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Home Brew Forums > Home Brewing Beer > DIY Projects > Other > DC transformer on computer fan, dangerous?
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Old 06-07-2010, 03:52 AM   #1
262andbrew
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Default DC transformer on computer fan, dangerous?

Should a 120 mm computer fan have fried a 500mA dc power converter?

OK, I admit it, I am an electrical hack. I can make a good splice and connect red to red, but the numbers escape me and I need a bit of guidance.

My application is for a keezer, but I think it could be applicable for the stir plate builds.

I had an old computer case fan and a switchable ac-dc converter in the junk drawer and wanted to create a bit of air flow in the keezer. The converter was rated with max 500mA, has a switch to change voltage. It worked at 12V, it worked at 9, 7.5, 6 and 4.5V. I settled on 6 and left it on full time (as debated in this thread: http://www.homebrewtalk.com/f51/elec...keezer-130423/ ). It ran for a week without a problem, but eventually my dc converter (actually transformer) gave out.

I have no specs or numbers on the fan . So I know we are feeling around in the dark, but do you think that if I splice in a cell phone charger that is 5.2V and up to 1.0 Amp I will:
a) be happy forever
b) burn down the house
c) have it run for a week and finally burn out too

The fan could be the problem, I really don't know. Would you chance it on a decommissioned cell phone charger?


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Old 06-07-2010, 04:16 AM   #2
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You could always hit up some garage sales. Find a 12v supply on a $2.00 cordless phone or something...

500mA at 12v is only 6 watts of power.


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Old 06-07-2010, 01:10 PM   #3
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If your fan drew less than 500mA before, you aren't going to be drawing more just because your power supply CAN supply more current. Computer fans are designed to run on 12V (I believe) so applying 5V will be just fine, no matter the source.

Current is dependent on the load for this type of circuit. If you apply 5V, it doesn't matter if the supply can supply a million amps or half an amp; it will draw the same amount. Voltage = Current * Load. So, if Voltage and the Load are the same, current will also be the same.

The only time you run into problems is that in general, adjustable power supplies that have a wide range are not really accurate in lower voltages. This means that you think you are supplying 5V, but it's really 5.5V. Your fan wouldn't have a problem, but ICs and such can.
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Old 06-07-2010, 02:04 PM   #4
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Cool, thanks fellas. Without knowing the draw of the fan one of my theories was just that it 'wanted' more than the 500mA, and if that was the case maybe that is what made it give out.

OR...I might have just been dealing with a weak/worn out/defective power supply. I salvaged another from the junk drawer last night and it seems to be running significantly cooler (the transformer itself). It's rated at 1000mA and 5.4V. Things are good inside and out...for now.
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Old 06-07-2010, 03:33 PM   #5
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I am not sure what the power rating of your fan is, but I think I measured my fans to draw less than 125mA each. I have one that says it's rated at 1.25W, which would give you ~100mA. Of course, this is assuming I am supplying 12V.

You probably just had a bad transformer, but your theory is correct. If you over current a power supply it will fail prematurely. I have had the smoke get out, but never an actual fire....... yet.... from a power supply anyway.
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Old 06-07-2010, 05:18 PM   #6
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Could have been a bad transformer. One thing to keep in mind though is inductive loads behave differently than resistive loads. By reducing the voltage you are increasing current consumption, the higher the load on the motor the more current you will draw. By reducing the voltage the fan blade itself relatively becomes a larger load requiring more power to turn it.
The calculation above is for a resistive load.

Could also have been the transformer was getting ready to go. I would just run the fan at 12VDC and if speed control was that important I'd use PWM.
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Old 06-07-2010, 06:05 PM   #7
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a generic 120mm fan pulls 410-500 milliamps at 12v. As CodeRage said, with inductive loads (motor) if you lower the voltage you'll increase the current. Motors have impedance, which is a combination of resistance and inductance, so the voltage/current relationship is not linear.
If i were to guesstimate what your looking at i would say the fan is 6watts, and at 6v you are looking at around 900ma. plus the I^2 R loss of the resister.

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Old 06-07-2010, 06:21 PM   #8
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Code- pretty sure pulse width modulation isn't the best solution for someone with limited electrical experience. A pot, resistor, or limiting voltage would be much easier and he wasn't trying to be exact on speed. Also, I realize the equations were off (approximations using non reactive components- I meant to say load rather than resistance), all you have to do is replace 'R' with 'impedance' and the resulting differentials and motor calculations. Granted, you are correct, but it's a muffin fan. My approximations are not far off, neglecting locked rotor. I figured that would be fairly negligible (at least average power wise), trusting it's only start up. A locked rotor current spike on a 1A source is unlikely to break the source- for a muffin fan anyway.
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Old 06-07-2010, 06:40 PM   #9
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ndsgr, I know what you mean. The pot/ voltage limiting is what possibly started the problem though. Even an LM317 would get nice and roasty at lower speeds. Though it is not a locked rotor where you would see peak current draw, the mass/velocity of the fan rolling at a slower speed does make the load appear bigger to the motor.

The DC chopping of the stator causing some inductive feedback at the higher currents is probably what baked the power supply. Gradually decaying over time with every switch.

I would put a 120VAC fan in there so you could tap right off of the compressor's power supply. One less thing to consume a wall socket at least. If you wanted to control the speed an off the shelf rheostat will get her done.
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Old 06-07-2010, 07:14 PM   #10
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You guys are good. So here is what I have learned:
1) 500mA would have probably (again we are short on info regarding the fan) been OK for me if I had not dropped the voltage to 6 volts.
3) I may have had a cheap transformer that was on it's last leg.
2) 1000mA puts me on the fence with the voltage in the 5-6V range.
3) If I want to spin it slower than full speed, I should just pony up and buy the 120V one and can put in a rheostat.
4) If you want to find a group of people with a wide variety of knowledge and skills (and enough patience to deal with me) - Go to HBT!!!

Thanks all.


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