What happens if I connect a computer fan reversed?...

[Carlos E.R.]

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

- nothing
- the electronics self destruct
- there is a reversed diode that shortcircuits the DC
- the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it


I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so a
mistake is possible.


I would love to reverse sometimes one of my fans, though.

--
Cheers, Carlos.

 

[Bertrand Sindri]

Carlos E.R. <robin_listas@es.invalid> wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one.
I know it has electronics to produce a rotating magnetic field from
DC, but what happens if I reverse the connection?

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.

- nothing
- the electronics self destruct
- there is a reversed diode that shortcircuits the DC
- the electronics detect the reversal and either keep working
correctly or reverse the rotation.

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.

> I would love to reverse sometimes one of my fans, though.

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.

 

[Sylvia Else]

On 28-June-23 9:38 pm, Carlos E.R. wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

 - nothing
 - the electronics self destruct
 - there is a reversed diode that shortcircuits the DC
 - the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it


I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so a
mistake is possible.


I would love to reverse sometimes one of my fans, though.

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.

 

[Martin Brown]

On 28/06/2023 12:38, Carlos E.R. wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

 - nothing
 - the electronics self destruct
 - there is a reversed diode that shortcircuits the DC
 - the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it

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.

I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so a
mistake is possible.


I would love to reverse sometimes one of my fans, though.

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.

--
Martin Brown

 

[Carlos E.R.]

On 2023-06-28 14:00, Bertrand Sindri wrote:

Carlos E.R. <robin_listas@es.invalid> wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one.
I know it has electronics to produce a rotating magnetic field from
DC, but what happens if I reverse the connection?

The actual answer is: it depends.

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

I feared so. Thanks for confirming.

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.

- nothing
- the electronics self destruct
- there is a reversed diode that shortcircuits the DC
- the electronics detect the reversal and either keep working
correctly or reverse the rotation.

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.

I would love to reverse sometimes one of my fans, though.

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.

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

--
Cheers, Carlos.

 

[Carlos E.R.]

On 2023-06-28 14:20, Sylvia Else wrote:

On 28-June-23 9:38 pm, Carlos E.R. wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

  - nothing
  - the electronics self destruct
  - there is a reversed diode that shortcircuits the DC
  - the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it


I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so
a mistake is possible.


I would love to reverse sometimes one of my fans, though.


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

I know. But...

--
Cheers, Carlos.

 

[Carlos E.R.]

On 2023-06-28 14:27, Martin Brown wrote:

On 28/06/2023 12:38, Carlos E.R. wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

  - nothing
  - the electronics self destruct
  - there is a reversed diode that shortcircuits the DC
  - the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it

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.

I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so
a mistake is possible.


I would love to reverse sometimes one of my fans, though.

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).

:-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.

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.

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.

--
Cheers, Carlos.

 

[DJ Delorie]

\"Carlos E.R.\" <robin_listas@es.invalid> writes:
> - the electronics self destruct

This is what happened to mine

 

[John Larkin]

On Wed, 28 Jun 2023 13:27:46 +0100, Martin Brown
<\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 28/06/2023 12:38, Carlos E.R. wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

 - nothing
 - the electronics self destruct
 - there is a reversed diode that shortcircuits the DC
 - the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it

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.

I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so a
mistake is possible.


I would love to reverse sometimes one of my fans, though.

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.

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.

 

[Fred Bloggs]

On Wednesday, June 28, 2023 at 7:42:18 AM UTC-4, Carlos E.R. wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

- nothing
- the electronics self destruct
- there is a reversed diode that shortcircuits the DC
- the electronics detect the reversal and either keep working
correctly or reverse the rotation.

Anybody tried? I don\'t want to try and burn it


I know that it is impossible to reverse the connections inside a
computer, but I use them for other things in the house, directly
powering them, and not all white cables mark visibly each polarity, so a
mistake is possible.

