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Thread: powering mini fans (power supply selection)

  1. #1
    Av8tor1's Avatar
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    powering mini fans (power supply selection) 101

    by special request...

    When choosing a power supply there are three variables you must consider.

    1. Voltage
    2. Type of voltage
    3. Amperage rating

    1&2). Voltage, your typical PC fans are made for 12 volts, and this is 12 vdc (second criteria). DC stands for direct current. The other one you are likely to encounter is AC, which stands for Alternating current. These two are not interchangable... its got to be DC output. The voltage rating we can vary a little and not cause any issue... but its best to stay with 12-13.8 volts.

    3). Amperage is another critical rating. This is a measurement of current flow... think of liquid water flowing through a line. If ths source of the water pressure does not have enough flow capacity, the pressure in the line will drop as flow rate increases... The same holds true for amperage and voltage. If your wall pack doesn't have enough amperage capacity, it wont be able to maintain its rated voltage under load. It will overheat from the excessive loading and soon fry itself.

    We can have a wall pack rated higher in amperage than we need but not lower.

    The rating that you will need depends on the fans that you run. Each fan will have its own rating... lets say yo have two fans. Fan "A" needs 12vdc @0.5 amps while Fan "B" needs 12vdc @0.75amps.

    For continuous use you should always use a power supply with an amperage rating twice what you need.
    So in our example we would need a wall pack rating of 12vdc @ an amperage of ( (0.5+0.75)x2) =2.5amps

    In critical applications (your helis are within inches of your 6 bulb T5)...it is always best to have at least 2 smaller fans and 2 power supplies instead of one large fan and supply. That way if one fails you will only lose part of your ventilation and not all of it.

    Think redundancy
    The cost of two smaller fans and two power supplies over one larger setup is much less then the cost of just one expensive plant.

    HTH's
    Av
    Last edited by Av8tor1; 09-10-2010 at 05:20 AM.

  2. #2
    Hello, I must be going... Not a Number's Avatar
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    Stickies or articles section might be the place for this post.
    Grand Hotel... always the same. People come, people go. Nothing ever happens.

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    Taliesin-DS's Avatar
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    I dont agree with one thing, voltage.
    Most pc fans are made to run on different speeds, this is done by lowering voltage.
    Most 12v pc fans are fine running om voltage as low as 3v.
    my growlist: http://terraforums.com/forums/showth...306#post976306
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    <Exo> @Talie......You are the lord of all things blah....

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    Av8tor1's Avatar
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    Personally, i don't like using voltage to regulate fan speed. Pulse width modulation is much better since you maintain maximum torque regardless of fan speed. This cannot be done by using voltage reduction with motors using a single speed winding'
    So while it may be successfully done by some, IMHO its not as efficient nor does it provide full torque at less then rated design speed :P

    My intent was "bullet proof instructions" for a newbie ...


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    Taliesin-DS's Avatar
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    I have no clue what all that rlly means :P
    But i got mine undervoltaged bec it blows waaaaay to hard at 12v.
    So losing some torque does not sound as a problem to me.
    my growlist: http://terraforums.com/forums/showth...306#post976306
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    <Exo> @Talie......You are the lord of all things blah....

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    Av8tor1's Avatar
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    Torque, HP and rpm's are related but different things... torque in this application may be defined as the twisting force applied to a shaft that has some resistance to rotation.

    Torque is not speed, but the "ummph" a motor has

    While reducing the voltage resulted in a reduction of motor speed which you needed. An additional side effect is the reduction in torque which can result in a motor start failure if an unexpected in increase in load exist. (such as trash build up on the fan blades etc etc)

    90% of the time it would never be an issue, but as you reduce the voltage lower and lower the likelihood of failure to start increases proportionally.
    When i have 25 helis within 6 inches of a 6 bulb T5, I want full torque.... Now you can have the best of both worlds, its a method called "Pulse Width Modulation".
    Here we "pulse" the current flow to the motor at full voltage. Since the voltage/current is pulsed at the max value we maintain max torque. But since we have less then a 100% duty cycle we also can have a reduction in motor RPM at the same time.

    Last edited by Av8tor1; 09-10-2010 at 05:16 AM.

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    Av8tor1's Avatar
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    Advanced PC fan speed regulation 201

    Not to muddy the waters but if youre feeling spunky and want to learn more about PWM

    A great lil intro...

    Last edited by Av8tor1; 09-10-2010 at 05:20 AM.

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