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Old 01-19-2011, 11:50 PM   #1
rocketman768
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I did a search on the forums, but I can't find a post about a simple PWM controller for stir-plates. I have tested it, and it works great. Any way, here it is:

Schematic & Build Diagram


In Action



  • R1 = 22k resistor
  • R2 = 100k pot (use the middle and one of the end pins)
  • R3 = 4.7-10k resistor
  • C1 = 2.2 micro-Farads capacitor
  • Q1 = N-channel MOSFET
All components available at mouser.com (parts list).

The 555 can be a cmos 555 (LMC555) or a regular 555 if you like. Q1 is any standard MOSFET (or NPN transistor, like 2n2222) with a current rating of at least whatever your motor is going to suck (mine draws 100 mA). The diodes are standard silicon diodes. The two wires going the pot are connected to its middle pin, and one of the two end pins (which one determines if turning clockwise increases or decreases speed). I have drawn two options for the output stage. I prefer option 1 because MOSFETS are much more efficient, but they can be hard to find in physical stores.

Make sure your Q1 is oriented in the right way (D,G,S for MOSFET, or C,B,E for NPN) by looking at the data sheet or packaging for the component!

I don't know if there's a discussion about the different speed-control methods, but let me put in my two cents about this design versus others.
Resistor voltage division sucks, because most of the power is being dissipated in the resistors. LM-317 regulators are a bit better, but they still cannot get the motor started at voltages less than about 4V, and the top available voltage is 3V less than the power supply, so you only get a narrow range of workability. PWM is the best I can see, because there is very little energy wasted, and the output is always 0V or Vcc, so you get the necessary torque to spin the motor at low speed, and you can push the motor all the way to full speed.

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Old 01-20-2011, 12:43 AM   #2
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the reason for not finding posts on this is that a simple potentiometer can easily control the speed of a DC fan. I have used PWM for fan control but it is overkill in this application..but hey around here overkill is norm.
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Old 01-20-2011, 12:44 AM   #3
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Quote:
Originally Posted by gunner65 View Post
the reason for not finding posts on this is that a simple potentiometer can easily control the speed of a DC fan. I have used PWM for fan control but it is overkill in this application..but hey around here overkill is norm.
That's what I was talking about when I said resistor voltage division. Until you turn the pot up to about 1/3 or 1/2, the fan won't move. I hate that, because the lower end of the speed range is gone.

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Old 01-20-2011, 12:47 AM   #4
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gonna tack solder it together tomorrow will let you know
pete
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Old 01-20-2011, 01:13 AM   #5
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Quote:
Originally Posted by rocketman768 View Post
That's what I was talking about when I said resistor voltage division. Until you turn the pot up to about 1/3 or 1/2, the fan won't move. I hate that, because the lower end of the speed range is gone.
Understood the reason just explaining why so few use this method. The lower end of the speed control isn't going to be that valuable in a stir plate so the loss in the pot doesn't really hurt as most people run them as high as they will go when aerating starters.
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Old 01-20-2011, 04:47 AM   #6
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Quote:
Originally Posted by gunner65 View Post
...most people run them as high as they will go when aerating starters.
If that were the case, they'd skip the pot and just hook a power supply up to the motor. Besides, I know mine throws the stir bar if I start it too fast depending on the size bar I'm using.

 
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Old 01-20-2011, 05:36 AM   #7
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your diagram just gives me a headache I am not good with that kind of stuff, but I was able to follow another thread that used radioshack parts and was very easy to build and cheap.

http://www.homebrewtalk.com/f51/my-s...y-build-86252/

-=Jason=-

 
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Old 01-20-2011, 06:28 PM   #8
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Quote:
Originally Posted by certaut View Post
gonna tack solder it together tomorrow will let you know
pete
I have tested it to make sure the output of the 555 works as suspected. Now, just need to get my mosfet and should be able to see if the rest works.

 
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Old 01-22-2011, 12:21 AM   #9
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Quote:
Originally Posted by rocketman768 View Post
I don't know if there's a discussion about the different speed-control methods, but let me put in my two cents about this design versus others.
Resistor voltage division sucks, because most of the power is being dissipated in the resistors. LM-317 regulators are a bit better, but they still cannot get the motor started at voltages less than about 4V. PWM is the best I can see, because there is very little energy wasted, and the output is always 0V or 9V, so you get the necessary torque to spin the motor at low speed.
When I saw the title of this thread I thought "Why would you use PWM for your stirplate, that's just overkill!" but this makes a lot of sense... I'm right there with you on being bugged by losing the lower end of the speed range, and it's really not that much more complicated than the other stirplate builds if you have the slightest experience with electronics.

I was planning on starting my stirplate build this weekend, and I think I've got all the parts (except possibly the MOSFET) on hand already. I'll let you know how it goes.

 
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Old 01-22-2011, 02:20 AM   #10
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Just tested this circuit out and it works great!

The low end of the motor is completely accessible, and it goes all the way to the high end!

However, I had to modify some of the values to suit my motor. I originally planned it to operate at > 20kHz so that you couldn't hear it. However, it appears that the motor I have doesn't do much until you get down around the 100 Hz range. So, I just swapped out the resistors and capacitors as follows:

R1 = 22k
R2 = 100k
C1 = 2.2 micro-F

I'll update this on the OP. The good thing is that now it's operating around 20 Hz, so the only thing you hear out of the motor is a tiny clicking sound, but I expect you won't even hear this when it goes in the box. SWEET.

 
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