The answer is most likely \"nothing\", if the circuit in the linked YT video is any indication:

https://www.youtube.com/watch?v=Im2GoXVy8d0

The circuit is easy enough to reconfigure, but you still have the basic problem of fan blade pitch fixed at CW or CCW. There are omnidirectional blades available, maybe you can hunt one down. Overloading the driver with the blade may burn it out. Sounds like you have a lot of these fans, so putting two opposite airflow back to back in your mounting and using a spdt to select the push or pull would be somewhat of a fix.

I would love to reverse sometimes one of my fans, though.

--
Cheers, Carlos.

 

[whit3rd]

On Wednesday, June 28, 2023 at 4:42:18 AM UTC-7, Carlos E.R. wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

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.

 

[Jasen Betts]

On 2023-06-28, Carlos E.R. <robin_listas@es.invalid> wrote:

Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

- nothing
- the electronics self destruct

yeah that one.

> Anybody tried? I don\'t want to try and burn it

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

--
Jasen.
🇺🇦 Слава Україні

 

[Jasen Betts]

On 2023-06-28, Carlos E.R. <robin_listas@es.invalid> wrote:

On 2023-06-28 14:27, Martin Brown wrote:

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.

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.

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.

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

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.

Some speed controls put a capacitor in series.

--
Jasen.
🇺🇦 Слава Україні

 

[Carlos E.R.]

On 2023-06-29 12:18, Jasen Betts wrote:

On 2023-06-28, Carlos E.R. <robin_listas@es.invalid> wrote:
On 2023-06-28 14:27, Martin Brown wrote:

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.

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.

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\".

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.

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

No, mine is a DC motor.

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.

Some speed controls put a capacitor in series.

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.


--
Cheers, Carlos.

 

[Joe Gwinn]

On Thu, 29 Jun 2023 12:49:08 +0200, \"Carlos E.R.\"
<robin_listas@es.invalid> wrote:

On 2023-06-29 12:18, Jasen Betts wrote:
On 2023-06-28, Carlos E.R. <robin_listas@es.invalid> wrote:
On 2023-06-28 14:27, Martin Brown wrote:

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.

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.

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\".


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.

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

No, mine is a DC motor.


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.

Some speed controls put a capacitor in series.


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

 

[John Larkin]

On Wed, 28 Jun 2023 16:12:10 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Wednesday, June 28, 2023 at 4:42:18?AM UTC-7, Carlos E.R. wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

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.

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.

 

[Fred Bloggs]

On Thursday, June 29, 2023 at 11:18:10 AM UTC-4, John Larkin wrote:

On Wed, 28 Jun 2023 16:12:10 -0700 (PDT), whit3rd <whi...@gmail.com
wrote:
On Wednesday, June 28, 2023 at 4:42:18?AM UTC-7, Carlos E.R. wrote:
Hi,

What happens if I connect a computer fan reversed? A brushless one. I
know it has electronics to produce a rotating magnetic field from DC,
but what happens if I reverse the connection?

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.
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.

You want to be able to filter the intake to prevent dispersing contaminants all over the enclosure, as well as keeping the enclosure at positive pressure, and that pretty much dictates the fan is located at the intake. The other way around has the contaminants entering the enclosure from everywhere.

 

[Carlos E.R.]

On 2023-06-29 16:02, Joe Gwinn wrote:

On Thu, 29 Jun 2023 12:49:08 +0200, \"Carlos E.R.\"
robin_listas@es.invalid> wrote:

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.

Some speed controls put a capacitor in series.


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.

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.

--
Cheers, Carlos.

 

[Joe Gwinn]

On Fri, 30 Jun 2023 12:50:03 +0200, \"Carlos E.R.\"
<robin_listas@es.invalid> wrote:

On 2023-06-29 16:02, Joe Gwinn wrote:
On Thu, 29 Jun 2023 12:49:08 +0200, \"Carlos E.R.\"
robin_listas@es.invalid> wrote:



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.

Some speed controls put a capacitor in series.


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.

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