Question for the electrical gurus

[stephane.savard]

I have a questions for the electrical guys out there, but it has nothing to do with airplanes or helicopters. Though it does involve spinning blades!

Okay, so I have an aquarium at home, and because of the heat, I had installed a small 12V fan to draw out the humidity and keep the temp lower. Bascially, I had soldered on a jack to the positive and negative leads of the fan, and use a 12V, 0.5A plug-in transformer to power it. It works great, but it's fairly noisy. I don't need it to be spinning that fast!

So with today's weather, I started cleaning out my old computer things, and found a 9V 1A plug-in transformer with the same power plug. I pluged in the fan, and it works, and spins much slower, while still being quite effective.

Here's my questions

1. can I safely use 9V to power a 12V fan? will it burn out the fan, or be dangerous?

2. The fan itself states 0.09 A on it, when plugged into a 0.5A transformer, does it draw 0.5A or 0.09A?

thanks for any answers to this questions :D

[stephane.savard]

An additional question:

I just noticed that the output of the 9V adapter is 9VAC instead of 9VDC, does that make a difference if the fan is DC? And why does it even work?

[Andrew Fernie]

Yes, it is fine to run at 9V (as long as your fish are happy with the slower speed).

It will draw the current it needs - if it was drawing 0.09A at 12V, it will draw less when hooked up to 9V. Your 0.5A supply will be coasting along.

Odd that it is working off an AC supply if it is a DC motor. It may be that there is a diode somewhere in the motor to prevent someone hooking it up backwards and having it blow in the wrong direction. However, if you want to convert your AC supply to a DC supply and be sure, you can put a diode in line. You can pick one up at DDO Electronique, or just wait until I see you at the field. You don't need anything special, anything that can handle the current is fine. There will be a band around one end of the diode. Solder the non banded end to one of the wires from your power supply and the banded end will be the positive terminal.

Andrew

[stephane.savard]

Thanks Andrew, if it works with the AC, I'll just leave it as is (that way, if I ever need to use that Linksys router someday, I can steal the power supply back from the squarium :) ). I know it must have that diode - when I first soldered on the red and black wires to the jack, I had put them on backwards. The fan simply did not run (instead of running backwards).

The fan does not really cool down the aquarium, but allows it to at least not go over room temperature. Basically, it just allows the air to circulate and prevents the buildup of moisture between the water and the tank's hood. That in itself allows the water to evaporate better and keeps the temp from skyrocketing.

The fish (and wife :) ) will likely be happier with less noise.

So it's then my understanding then that with the AC current, it's simply turning the fan on and off 60 times per second?

[Paul Weijers]

Hi Stephan,
No, it is not turning on and of 60x/second. As Andrew pointed out, and has you found out by reversing the DC connection, there must be a diode built in the motor housing. What the motor is seeing are half waves, smoothed out by the mechanical inertia. Want to make it a bit smoother and avoid possible interference with other electronic equipment, connect a 1 or 2 microfarad capacitor across the terminals. Watch for the polarity.
Paul

[Sunfly]

I agree with the statements above excepted the value of the capacitor.
A capacitor of 200 uF is the minimum value I would put there.
This is because we don't was the electrolytic capacitors to fully charge and discharge repetitively. This will be over the power factor of such component and it will deteriorate or explode after a while... can't predict when.
According to the load you have there a 200 uF capacitor will keep about 80% of its charge between the cycles. You might as well use a 470 uF cap as this is a very common value. So a 20V cap with a value of 470 uF would be your deal.
This will have the effect of rising the average voltage on the motor and it will speed it up a bit. If you want to lower the speed by about the half then I suggest you to put a 100 Ohms 0.5 Watt resistance in series with the motor.

Find the schematic in the PDF attached.

The resistor will dissipate about 0.25 Watt but you need half a Watt.
That is how we calculate circuits in good electronics design.:D

Enjoy !

Cheers

[stephane.savard]

Okay, I didn't understand much of that. ha!

I've got it hooked up now, and it's been going for 24 hours. The fan makes no noise at all as it turns, and putting my hand above it, I can barely feel any air moving. But, all condensation has dissapeared from inside the aquarium's hood, so it's working perfectly :)

[Sunfly]

Ok But if you put a too small of an electrolytic capacitor there it may explode, leak and make a mess.
If that smaller ( 1 or 2 uF ) capacitor is over the water when it does that mess
your fishes may not cope with the chemicals and die.

[Sunfly]

My theory and experience with electronics is:
1) If it is working it is not necessarily OK.
2) It is OK normally when you do everything to known good specs and if it works in the worst conditions imaginable.
3) Murphy's law is the law above every other law.
The first statement of Murphy's law is:"If something can go wrong it will go wrong"

:D

Cheers,