Simple Electronics Wiring Question

Please be kind. I used to know the answer to something so basic, and kinda think I still do, but several people I’ve asked have come to the opposite conclusion, albeit without a great deal of conviction.

Anyhoo, for reasons explained in the margin,* I’m planning to hook up two small CPU fans, which run on 12 volts DC, to a single AC/DC converter. The question is, do I hook them up in series or parallel? In series would mean connecting a hot and cold of each, then connecting the loose hot of one and the loose cold of the other to the power supply. Parallel would mean connecting both hots to the hot of the power supply and both colds to the cold. Intuition/memory says I should do it in series. Several not-very-sharp electronics store types (we’re talking Radio Shack here) say parallel. I’d reallly appreciate hearing from someone who knows what they’re talking about.

  • What this is all about is that I have a storage shed with no windows that’s getting too hot. What it does have is four small three inch circular louves, two each on opposite walls near the peak of the roof. My objective is to exhaust air out the two louvres on one end, drawing air through the other two. I have no illusion that this will keep the shed cool, but I do hope to reduce the amount of heat trapped. This is a rental, so cutting a hole to install a proper exhaust fan isn’t an option. Inquiries at all sorts of hardware and fan supply stores have convinced me there’s no off-the-shelf product that does what I want, so I’m jury-rigging the solution just described.

Parallel, assuming you want them to actually see 12v. If you hook them up in series, the voltage drop across each (assuming they’re identical fans) will be 6v. Probably they’d work that way, but they’ll spin much slower.

Parallel. That way ensures that each fan has adequate curent supplying it, and that the voltage across is each fan is correct.

It ensures that both fans will receive the same voltage. It does not ensure that the current will be sufficient.

The current is dependant on the adapter. If it’s a 1A adapter, with two 500mA or less fans, you’ll be fine. However, if the current draw of both fans added together is greater than the rated amperage of the adapter, you’re going to poof the adapter.

And “poof” is a recognized electronics technical term. Just ask any bench technician.

You misunderstand. Implicit in my post was the assumption that the supply would be able to source adequate current. I only intended to differentiate between the current draw in parallel and the half current draw in series.

Easiest way to look at it is to consider all your other plug-in appliances, including lamps and televisions and the computer itself - all plugged in parallel to your house current.

Sounds like the shed already has AC. Why not use AC fans? They would be more efficient. (The only drawback is wiring. If this is a permanent installation, the AC wiring would have to conform to NEC rules.)

Super minor nitpicks:

  1. Home appliances are plugged in parallel to your household voltage.

  2. The step-down transformer has a center tap. This means roughly half your appliances are tied in parallel, and the other half are tied in parallel.

I wondered about that too, but I’m guessing he’s trying to match the size of the 3" louvers and hasn’t found a source for tiny little AC fans. Cutting a bigger hole and putting in a proper exhaust fan is the obvious preferred solution, but the OP indicates that this ins’t possible.

Thanks, all. Glad I asked. In my defense, none of the electronics-store-types even understood the question till I had diagrammed it for them.

As for the ancillary points, yes, the adapter has more than enough milliamps to cover both fans (three times, even). And, yes, I would have preferred hard-wired mini-fans (would’ve been cheaper, if nothing else), but couldn’t find 'em.

The power adapter output is 12V, correct? (Everyone seems to be assuming that, but the OP hasn’t said it explicitely.)

From here.

115V Axial fan

Lot’s of different options available.

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So… they’re all tied in parallel? :smiley:

Thanks, Danalan. That one’s too big - note that I need fans that will vent three inch holes - but found one here that’ll fit. (Found axial fans at one of the places I looked, btw, but they said none were made smaller than four inches.) Looking at this, though, it seems to me I’ll get more airflow with CPU fans, as the blades are larger (see here). Thoughts? Incidentally, I already have the parts for my original plan, but have no problem returning them if this is the better solution.

As for the DC adapter, yes, it’s 12V.

Another thing to keep in mind:

Exhaust fans for building envelopes are designed to withstand condensation and incidental water. I am guessing that small compute fans are not.

If I may offer a different direction. I have the same situation and I simply installed a small shelf near the peak and put one of those small double fan setups that are intended to go in a parially closed window (about 9-10" tall and maybe 24" wide) . It has a heat sensor in it that turns on and off as needed, so it rarely runs at night.