PC fan speed adjustment options.

[David_]

Hello.

I am building a magnetic blender, it's really nothing more than an PC fan with a hard-disk magnet attach to it and then placed in such a way that you can place a glass beaker on top of the fan, of course without touching the fan, then you place the liquid to be mixed into the beaker as well as a little peace or rod of a metal that responds to the hard-disk magnet, so when you turn on the PC fan the fan's rotation is causing the metal rod in the liquid to spin thus mixing the liquid.

I have been reading about controlling PC fans and I can think of a few different ways but I tried using one of those 4-wire PC fans and I was somewhat disappointed regarding the range of adjustable speed.

And my plan with that was to use a microcontroller to output a adjustable duty cycle PWM signal and pass that to the fan control wire with a transistor since it requires a higher voltage than what the MCU can deliver. And then have a button for fan on/off and then an encoder or tvo buttons to set the speed.

But then I feel that this would be a lot of trouble in order to accomplish what I need to accomplish.

But what is the difference between using a 4-wire fan and controlling the speed by adjusting the duty cycle of the PWM signal feed to the control wire and using a simple 2-wire fan and switch it's supply line on/off at varying rates?

Do you know of any small simple circuit to do that?

Regards

 

[c_mitra]

What you are referring to "magnetic blender"- we use in the lab a lot- magnetic stirrer- for mixing liquids. They need a lot more power than a simple PC fan can provide.

Viscosity of air and water are greatly different; you are using a motor that is designed to run at high speed and low load to drive a high load at low speed.

But I really do not know anything about 4 wire and 2 wire fans used in PCs.

 

[David_]

Right, a magnetic stirrer, that's what I meant.
I see your point about a fan not being powerful enough, but I really do think it will work.

First of, the power of a fan is rather individual and I know of PC fans that are very weak while some others are more powerful.
And second, I will begin by cutting of the blades of the fan. So it will not need to put power into performing it's original task.

Although a motor is still to be prefered (and I own a lot of small motors, the problem is the question of how to mount it, I have pulled smaller motors out of lots of things and some of them are for sure usable)
The liquid and the amounts of liquid that are to be stirred aren't very large nor very dense. Ok the liquid is sort of dense, this stirrer is meant for mixing liquids for personal vaporizer (e-cigg) which consists of 30% PG(Propylene Glycol) and 70% VG(Vegetable Glycerin). PG is almost like water while VG is really thick like a light syrup, when you mix these two liquids and adds flavour concentrates the mix needs to steep for anything between 1day to 30 days or more, depending on what flavour you are using. And I have a recipe for a liquid that needs to steep(I'm not sure if steep is a ordinary word or if it is made up by vapers although it's more likely to have been used by brewers long before vaping came along, but to steep a liquid is to leave it to mature).

And I have heard and want to find out if it''s true that mixing the liquid in a magnetic stirrer for some time can decrease the time needed to develop the flavours of a mix, one of my favorite recipe's needs 4 weeks to properly develop, while others need 7 days. And it would be great if I could put the mix into a magnetic stirrer for a number of hours or a day or two and be done with it.

If it's even possible to speed up the process like that I don't know but I want to find out, and I think that I have a fan that would be sufficient to test this.

If the results is positive than I would like to proceed to build a real motorised version, but I'm curious, what kind of motor is best suited for a task like this?
Running at hours and being able to be controlled regarding to speed?

Oh and I might just say that steeping with heat is not something I want to do since it ruinse the shelf time of the juice.

 

[zouzanda]

hope this skematic will help you. you just need to program the mcu

 

[c_mitra]

The liquid and the amounts of liquid that are to be stirred aren't very large nor very dense. Ok the liquid is sort of dense, this stirrer is meant for mixing liquids for personal vaporizer (e-cigg) which consists of 30% PG(Propylene Glycol) and 70% VG(Vegetable Glycerin). PG is almost like water while VG is really thick like a light syrup

Let me clarify: propylene glycol is more viscous than water. Glycerin is very viscous.

If you try to stir with a common stirrer, it will create voids (empty space filled with vapour) within which the stirrer blades will rotate happily. The correct way is to reduce the speed to about 30 -100 RPM so that the stirrer will have more power.

It is like using an aircraft engine to drive a speedboat. Both have blades but they look widely different. They work in different speed regions.

