Question about DC voltage, converters and case fans

About a year back I built a desk unit that houses all of my computer equipment. The main desk “compartment” gets pretty warm unless I keep the doors open. To solve this problem I want to cut a hole in the side of the desk and install a computer case fan that will ventilate the space.

At this point I have one 12V Case fan that draws .08 amps. My question is how to power it. I have a box full of old AC adapters from things like cd players, cell phones, etc. Can I use any of these and if so, what should I consider if the voltage/amps don’t match up exactly?

Will using something other than a 12V converter be ok? I think this should only affect the fan speed…

What if the converter can supply considerably more amperage than is required (.5 A)? Does the extra capacity just sit unused or does this “under loading” do any damage?

Any help is appreciated!


120V fans aren’t that expensive. Maybe something like this would be easier and cheaper then using a DC fan and a power supply, unless you already have both of the latter.

Think of an ordinary 120v wall socket - rated for 15 or 20 amps, but you can safely plug in a table lamp that draws less than an amp. Extra current doesn’t flow from the supply to the load unless the load demands it.

Try for as close to 80mA as possible. Those little fans might work with less, but you risk overloading (and overheating) the wall wart if it’s too far underrated. I think most DC supplies are 12V at 100mA so you should be fine with one of those.

Couldn’t you power the fan from your computer’s power supply?

Thanks for the replies!

Running it off the computer would be difficult as the fan will be about 2 1/2 feet away, plus the computer can slide in and out of the desk (or be removed completely - and I want the fan to stay mounted to the desk).

The AC fan is an option - but I hadn’t seen any locally available and I already have the fan and the random converters - so I’ll try and make those work.

OK- Here’s a new question. For the small amount of current that the fan will pull, is it possible to hook up an old mercury dial thermostat directly in line between the fan and power source or does it need to be connected via a more traditional relay setup?

Do you already have a temperature controller? Or do you want to purchase a stand-alone controller? Or do you want to build one?

Have nothing at this point for temperature control. But I think I have an old Honeywell mercury type thermostat lying around somewhere.

I’m not adverse to buying something, I just figure that using some spare stuff I already have on hand would help me justify to my better half all those boxes of parts I refuse to throw away “because I may someday use them” …

I could be wrong, but I think most HVAC thermostats run off 24V.

The simplest solution is to use a bimetallic thermostat switch in series with the fan. But most are NC, and open when the temperature rises above a certain level. You want the opposite. Fortunately, a NC can be easily “converted” to a NO with the use of a relay.

I’m not sure where to get a bimetallic thermostat. Perhaps google can come up with something.

Crafter Man - I forgot about NO/NC distinction! Wish I read your message before I spent 45 minutes rummaging through boxes looking for that thermostat.

Last night I hooked up the fan to an old DC converter previously used for a cell phone. Works like a charm.

For temperature control, I’m going to build this little circuit:

That’s pretty cool.

Myself, I would have just turned the thermostat upside-down. :wink:

Most space heaters use them. If there’s not one gathering dust in the basement, you can pick up a used one at Goodwill or the like for a couple of bucks.

I checked that circuit out. It will work, but I don’t much care for it for a couple of reasons:

  1. There’s no hysteresis. Depending on the thermal time constant and heat capacity of the system, there’s a good chance the fan will be frequently cycling on and off. This will be annoying.

  2. The MOSFET will be operating in its resistive region, and will therefore dissipate some heat. A bimetallic sensor or reed relay may be more efficient. (I could be wrong about this point. You’d have to do the math to make sure.)