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  • help with 12v fan speed control

    I’m looking to add a speed controller to a 12V fan.

    Will a simple rotary switch allow me to do this on a single speed fan?

    Will a 120v ceiling fan switch work?

    I need an education

    Thanks

    Mike
    Mike Hunter

    www.mikehunterrestorations.com

  • #2
    Just go to a salvage yard and get the fan switch and resistor pack out of anything,,,

    I don't think a 120 volt C. fan switch will work as your resistance values will be way off...

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    • #3
      Any kind of variable resistor within the amperage range of the fan should work.
      Larry - west coast of Canada

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      • #4
        I think you really need to know what type of 12v motor you have first.
        Gene

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        • #5
          You can't use the 120v type. What are you trying to run - a tiny computer fan or a big blower from a car? Car use resistor packs as mentioned above but they are energy pigs and must be mounted in the airflow or they burn up. With constant load speed is proportional (roughly) to voltage and adjustable power supply is the easiest method. Ebay overflows with "Constant voltage" 12v LED drivers from tiny to huge (power), and they are small/cheap.

          Like this (and there are USA sources), but we need to know more about your motor :



          New York shipper I've dealt with - good guy.




          Of course... if you wan to run the 12v fan off 120v, then you will need a mains power supply instead (with adjustable output) or in combination with the above , but those abound on Ebay/Amazon etc also.
          Last edited by lakeside53; 11-30-2014, 01:37 PM.

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          • #6
            Will a ceiling fan switch work -- no. The ceiling fan switch is designed simply to connect different sets of wires within the ceiling fan. Think of it as the control for a three way light bulb.

            I assume you are talking about a 12v DC computer type fan. If, so, you just need to change the voltage applied to the fan. Do you want 2 or three fixed speeds or variable speed. For variable speed, Amazon sells a 120vac input, variable voltage DC output for about $10. For a variety of fixed different speeds, a rotary switch and a few resistors will do the job. Find the current draw of the fan and calculate the value of the resistors you need using ohms law E=IR, where E is the voltage desired, I is the current requirement of the fan in amperes, and R is the value of the resistor.

            Say the fan draws 250mA and you want (about) 3/4 speed. R=E/I so the value of the resistor (in ohms) would be 9 divided by 0.250 (milliamperes converted to amperes). The needed resistor would be 36 ohms. Next calculate how big a resistor is needed. P=IE Power, in watts, equals current times voltage. Or rather than a switch and resistors, get a 3, or better yet, a 5 watt rheostat. Simplest way to get variable speed, technically, a big waste of power, but in your application, who cares.
            Kevin

            More tools than sense.

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            • #7
              The DC fans now used in P.C.'s are generally BLDC (Brushless DC) and have a small controller in them.
              So it is virtually impossible to vary speed, if it is a DC brushed fan then there are $5-$10 PWM controllers on ebay.
              Max.

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              • #8
                First, thanks for all the input, much appreciated.

                It’s not a computer fan, it’s a marine bilge blower motor; the type used to exhaust explosive fumes from the bilge/engine room.

                Bottom line is that I want more fresh air ventilation in the aft cabin. I plan on putting a fresh air intake above deck and venting to the cabin. Right now the motor is single speed, 130 (advertised) CFM, I want a way to control the speed, either a 3 speed switch or a rheostat type switch.
                Stated amp draw is 2.5A (12V), recommended 5A fuse

                An efficient 12 volt, continuous duty, high volume air flow bilge blower for extracting potentially hazardous gasses from boat bilges and engine compartments.


                Mike
                Mike Hunter

                www.mikehunterrestorations.com

                Comment


                • #9
                  One of those modules I linked will work fine. If you use a car type resistor bank and switch (way more work that the LED driver module - just wire an external potentiometer if you need it) make sure you mount the resistor in the airstream - they get quite hot and will burn out if not. Personally, on a boat I'd stay away from hot things and wasting energy.

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                  • #10
                    This is a bilge blower suitable for potentially explosive atmospheres, are we sure it is not a brushless DC motor?
                    Last edited by The Artful Bodger; 11-30-2014, 03:14 PM.

