Digital dc pump control

Comments

• Chris Savage Parallax Engineering Posts: 14,406

  2005-02-21 19:25 edited 2005-02-21 19:25

  If you're going to be using a DAC, that's half the equation right there.· Now all you need is a power transistor to control the voltage.· The output of the DAC can drive the BASE of the transistor to control how much voltage the motor sees.· You will need to calculate for the resistor between the DAC and the transistor BASE, based on what your DAC is putting out for voltage.
  
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• steve_b Posts: 1,563

  2005-02-21 21:05 edited 2005-02-21 21:05

  What kind of speed control do you have with a DC motor by varying the voltage.
  
  I remember mucking around a long time ago trying to get a friends "spy recorder device" to record for more than the 90minute tape allowed.· Playing with the voltage did a little bit....but I hit the cutoff voltage before I slowed the motor down enough.
  
  Wouldn't PWM be more what is needed for speed control?!
  
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  Steve
  http://members.rogers.com/steve.brady
  "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
• dmm Posts: 2

  2005-02-21 21:37 edited 2005-02-21 21:37

  Speed control was perhaps a bad term. Vacuum/flow control might be more accurate. The pump I was looking at (Clark 015 http://www.clarksol.com/html/prodspecs015Pump.htm ) shows flow vs. vacuum/pressure curves for several operating voltages. I thought maybe I could back off the voltage when vacuum demands aren't so high to save power (it is a portable application).
  
  I will look into the transistor idea Chris. I just don't play with analog electronics enough to know how to do that off the top of my head. I will take myself to school to figure it out.
  
  Perhaps I will try the PWM idea also, using a transistor as a switch?
  
  Thanks
  Dean
• Chris Savage Parallax Engineering Posts: 14,406

  2005-02-21 22:44 edited 2005-02-21 22:44

  Based on your question I gave the closest answer I could.· In many applications, including pumps, you can vary the speed of the motor by varying the voltage.· It really depends on a lot of factors, including how much of a load is on the motor (Causing it to stall at lower voltages).· For simply varying voltage you can tie the output of your DAC through a resistor into the base of an NPN Power Transistor.· The Emitter would be tied to ground.· The collector would be tied to one side of the motor, and the other to the positive supply, which would presumably be at the upper end of the motors voltage input limit.· By varying the DAC voltage, you can vary the voltage through the motor.· However, the base voltage will be pretty low, and be a narrow range between what it takes to keep the motor going at it's lowest speed without stalling, and what it takes to drive the transistor to max saturation for full speed.
  
  Steve's suggestion is usually better for driving motors.· It's certainly more efficient.· And you could use a pre-fab motor controller.· Parallax even carries some.· My suggestion was simply based on your need to vary voltage.· You need to decide what's the best course of action for your project.· I don't know the details.· Good luck!
  
  
  
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• Charles Posts: 3

  2005-03-01 02:03 edited 2005-03-01 02:03

  Maybe you could use a 555 timer set up to vary the duty factor of the output square wave to control a pass transistor or scr so that instead of reducing the voltage, you would be reducing just the current.