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What is the cause of this?

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Marks256

New Member
Ok, i just got board, and started playing with a CPU cooling fan. After getting board with putting in about 18vdc(2 9v batteries in a series), i started hooking up more and more batteries until i had 5 batteries, or about 45vdc hooked up to it. After about two seconds of REALLY high power, the fan started to slow down.

What is the cause of this?

The reason i ask this, is because it did not wreck the fan. I took the fifth battery off, and it worked perfect again. So, did it start to get hot, and dissipate the power as heat, or what?
 

audioguru

Well-Known Member
Most Helpful Member
The fan motor has many turns of copper wire. Copper wire increases its resistance when it gets hot which would reduce the current, a little.

Probably the high current drained the 5th battery to zero and it couldn't pass the current from the other batteries.
 

Sceadwian

Banned
One or more of your 9 volt batteries was probably weak. 9 volt batteries are themselves actually 6 1.5 volt alkaline cells themselves in series already inside the package, that's why they have such horrible energy density compared to larger cells. Put 5 of those in series and what you actually have is 30 individual cells in series. Batteries don't work so good like that, the internal resistance don't ballance at all and one or more cells or sub batteries will heat up. This isn't normally a problem but you're talking 40+ volts now across 30 cells, you're lucky the weak cell didn't explode or at least leak.
Needless to say, forcing 4 times the rated voltage into a device is not a good idea and you should probably tone down your experimentation a little bit unless you have proper safeguards in place. The least I would recommend is saftey glasses and some kind of 'blast shield' It might seem paranoid but in electronics when things go bad they go bad in a hurry, and 40 volts is easily lethal. It's not likley to occur even if you do something really stupid, but it's like driving a car, only stupid people don't wear seat belts.
 
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Hero999

Banned
Perhaps the fans have built in thermal protection which started to lower the current as the motor got hotter.
 

Marks256

New Member
Sceadwain said:
saftey glasses and some kind of 'blast shield'
Good point, but i wear glasses. :D Sure, that isn't much protection, but it is a start. I think i am done experimenting. My finger hit one of the blades(while the fan was going at high speed), and it literally obliterated the fan blade. Luckily my finger is still fine; here is some proof: UYHNM!!!!!! That is right, i can still type those letters!!!!!!! :D :D :D :D :D :D :D :D D: :D :D :)
 

RODALCO

Well-Known Member
Your fifth battery was probably weak.
Also you were very close in frying your 12 volts fan.
But ok, if you were experimenting to prove the fact that the 5th battery was weak, did you swap it over with one of the other ones just to prove the fact.

It may be a good idea to look at a good variable dc powersupply because power from batteries is not cheap.

Roughly 1 kWh from batteries will cost you about $75.00 !!
 

Dr.EM

New Member
I have also observed this effect. I was using 120mm computer case fans rated 12v. I had them connected to my variable supply and I could crank it to about 24v with the expected increase in speed, but going over this toward 30v caused them to slow down again. It is something in the fan that does it. They have electronics for the brushless control, perhaps they incorperate crude cuirrent limiting too (for all the modders out there :D)
 

Hero999

Banned
This is probably the case. If you slow a fan down with your finger until it stops you'll notice it gets pulsed on every second or so. Exactly the same thing is probably happening when you overvoltage it, the current increases beyond a certain point so the power is cut of, then it slows down so the power turns on a again. It's kind of like PWM.
 

Marks256

New Member
The fan was going to get thrown away anyways...
 

HiTech

Well-Known Member
Marks256 said:
The fan was going to get thrown away anyways...
Why? You should keep it to construct the coolest hat on the planet, often envied by geeks attending computer shows. Wearing a hat like that really lures in the women.

 

audioguru

Well-Known Member
Most Helpful Member
But you must wear the COOL hat backwards so it blows down the back of your neck.
 

Marks256

New Member
That hat is pretty cool... Literally.... :D
 

windozeuser

New Member
Sceadwian said:
The least I would recommend is saftey glasses and some kind of 'blast shield' It might seem paranoid but in electronics when things go bad they go bad in a hurry, and 40 volts is easily lethal. It's not likley to occur even if you do something really stupid, but it's like driving a car, only stupid people don't wear seat belts.
Mythbusters are so cool :p

"EVERYBODY RUN BEHIND THE PSEUDO BULLET PROOF GLASS!"
 

Sceadwian

Banned
I'm surprised Adam hasn't killed himself on camera yet =) There have been some pretty serioues atempts! But oh not a dull moment that's for sure.
 

Marks256

New Member
I am hopping to get satallite TV real soon so i can start to watch Myth Busters on a regular basis. I own a 5 disk DVD thing of MB, but there was only like 15 episodes on them, which i watched all in one afternoon. :) Good stuff.... I personally liked the JATO Car, and the Bug Bomb House. Lol.
 
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