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                    • #11
                      Iuse fands of this sort of size., and control with a damper.

                      Originally posted by lakeside53 View Post
                      One of those modules I linked will work fine. If you use a car type resistor bank and switch (way more work that the LED driver module - just wire an external potentiometer if you need it) make sure you mount the resistor in the airstream - they get quite hot and will burn out if not. Personally, on a boat I'd stay away from hot things and wasting energy.
                      I use 12 v fans of this sort of size to raise steam on coal fired model steam engines. They do not object at all to having their output controlled by having an adjustable damper over the output. Maybe this would be an easier and perhaps safer andser for your circumstances. Regards David Powell.

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                      • #12
                        Mike Hunter, hopefully this will help both of us as I'm not trying to hijack your thread but had the same question...

                        These are the parts I've ordered or have for an upcoming project, hoping someone will say whether they will play ok with each other:

                        12 Volt DC - 5 Amp UL Listed (12V 5A UL) Power Supply


                        RioRand RRCCM2SPC Adjustable DC Motor Speed PWM Controller 10V 12V 24V 30V Reversing Switch 120W - $11.99 & FREE Shipping


                        Cooler Master JetFlo 120 cooling fan


                        The power supply is UL listed @ 5A, the fans (4 of them) draw 0.4A each, just not sure if controller will be peachy or not. Ok if not in my case... but adjusting the speed would be nice.

                        Comment


                        • #13
                          That's about a 30 watt or so motor. If you use a resistor to drop the rpm, you would be dissipating about 15 watts or so if you set it up to run on about half voltage. The current draw will drop somewhat because of the reduced air flow at lower rpm, but essentially you will be drawing about 2 amps or so. The resistor value would then become about 3 ohms. Three 10 watt, 1 ohm resistors could be wired in series to give you the dissipation ability without the parts getting too warm. That would then give you an easy way to have a speed range. You would have full power by applying 12v directly, and you would have lowest power by using the full length of the resistor pack in series. Other speeds would be by bypassing one or more of the series resistors.

                          If you wanted a slower rpm than this would give, then make the resistors 1.5 ohms each. I would put them in an array that mounts in the air stream so that the parts don't get more than warm. Conversely you could mount the resistors on an aluminum plate using silicon caulking, then add the wiring and silicon all the exposed leads and connections too. Anything done like this I like to give a week or more for the silicon to cure before subjecting it to the elements. You will need a multi-position switch also of course.

                          The bilge blower you referred to is not likely to be a brushless type, otherwise it would most probably have been touted as such. Most likely it's a sealed brush type motor. Either way, a brushless motor will run on reduced voltage to a certain point, below which it won't function. Computer fans can be slowed this way, and I have yet to find one which can't be slowed almost to the point where you can't hear it running, yet it still runs. The bilge motor is much larger of course, but if it is indeed brushless, it will still run at a reduced voltage though it may have a low voltage cutout point which is above the lowest rpm you might like to have. I'm virtually certain though that it is a normal brush type motor.

                          You certainly can use a pwm module to control speed. If you didn't want a potentiometer control with it's smooth, full range characteristic, you could replace the control with a few series resistors and tap the control voltages from that. I can see where that might be better, as you'd have positive steps in the rpms which would be more or less pre-set. To set it up this way, you'd have to adjust the pot for the speeds you want, then measure the resistances across the pot for each speed, then do some math and make up the series string of resistors to replace the pot. Probably more fiddling around than you'd want to do.

                          One good reason why I'd go the resistor route is that in a marine environment, electronics need to be really well sealed, and corrosion is going to take some hold anyway. The simpler the system, the more reliable-
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            Originally posted by caveBob View Post
                            These are the parts I've ordered or have for an upcoming project, hoping someone will say whether they will play ok with each other:
                            So as not to hijack the thread, a short answer. Yes, they'll work together just fine.

                            PS: Mike - what Darryl said, but I think I'd go with the PWM module and resistors and switch.
                            Last edited by KJ1I; 12-01-2014, 02:24 PM. Reason: Added PS
                            Kevin

                            More tools than sense.

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