Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.

wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.

When the fan gets 6v at 75mA, the resistor drops 6v 75mA (12v psu). R
is then
6/0.075 = 80 ohms. So a 100ohm pot and 12v psu would give you good
control.

75mA x 6v = 0.45 watts, pretty much any pot can handle that.


NT

On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

I don't know how the fan would behave, PC fans have brushless solid
state DC motors they might need almost the full 12 v to run the
electronics. Also 0.15A will be the average, they might draw their
current in short heavy pulses.

However 50 ohms would drop 7.5volts at 0.15 A.

Might be cheaper to get a 50 ohm and a 25 ohm resistor and try it with
either one in circuit, or both in parallel (= 17 ohms) and see how it
goes.

Derek

In article ,
Derek Geldard writes:
On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

I don't know how the fan would behave, PC fans have brushless solid
state DC motors they might need almost the full 12 v to run the
electronics. Also 0.15A will be the average, they might draw their
current in short heavy pulses.

They're often run in PCs at reduced voltage. Minimum is generally
7V to guarantee starting. With intelligent control, (higher starting
voltage, then dropping back), they can be run significantly lower
than this. In practice, I've found very few 12V ones which won't
start even on 5V.

Not sure they'll have the ideal torque profile for a stirrer.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Sat, 15 Nov 2008 19:01:39 +0000, Derek Geldard wrote:

On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC depending
on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

I don't know how the fan would behave, PC fans have brushless solid
state DC motors they might need almost the full 12 v to run the
electronics. Also 0.15A will be the average, they might draw their
current in short heavy pulses.

However 50 ohms would drop 7.5volts at 0.15 A.

Might be cheaper to get a 50 ohm and a 25 ohm resistor and try it with
either one in circuit, or both in parallel (= 17 ohms) and see how it
goes.



The "proper" way to speed control these is using PWM. The OP may be able
to get a PWM controller for a case fan and just run everything off 12v as
its supposed to. The problem with using a reduced supply voltage is that
the torque falls off drastically. When used as a fan this *appears* to be
ok, but the airflow is actually far less than the speed would suggest.
Using the fan as a stirrer is more likely to just stall it. Note that PWM
can run the fan much slower (although it is much noisier) than a dropper
resistor.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.

wrote in message
...
Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.


Is this any good to you? http://brewiki.org/StirPlate

In message , Derek Geldard
writes
On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

I don't know how the fan would behave, PC fans have brushless solid
state DC motors they might need almost the full 12 v to run the
electronics. Also 0.15A will be the average, they might draw their
current in short heavy pulses.

However 50 ohms would drop 7.5volts at 0.15 A.

Might be cheaper to get a 50 ohm and a 25 ohm resistor and try it with
either one in circuit, or both in parallel (= 17 ohms) and see how it
goes.

Prolly better to knock up a cheap and nasty controller using a voltage
regulator IC with the pot controlling the voltage

--
geoff

Thanks guys...By all accounts there are lots of these in operation
without any major issues such as stalling etc...I'm just keen to
understand how to size in the pot given our local suppliers don't have
a huge range to choose from. They start at 500ohms and up..I can
source a 200ohm one online.


On Nov 16, 8:39*am, "BigWallop"
wrote:
wrote in message

...

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.

Is this any good to you? *http://brewiki.org/StirPlate

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).
Fan - Roughly 12vdc/ 0.15A/ 1.8Watts
Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.
Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit
Thanks for your help
Ged.
Is this any good to you? http://brewiki.org/StirPlate


If you want to control fan speed why not use one of these?

In article
,
wrote:
75mA x 6v = 0.45 watts, pretty much any pot can handle that.

Think you'll find most common ones are 1/4 watt.

--
*Did you ever notice when you blow in a dog's face he gets mad at you? *

Dave Plowman London SW
To e-mail, change noise into sound.

On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

--
Frank Erskine

wrote:

Thanks guys...By all accounts there are lots of these in operation
without any major issues such as stalling etc...I'm just keen to
understand how to size in the pot given our local suppliers don't have
a huge range to choose from. They start at 500ohms and up..I can
source a 200ohm one online.


On Nov 16, 8:39�am, "BigWallop"
wrote:
wrote in message

....

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.

Is this any good to you? �http://brewiki.org/StirPlate


200ohm pot with a 220 ohm R in parallel with it would be perfect.


NT

wrote:
wrote:

Thanks guys...By all accounts there are lots of these in operation
without any major issues such as stalling etc...I'm just keen to
understand how to size in the pot given our local suppliers don't have
a huge range to choose from. They start at 500ohms and up..I can
source a 200ohm one online.


On Nov 16, 8:39�am, "BigWallop"
wrote:
wrote in message

...

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

Thanks for your help

Ged.

Is this any good to you? �http://brewiki.org/StirPlate


200ohm pot with a 220 ohm R in parallel with it would be perfect.


NT

You can get a fair bit more torque from motors run at low speed by
using a 2nd external motor to fan air thru the 1st motor. The 2nd one
stays at full speed.


