Powering 80mm fans outside a computer

[Paddy]

I am going to install fans into the hood of my fishtank - the new lights i've installed are getting quite hot, and there is a danger that the acyrilic might crack.

I was going to use standard 80mm PC fans as I have loads lying about, but how do I power them? I have an old AT PSU, but i'd prefer not to have to house that clumpy thing... is there an alternative?

Cheers!

[GNEP]

You should be able to run them on anything from 5 to 12 V from a wall adaptor if you have a suitable one spare - the fans only take a Watt or 2 each. Cut the plug off and solder a molex socket on

[gt40]

you may find the life of the fans may be short - they are not designed for humid locations.
otherwise - what GNEP said
and you can always wire them to the same switch as the lights.

[LvR]

Run them off AC.....................
















Yeah and you all can get up off the floor now and stop laughing!

Slap a diode and a cap in series with the fan and there you go.

Impedance of a cap is 1/(2 x pi x f x c). If you first measure the current drawn by the fan when running at 12V, you multiply that by the impedance in order to get enough voltage drop across it to have just about 12 volt on the fan.

Lets say the fan draws about 80mA then you need to drop 120 - 12 - 0.7 = 107.3V across the capacitor. Using the impedance equation in reverse - you need a cap of about 2uF rated at 160V in series with a 1N4007 diode

[Paddy]

cool - so any 5-12v transformer will work? Do i have to worry about wattage?

[GNEP]

Nah should be fine - as I said, power draw should be in the region of a watt or two, so as long as the adaptor can supply 150mA you'll be fine (using the old I=P/V equation @12V; at less V, assume R is constant (good enough) and so I is proportional to V ie gets less too )

[Wombat]

Is it a salt water fish tank?

Egads, the fans could just become a short!

[Paddy]

yeah, its a tad salty

[Fat Tone]

I suppose you could mount the fans somewhere safe from the humidty and do a little ducting.

[gt40]

Or have them blow instead of suck.

[Fat Tone]

That's what I meant!

[Paddy]

The hood is open at the back so i'll have the fans blow cold air into the hood in the hope that it circulates to the back.

[gt40]

Just out of curiousity, how warm does the water get if the lights are so hot?

[Paddy]

The tank is thermostatically heated to 28'C. There is no active cooling system (other than the heaters turning off) so surface air flow is paramount. Well perhaps not in the middle of winter, but youknow what I mean