It is good to use a electronic speed controller with one of your motors and use a permanent magnet mounted on the motor to turn the stirrer. The same idea is used in lab stirrers.

You should use the max speed such that (i) no air is sucked in the vortex (ii) no voids are seen near the stirrer blades. In such cases you can expect about 50% reduction in time.

 

[David_]

Aha, now the shortcomings of fans became much more apparent to me.

So maybe I should ditch the fan idea and instead put time into figuring out how to mount motors, perhaps I should just buy a motor which is delivered with some sort of mounting options.

About that MCU schematic, it looks dead simple, and I assume that PWM would be used.

But if I where to look at ebay or something for a cheap motor is there any particular specification that is important, I realize that I should begin by reading up on motors in general.

 

[c_mitra]

...perhaps I should just buy a motor which is delivered with some sort of mounting options....

It is real hell to drill a hole in a magnet so you will need a clamp that can hold a cylindrical or rectangular magnetic and can be fixed with the motor shaft. I presume it is easier said than done but I guess it is best to fix a circular plate to the motor shaft and fix the magnets on the circular disk with small clamps or something...

If you are using volatile organic solvents or things that can catch fire, it is better to use motors without brushes (brushes produce lots of sparks that can be dangerous). Most cheap universal motors are using brushes and should be avoided.

Most of the computer cabinet and CPU fans are BLDC motors and they have the drive circuitry within the fan itself. Perhaps the circuitry can be freely copied...

 

[David_]

I really don't think that the solutions I will be mixing classify as volatile or flammable, I know that the some of the flavor concentrates used is dissolved in alcohol but in any case that isn't more than a few % of the solution.

But still, I had planed to 3D print a in the end as good as air-tight box(although with dedicated openings for ventilation) in which the motors will placed.

I say motors because I have thought up a funny idea that I want to explore, you see I am using 20ml plastic bottles to mix when I am experimenting and looking at the bottom of those plastic bottles I don't see why a suitably shaped metal peace wouldn't be able to spin freely in the bottle.

I have to figure out what kind of metal object to be used, a metal that wouldn't interact with the liquid, I am thinking about the nicotine. I write with a permanent marker on the bottles, and if I take nicotine free liquid and washes over the written text with it nothing happens, but if I do the same with a liquid containing 6mg nicotine per ml then the text written with a permanent marker washes away.
I might be mistaken but that at least indicates to me that nicotine is sort of aggressive, I don't know enough about material interactions to even know what language to use.

As far as real magnetic stirrers go my newest idea might not be much like such a device, I don't know but as long as the liquids are mixed, the idea is then to design a fairly large device to be 3D printed on top of which there's x number of holes in which these 20ml bottles fit so they will be firmly placed(with a little metal bar inside the bottle).
Then in one corner of the top side there is a 2*16 char LCD or a 128*64 pixel display or some LCD or OLED display & a few buttons, the buttons being a user interface to choose which motors to activate, and for how long. Operated with an MCU, which is controlling a PWM signal to the drivers of each of the motors.

There is small motors with magnet mounted under each of those bottle holes, with a coupe of mm of plastic in between the magnet and the bottles underside.

Do you see any obvious "not going to work" situation with this idea, I at least think it sounds as a great idea as I would be able to mix 8 bottles of experimental flavors and then place them all in the devices bottles holes, activate the motors and let it run for a few hours.

Maybe I should also have a sort of lid that is put over the bottles to definitely hold them down in place.

 

[FlapJack]

For the low RPM you need, a stepper motor could work, i am thinking NEMA 17. You would have to program a micro to run the motor, or buy a stepper motor drive.

https://www.mpja.com/Stepper-Motors-and-Drivers/departments/101/

 

[c_mitra]

Do you see any obvious "not going to work" situation with this idea, ...
Maybe I should also have a sort of lid that is put over the bottles to definitely hold them down in place.

There is nothing wrong with your idea and it should work as you describe. You need some parameter to control the speed of the motors so that they run efficiently. When you fix the magnets, make sure that they are nicely balanced else the whole setup will create a hell of noise.

I once made a small coil (actually three coils) that were wired like the stator coils of a three phase motor. They were fed with square pulses that are 120degees apart. A small magnet, nicely wrapped in a teflon film (I used the thread sealing tape) acted as the rotor.

It worked beautifully and I had a nice (make that cute) electronic phase control magnetic stirrer.