Noob Question on Wall Warts...

[[email protected]]

Hi,

I want to hook up a 120mm case fan to a power supply. I'm wondering if
I can just use a 12v wall wart transformer. The only thing I don't
understand, is the wattage for the fan is 4watts, but most wall warts
only output about 500-800ma.


I also want to hook up a potentiometer to the 12v circuit, so I can
vary
the speed of the fan. Which type of potentiometer should I use? 500ohm?


Thanks.

 

[Michael Black]

Hi,

I want to hook up a 120mm case fan to a power supply. I'm wondering if
I can just use a 12v wall wart transformer. The only thing I don't
understand, is the wattage for the fan is 4watts, but most wall warts
only output about 500-800ma.

Divide the wattage by the voltage, and that will give the current, if
the AC adaptor doesn't specifically say what the amperage is. 4 watts is
only 333mA. And that figure seems high, I've pulled fans out of
junked computer power supplies and made tiny fans for cooling me
out of them, and the ones I've looked over draw less current.

I'd say in this case things aren't too finicky. If the AC adaptor
supplies more current than the fan requires, then it's fine. Often,
the AC adaptors will not have good regulation, so the voltage goes
up if the current drawn is less than the rating, but those fans
are cheap and if they have a shortened life time, you can easily
pull another one out of another supply. I dropped one a few
weeks ago, broke one of the plastic fan blades, and had the
fan back into operation in minutes, simply by pulling a different
fan out of the box of fans and soldering it to the AC adaptor.

Michael

 

[jasen]

Hi,

I want to hook up a 120mm case fan to a power supply. I'm wondering if
I can just use a 12v wall wart transformer. The only thing I don't
understand, is the wattage for the fan is 4watts, but most wall warts
only output about 500-800ma.

500 mA at 12V is 6 watts, so that won't be a problem, (excecc mA capacity
will stay inside the wallwart)

I also want to hook up a potentiometer to the 12v circuit, so I can
vary
the speed of the fan. Which type of potentiometer should I use? 500ohm?

yes, or lower.

Bye.
Jasen

 

[Bob Masta]

Hi,

I want to hook up a 120mm case fan to a power supply. I'm wondering if
I can just use a 12v wall wart transformer. The only thing I don't
understand, is the wattage for the fan is 4watts, but most wall warts
only output about 500-800ma.


I also want to hook up a potentiometer to the 12v circuit, so I can
vary
the speed of the fan. Which type of potentiometer should I use? 500ohm?


Thanks.

You need two simple relationships: Power = Volts * Amps (P= V*I)
and Ohm's Law: Volts = Amps * Ohms (V = I*R)
Rearrange as needed to find the missing value.

A 12 volt fan consuming 4 watts is drawing 4/12 = 333 mA (1/3 A)

The resistance of the fan is 12/0.333 = 36 ohms. If you put
a 36 ohm resistor in series with the fan, the total resistance
will be doubled so the current will be cut in half (12 / 72 = 0.1667).
The total power will be 12 * 0.1667 = 2 watts. So your speed
control pot will need to dissipate that much, which is a pretty
hefty amount... you'll need a wire-wound power type.

Another approach is to use a standard pot to control a transistor.
You can use a power transistor on a heat sink to dissipate the
excess power you will be wasting, OR you can use the pot to
control a pulse train that drives the transistor. In that case, the
transistor will consume little power since it's either fully off (no
current) or full on (low voltage drop) most of the time. To control
the speed, control the duty cycle of the pulse train.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

[[email protected]]

Thanks for your replies. I've never heard of either a wire wound pot or
a heat-sinked power transistor. Which setup is the cheapest parts? Im
not too concerned with the overall power cost of running it, but with
the lowest component prices.

 

[Bob Masta]

Thanks for your replies. I've never heard of either a wire wound pot or
a heat-sinked power transistor. Which setup is the cheapest parts? Im
not too concerned with the overall power cost of running it, but with
the lowest component prices.

For those of us with well-stocked junk boxes, the pulsed transistor
would probably be cheapest to build, since it would use a cheap
readily-available pot and the transistor probably wouldn't need a
heat sink. But it would be more complex to build since it would
need a pulse generator circuit. Since I assume that as a "noob"
you don't have a lot of parts on hand already, you'd have to go
out and buy those. So the wire-wound pot is probably the best
choice here, and it's certainly the simplest. You don't need to
specifically look for "wire wound", just the power rating. You
might be able to find a "speaker fader" pot that will do the
job. These are sold as volume controls that are inserted between
the amplifier output and the speaker, so they are made to
handle some power.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator