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Old 01-24-2008, 04:07 PM   #1
CrookedChris
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Default Yet another stir plate thread... with a For Sale feeler

After getting frustrated with the speed control limitations of brushless computer fans and just feeling uneasy about the lack of robustness in the speed control methods being implemented by the nice people on this forum I decided to go a step further in making my own stir plate.

Don't get me wrong, I don't think there's anything intrinsically wrong or unsafe with the designs I've seen implemented here. It's just that as an anal retentive electrical engineer I wanted finer control from 0 to full speed with no dead bands in the speed control knob and a robust design that will last me forever.

So I've implemented a very simple PWM (pulse width modulation) speed controller with a brushed DC motor (not a fan). My electronics provide ramp-up control, max speed limitation, and protection of the electronics from voltage spikes created by the motor (back EMF spikes). The brushed motor is a little louder than a brushless fan but the noise is dampened by the enclosure and is completely drowned out by the sound of the stir bar when stirring liquid.

I've made mine and it's working great. I haven't put together a schematic drawing or anything yet, just scribbles on scrap napkins and bits of paper. If people on this board are interested in my design I'll put together a schematic, bill of materials, and build instructions and post them to this thread. If no one's interested, I won't waste the time. So if you want to see it, speak up.

Additionally, if any of you guys (or gals) are really interested I would consider putting together ready to assemble kits of parts for sale. I'm not sure what it would cost yet. I'd just take the cost of all the parts (including a professionally manufactured circuit board and stir-bar) and mark it up a bit for my time and effort assembling the kits and dropping them off to be shipped. This option might be cheaper than getting all the parts yourself because of the volume discounts I might be able to get on the parts if enough people are interested.

I'll try to post pictures sometime in the next couple days but mine looks externally like all the others built in a Radio Shack project box. Inside there's a little circuit board and a bit of PVC pipe to hold the motor in place. On the end of the motor's shaft is a small plastic disk to which rare earth magnets are glued.

Also, in case anyone didn't know a small (too small for a 2L starter?) professionally manufactured stir plate can be purchased on-line for less than $90. But what fun is that? Hanna HI 190M Magnetic Stirrer



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Old 01-24-2008, 07:36 PM   #2
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Well, I'm not interested in buying/building one as I built a simple 317 based controller, but I'd be interested to hear a quick summary of what you did. I'm just a hobbiest in electronics, and am always eager to see how something is done so I can use it later on.

Like with the spike from the motor you're talking about, I just put in a snubber diode, did you do something more complicated? I can set my max voltage by changing out one resistor that's in parallel with the pot, but it's going to be a whole different ballgame with PWM. Speaking of which, are you using it like a switch mode powersupply, or more as a duty cycle control for the motor?

Sorry for all of the questions, like I said, I love to learn something new.


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Old 01-24-2008, 07:52 PM   #3
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I've been meaning to do something like this for quite a while, but as I've already got an adjustable-output bench power supply it's easy to just use that, so my motivation has been lacking.

Just out of curiosity, why did you use a brushed motor, instead of just PWM-controlling a fan instead? Not that it's a big deal or anything, but fans are just so convenient because of how easily they can be adapted to make stir plates.
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Old 01-24-2008, 10:32 PM   #4
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Scimmia. Nope I just put in a diode, just like you did. Super simple solution for a simple problem. Maybe I missed it, very possible, but I didn't see that addition on other designs. The PWM is a duty cycle controller. My design is very simple and I also adjust the max speed using a single resistor. Don't apologize for asking the questions, they're good questions.

Funkenjaeger, I saw your post about closed loop control using a PIC and the speed feedback on the fan. That's definitely a good design and more sophisticated than mine. Mine's all analog. I did try PWM control w/ a fan but it still didn't deliver the results I wanted. Let me elaborate:

At first I thought that the voltage regulator method everybody was using looked super simple, required very few components, and I had all the parts on hand so I gave it a shot with a few different computer fans I had on hand. The minimum speed on my one 120 mm fan was too fast and threw the bar constantly. Additionally, a large portion of the POT rotation created voltages below the minimum operating voltage of the 120mm fan so I had this huge range of uselessness and a small range of POT rotation that just resulted in the fan spinning way too fast. The 80mm fans I had on hand (very old stuff) worked a lot better but were just too slow @ 12 V to get a really vigorous stir going in 2L.

I thought about wiring a resistor in series with the POT to eliminate this dead band but then I decided to quit putzing with the voltage regulator and just move to a PWM.

I built the PWM and tried it with the 120MM fan which improved the situation. That is, I could run the fan at a lower speed but low speed operation was touchy and rather twitchy. The problem with brushless motors (like computer fans) is that they use electronics to switch the magnetic fields as necessary to create a rotating magnetic field (whcih is done mechanically in a brushed motor) and when the voltage drops below the voltage needed to run those electronics (likely 5V in most cases) the fan just stops. Brushless motors are usually meant to be controlled by feeding a desired speed value to the controller or, in the case of a computer fan, to run constantly at one speed preset in the controls. That is, using PWM or reduced voltage to adjust the speed of a computer fan works to a certain extent (by reducing the current through the coils and thus the magnetic field) but that's not generally how they're designed to operate.

I acknowledged that my problems might be solved by getting a different fan. I was going to do just that when it hit me: "Why use the fan at all?" That's when I got a brushed DC motor.

This led to the design I'm using now. The motor is super smooth from 0 RPM to top speed and I can make really fine adjustments. I can turn it on withthe POT turned all the way up and watch the stir bar slowly spin to full speed without worrying about the throw bar getting tossed. Additionally, the plastic disk I made to mount the magnets to the motor made it really easy to get them balanced perfectly.

As far as using a fan in general, I don't really see the motivation. Yes they have a big mounting platform for magnets but it really wasn't a big deal to drill a plastic disk to fit on the shaft of the motor. Additionally as you can see working with the fans I had was kind of a pain.

I want to reemphasize that I'm not dissing anyone else's design. I started from MrSaLTy's design and I really don't mean to imply that I think mine is drastically better than any of those. And surely Funkenjaeger's PIC based closed loop control is much more sophisticated than my open loop PWM. My method may not be for everybody but to me it's simple, elegant, and user friendly. Feel free to take it or leave it.

I'll post a schematic tonight for anyone that's interested.

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Old 01-24-2008, 11:25 PM   #5
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You should post some pics of your motor/plate/magnet setup. I agree that as a motor, the fan really isn't ideal because of the poor low-speed operation, but at the same time, it's hard to come up with an alternative motor setup that would be anywhere near as quick and easy to set up - not that it's not WORTH extra effort per se, but it's hard to beat building a stir plate from start to finish in like 20 minutes - so if you've got something simple figured out, I'd definitely like to see it.

That's also the main reason I haven't gotten around to finishing the PIC-controlled closed-loop thing, I can't get up the motivation since my current super-simple setup does everything I need already.

I'd be interested in seeing your design though.

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Old 01-24-2008, 11:39 PM   #6
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Quote:
Originally Posted by CrookedChris
Scimmia. Nope I just put in a diode, just like you did. Super simple solution for a simple problem. Maybe I missed it, very possible, but I didn't see that addition on other designs. The PWM is a duty cycle controller. My design is very simple and I also adjust the max speed using a single resistor. Don't apologize for asking the questions, they're good questions.
You didn't really miss anything, I've just commented in a few of the theads that you should really add that diode for protection. Never posted a new schematic or anything, didn't figure it was worth it.

Quote:
Originally Posted by CrookedChris
At first I thought that the voltage regulator method everybody was using looked super simple, required very few components, and I had all the parts on hand so I gave it a shot with a few different computer fans I had on hand. The minimum speed on my one 120 mm fan was too fast and threw the bar constantly. Additionally, a large portion of the POT rotation created voltages below the minimum operating voltage of the 120mm fan so I had this huge range of uselessness and a small range of POT rotation that just resulted in the fan spinning way too fast. The 80mm fans I had on hand (very old stuff) worked a lot better but were just too slow @ 12 V to get a really vigorous stir going in 2L.
And ironically, I'm using a 92mm fan, and have good control with the voltage topping out at 9V. I do still have the dead spot at the lower end, though.

Quote:
Originally Posted by CrookedChris
I thought about wiring a resistor in series with the POT to eliminate this dead band
I figured I would do that next time I opened the thing up. I found that my fan doesn't kick in until ~2.5V.
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Old 01-25-2008, 01:03 AM   #7
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CrookedChris, thanks to your inspiration I threw together a simple 555-timer-based PWM fan controller tonight. Essentially just "Nomad's 555 circuit" at this site:
http://www.cpemma.co.uk/pwm.html
but with a bunch of component values tweaked to best suit the stir plate use.

Seems to be working great with my 120mm fan-based stir plate, it covers the full range from barely-spinning to insane-vortex-almost-throwing-stir-bar, with no 'dead zone' in the adjustment, and starts up at any speed without needing a 'boost'.

Is your circuit anything like that one?
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Old 01-25-2008, 01:10 AM   #8
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Um, why not just buy a stir plate from a local used lab equipment vendor? I wouldn't purchase anything from the place i'm at but there ARE a lot of good ones out there that won't screw their customers!
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Old 01-25-2008, 01:43 AM   #9
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Quote:
Originally Posted by Melana
Um, why not just buy a stir plate from a local used lab equipment vendor? ...
Why not just buy beer at the corner store?
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Old 01-25-2008, 02:36 AM   #10
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Quote:
Originally Posted by evanmars
Why not just buy beer at the corner store?
Well said

And on top of that, even used lab stir plates can sell for something in the neighborhood of $100, and they're usually big, heavy cast-iron things. My stir plate cost me absolutely nothing except a few bucks for the stir bar because I already had parts laying around. Even if you didn't already have parts you could conceivably build one for $20-30, that'd also be much smaller and lighter than the lab ones.


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