and on at leisure....i'd like to connect them to a constant 12v source in my car, like i did my new cd deck...

well adul reminded me that i would need a resistor of sorts because the amperage my car puts out would be wya too much for the fans and would blow them out real fast.....so im wondering what kind of resistor i need!?

adul suggested i start w/ a 1k ohm 2watt resistor and if that doesn't let enogh juice thru then bump down to an 800 ohm 2w resistor.....

what do you guys think?

this resister will be going to the ON part of the switch, so the current will go thru the switch, then the resistor and the power from there will go to the fans.....

help?!

 

[shabby]

Resistor? I think you mean a fuse.
After looking at some specs of 5" fans, most are .5 to .66 amps, you'd probably need a 1.5 amp fuse.

 

[etech]

sheesh

It's not the current it's the voltage. You could put it on the output of a nuclear reator at 12v and it would not draw any more current. Now having said that, you cars alternator will put out up to 13.6v when charging the battery. That is not enough to hurt most 12v fans. If you wanted to you could hook it up through a fuse, most 120mm fans will draw around 1/2 amp(usually a little less). Shabby is correct a 1.5 amp fuse would be correct if you tink you need one. Most newer fans that I have seen have locked rotor protection built in.


I think adul does not know much about electronics.

 

[Angrymarshmello]

I smell flames...

 

[xero]

come on here guys....these fans are 12v brushless fans i got from radioshack.....and they work great

i have a 105 amp alternator on my car...it puts out 14.4v when recharging the battery while the car is given gas...

 

[jaygee]

12 Volts is 12 volts, whether it come from a lantern battery or your car system. The voltage in a car system can hit 13.8 volts. So if you can, see if you fans can take the 13.8 V. They'll be overclocked a tad. If not, and they draw, say .67 amp you will have to use a series resistor to drop the extra 1.8 V. 1.8 V / .67 A is a whopping 2.67 ohm. The Power rating should be I * V, or .67 A * 1.8 V or 1.21 W, coulda hefty for a such a small resistance. But not undoable.

An 800 ohm resistor would limit the current to 13.8 V / (800 ohms + the resistance of the fan). The resistance of the fan is neglible, it's resistance at full load it 12 V / .67 A = 18 ohms. 13.8 V / 800 ohms is about 18 mA, I doubt the fan would turn.

My suggestion, hook the fans directly to your car system. Let them run fast. I works for computers.

I hope I've made this as clear as the sailing here on the Garden State Parkway.

 

[Imaginer]

".these fans are 12v brushless fans i got from radioshack....."

How the heck did you get those things to work? :Q

 

[xero]

jaygee, i dont need the fans to be "overclocked" and i'd rather not do that to them...

so you're saying i should pretty much just get a 3-ohm resistor and wire it in to the fan power wires?


if not...then....well im confused.....i dont know much about this kinda stuff.....so you lost me...

 

[BigToque]

lol... now were into overclocking our fans

Add a tad more voltage and see if we can push a few extra RPMs into that motor

 

[Soybomb]

I'm pretty sure jaygee is saying the same thing I'm about to. Dont listen to adul Find a power lead thats switched with your key, most likely the same one your amp is on and plug them directly into it. 13.8 volts isnt gonna kill a 12v fan.....