What happens if I connect a computer fan reversed?

The actual answer is: it depends.

And what it depends upon is the design of the DC drive electronics.

I accidentally reversed one once while wiring a custom connector. Upon power application, the fan driver released the magic smoke and that fan never worked again.

Any of the above is possible -- depending upon the driver board design.

Self destruct is most likely given the level of "build down to a price" in the computer fan market.

Unless you purchased one that explicitly stated that reverse polarity produced reverse rotation you should not expect a computer fan to reverse direction with reversed polarity. Absent explicit indications they are all single direction spin only.

Simply reverse the physical position of the fan (i.e., flip it over) to reverse the airflow.

Even if you get the fan to run backwards, it won't function very well, because the blade angle of attack will be wrong.

Sylvia.

If it is an incredibly ancient one with no electronics in it at all then it might run backwards but the fan is designed to work well one way round.

Turn it over or mount it so that it can be removed and flipped.

I presume this is to have fan cooling of your office desk. I have an annoyingly powerful floor standing one that has settings that are roughly storm force, hurricane and tornado. Even on its slowest setting it rearranges all the loose papers in my office (and there are lots).

I tried putting a drill speed controller in series with it but the smart electronics inside didn't like having a truncated off waveform it either worked on the full waveform or refused to run at all.

I feared so. Thanks for confirming.

Yeah, well, I hoped to flip a switch as my mood flips :-D

I know. But...

:-D

The purpose of reverting direction is for a fan inserted on the window, normally to push fresh outside air into the bathroom. Some times I'd like to exhaust air for a while, specially when smelly. I could also do this to clean the filter (intended to stop mosquitoes from entering), pushing air in the "wrong" direction, which would push the dust and insects outside.

On the other hand, ceiling fans are wonderful. They are often designed to revert direction, and then the air flows against the ceiling and then down the walls, resulting in a nice breeze. They call it "winter mode", but I use that for sleeping in summer and not catching a cold.

Many (those with no electronics) 230 V ac fans I can lower their power dramatically by inserting a 220:110 autotransformer, or if not available, a filament bulb in series. The tornado subsides and becomes a silent breeze, with three adjustable speeds.

These fans have a capacitor and several coils, plus a switch that I have never decoded, to handle speed changes. No electronics. Maybe they are becoming extinct.

This is what happened to mine :-P

A BLDC fan can usually be throttled by adjusting the DC power supply voltage. I've done that with a DAC and a power opamp.

Reversing the voltage will not work and probably blow it up.

As others have noted, there's a LOT of different motor designs that might lie inside 'a computer fan', including piezoelectric oscillate=two=blades up to synchronous AC motors.

More disturbing, though, is the ducted fans that create net airflow, usually OUT of the enclosure at the power supply. Some boxes have a second fan, which (in order not to fight that airflow) should pump air IN to the box. Servers with good design will typically take cool air from the front face of a 3U RETMA-rack box, and eject warm air out the rear face.

You really don't want one fan reversed, if there's multiples in series. Cooling is achieved by ventilating of the whole box, not just spinning fan blades.

Centrifugal fans (air movers in HVAC) don't have any airflow-direction reverse, which makes them almost idiotproof.

yeah that one.

open it up and look inside - the circuit is very simple, (start by taking off the label, then carefully remove the circlip)

They make AC fans in the computer fan shape. I have not examined their motors but I assume they are shaded pole motors, so not electrically reversable.

These are capacitor run motors and by switching the location of the power feed the rotation of the field can be reversed.

Some speed controls put a capacitor in series.

Yes, I have some of those. I buy them when getting the DC there is not convenient. They tend to be powerful and noisy. And "thicker".

No, mine is a DC motor.

Hum. I have not seen that. But I have seen some AC to DC tiny power supplies that use a capacitor instead of a resistor to drop the ac voltage a lot.

The switched-capacitor-in-series scheme is common in the US for speed controls (Off, High, Medium, Low) of ceiling fans.

Cheap, and no annoying buzz.

Joe Gwinn

I bolt mosfets to CPU coolers and blow the hot air out the back of a rackmount enclosure. A K199 gives a bit under 0.25 K/W, better than a giant expensive heat sink.

The direction of air flow in rackmount boxes is sort of a religious war. One of our customers insists on rear intake hand-warmer mode, and one wants the opposite.

Ah, that's a different one that what I was thinking about.

Reminds me. I have one table top fan that has 4 positions: off, low, medium, fast, and silent (in that order).

The silent one has a buzz, so much that sometimes I prefer to use the "low" position.

It may have a rectifier diode and capacitor in series at that setting.

Joe Gwinn