NT

"Frank Erskine" wrote in message
...
On Sat, 15 Nov 2008 09:57:51 -0800 (PST), wrote:

Can someone tell how I would size a potentiometer for a circuit to
control a simple 12v fan ( ex pc).

Fan - Roughly 12vdc/ 0.15A/ 1.8Watts

Input Power - Old movile phone charger - 6v upto around 12v DC
depending on model I find in the garage.

Am making a magnetic stir plate and using the fan with a couple of
magnets stuck onto it as the main drive unit

If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

What a memory

In article ,
mick writes:
The "proper" way to speed control these is using PWM.

These are not simple motors, but contain a driver circuit to generate
rotating magnetic fields. It's response to a PWM supply is not well
defined, and you'll get different fans behaving differently.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Sat, 15 Nov 2008 23:59:04 +0000, Frank Erskine
wrote:


If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

Ah, yes I remember it well.

The problem was trying to overcome stiction so that a model train
would pull away smoothly as opposed to sullenly sitting there, then
hareing off without warning and coming off the tracks at the first
bend which is what you got with a traditional (Hornby) rheostatic
controller.

Derek

On Sun, 16 Nov 2008 08:30:06 +0000, Derek Geldard
wrote:

On Sat, 15 Nov 2008 23:59:04 +0000, Frank Erskine
wrote:


If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

Ah, yes I remember it well.

The problem was trying to overcome stiction so that a model train
would pull away smoothly as opposed to sullenly sitting there, then
hareing off without warning and coming off the tracks at the first
bend which is what you got with a traditional (Hornby) rheostatic
controller.

I built one, and it worked very well, although at 'minimum' speed
there was still a tiny pulse from the controller, which made the train
creep very slowly at a 'station', especially if it didn't have many
wagons/coaches.

--
Frank Erskine

On Sat, 15 Nov 2008 23:35:22 +0000 (GMT), Dave Plowman (News) wrote:

75mA x 6v = 0.45 watts, pretty much any pot can handle that.

Think you'll find most common ones are 1/4 watt.

I was wondering about the disipation as well 1/2 watt does feel a bit high
to me. Also bear in mind the rated disipation would be across the whole
track not just a fraction of it.

--
Cheers
Dave.

In article ,
Frank Erskine writes:
On Sun, 16 Nov 2008 08:30:06 +0000, Derek Geldard
wrote:

On Sat, 15 Nov 2008 23:59:04 +0000, Frank Erskine
wrote:


If you can get hold of Wireless World magazine for (ISTR) September
1967 there's an article on speed control for small dc motors.
The idea is that you apply variable pulse-width full voltage to the
motor, and between pulses measure the back-emf of the motor to measure
its speed. The b.e.m.f. is used to control the speed to pretty fine
limits, taking into account varying load on the motor.
The idea was based around a controller for model railways :-)

Ah, yes I remember it well.

The problem was trying to overcome stiction so that a model train
would pull away smoothly as opposed to sullenly sitting there, then
hareing off without warning and coming off the tracks at the first
bend which is what you got with a traditional (Hornby) rheostatic
controller.

I built one, and it worked very well, although at 'minimum' speed
there was still a tiny pulse from the controller, which made the train
creep very slowly at a 'station', especially if it didn't have many
wagons/coaches.

Everyday Electronics did one sometime in the 1970's.

Nowadays, you can buy a chip which does it all for you.
A couple of years ago, I designed a circuit to drive a
motorised potentiometer and used the A3953 which is a
full bridge PWM motor controller. (Only need full-bridge
if you want to have reverse control too.)

However, as I said in another post, PC fans are not
simple motors, and they aren't designed for PWM control
and certainly not reversing. (I did permanently reverse
one once, by carefully disassembling and swapping around
the motor field winding connections.)

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

In article ,
Andrew Gabriel wrote:
However, as I said in another post, PC fans are not
simple motors, and they aren't designed for PWM control
and certainly not reversing. (I did permanently reverse
one once, by carefully disassembling and swapping around
the motor field winding connections.)

Surely most can simply be mounted the other way round?

--
*If horrific means to make horrible, does terrific mean to make terrible?

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
"Dave Plowman (News)" writes:
In article ,
Andrew Gabriel wrote:
However, as I said in another post, PC fans are not
simple motors, and they aren't designed for PWM control
and certainly not reversing. (I did permanently reverse
one once, by carefully disassembling and swapping around
the motor field winding connections.)

Surely most can simply be mounted the other way round?

Long storey...
I was butchering one to go into one of those powered Peltier
cool boxes. The original fan motors are bush ones with thin
sprung metel brushes, which give a few month's continous
use only. OK for taking on holday a couple of weeks a year,
but useless for continuous use.

I took a good quality Papst fan, snapped off all of the
frame, leaving just the impeller mounted on the flat motor
base, which was fixed to where the original motor went.
Trouble was it turned the wrong way. The original fan was
more centrifugal than axial, and by reversing the direction
of the new fan, and having removed the frame, it too works
well as a centrifugal fan (probably better than it did as
an axial fan). It's done about 12 years continuous service,
mostly as a small fridge for keeping the team's milk fresh
in a number of offices I've worked in. It still works,
but isn't in use now.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]