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reducing 12V+ source voltage with a potentiometer

J

Joseph

Jan 1, 1970
0
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.


All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it. So
I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have
on hand a potentiometer that say 15K ohm.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.
 
P

Pooh Bear

Jan 1, 1970
0
Joseph said:
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.

All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it.

I suspect it actually reads 0.08A
So I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have

on hand a potentiometer that say 15K ohm.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.

It's not the ideal way to control a fan but it should work ok. You'll need
a 'pot' capable of dissipating a watt or so. Cermet types are likely to be
best suited. Or try a fixed resistor of anywhere say between 150 and 470
ohms and see what the effect is like.

Graham
 
P

petrus bitbyter

Jan 1, 1970
0
Joseph said:
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.


All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it. So
I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have
on hand a potentiometer that say 15K ohm.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.

There are several ways to control a 12V fans speed but a potmeter is the
worsed. You can try to find a linear voltage controler or a PWM circuit. I
have no circuits here at the moment but a little googling will make you find
what you need.

petrus bitbyter
 
S

Spehro Pefhany

Jan 1, 1970
0
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.


All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it. So
I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have
on hand a potentiometer that say 15K ohm.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.

Try this circuit: http://cache.national.com/ds/LM/LM117.pdf

(first schematic, but with a 2K pot rather than a 5K). You don't need
the output cap, but I suggest putting the input cap on there, so 4
parts. It might need a small heatsink, depending on the current the
fan draws.


Best regards,
Spehro Pefhany
 
J

John Crighton

Jan 1, 1970
0
There are several ways to control a 12V fans speed but a potmeter is the
worsed. You can try to find a linear voltage controler or a PWM circuit. I
have no circuits here at the moment but a little googling will make you find
what you need.

petrus bitbyter

You are correct about a little googling.
Here is a site that Joseph might find interesting.
http://casemods.pointofnoreturn.org/pwm/index.html
and another which is pretty good
http://www.cpemma.co.uk/index.html
On that last site under "other fixed voltages"
http://www.cpemma.co.uk/diodes.html
here is what they say
"Semiconductor junctions always drop some voltage, and diodes are no
exception. A 1N4001 silicon diode drops about 0.75v at typical fan
currents (100-350mA), so one or more diodes in series with the load
can be used to take the edge off a noisy fan. Two diodes will reduce
the fan voltage to about 10.5v."

Joseph that idea of adding diodes just might
make your fan less noisy and still keep your
mother board happy. Experiment with the
number of diodes in series with the fan.

Have Fun,
John Crighton
Sydney
 
Joseph said:
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.


All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it. So
I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have
on hand a potentiometer that say 15K ohm.

thats the approx R of the fan, not the R of the pot you want. But its
not far off. A bit higher would give more v reduction range.

I'd double check your fan ratings though, that does sound high.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.

Its the simplest way - for a one off theres no reason to get into
complex electronics whena simple cheap pot does the trick. Either use
a 1w pot or else put your chosen pot in the airstream. Or you can just
use a bit of resistance wire and a croc clip etc to tap off where
wanted. R wire: heater elements, light bulb filaments, etc.


NT
 
Joseph said:
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please
help.

theres a simpler way that might do what you want: connect the fan to 7v
instead of 12v. +to +12v, - to 5v.

NT
 
G

Gerard Bok

Jan 1, 1970
0
Try this circuit: http://cache.national.com/ds/LM/LM117.pdf

(first schematic, but with a 2K pot rather than a 5K). You don't need
the output cap, but I suggest putting the input cap on there, so 4
parts. It might need a small heatsink, depending on the current the
fan draws.

But keep in mind that using this regulator will leave only about
10 volts for your fan.
Which means it will not reach full speed, not even if you need it
to keep the parts cool.
 
L

Luhan Monat

Jan 1, 1970
0
Joseph said:
I would like to reduce the 12V+ source voltage going into computer fans
with potentiometers place between the 12V+ and the fan. I am unsure
what Ohm rating potentiometer I need to get could someone please help.


All I remember from physics class was that V=IR formula. So I guess
that means R=V/I = 12/I The fan has a label that says 0.8A on it. So
I guess that means R=12/0.8 = 15 Ohms max ? Kinda small? I only have
on hand a potentiometer that say 15K ohm.

Could someone please share me their experience using pots to
dynamically reduce a 12V source voltage.

Cheap and dirty. Put a 12vlot lamp in series with the motor. For
faster motor speeds, use higher current lamps. Moundt a 1-hole mounted
lamp holder on the back of the machine for easy lamp changes. Also
provides some light back there when switching cables and such.
 
B

Barry Jones

Jan 1, 1970
0
Joseph wrote:


help.

theres a simpler way that might do what you want: connect the fan to 7v
instead of 12v. +to +12v, - to 5v.

NT
I think this is the best way. You could mount a switch on the case, and
switch between 12 volts, 7 volts or 5 volts, all easily available. That
should give enough options. How about a 3T2P switch for simplicity?

Although I have to admit, a non automated system that cuts the fan speed
makes me nervous. I'm sure I'd forget about it and leave it on low speed
while putting a heavy load on the system.
 
S

S

Jan 1, 1970
0
I used a rheostat to reduce fan speed--it absorbs heat well and can be
mounted on the front of the case with a semi-attractive knob! make sure you
get one thats rated for the number of watts you intend to run through it.
 
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