Are brush motors bidirectional?

Discussion in 'General Electronics Chat' started by GunsNTulips, Jan 7, 2014.

  1. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    Some must be, but so far the ones I've tested run one direction much easier than another. I've been playing with an H bridge I set up with 2N2222 transistors and 22KOhm resistors to the base. I used an old Atari joystick as my switch. I have a power supply, and I was going to try and characterize the bridge for current and voltage to take notes. I'm just playing basically.

    At first, I had an old 5 CD changer tray hooked up so I could move it around and around, but it would always move one direction better than the other. Voltages across the motor corresponded to speed, but I had assumed that the voltage would depend primarily on the transistors. I assumed the difference was because I had a poor connection someplace, so I soldered stuff together, but nothing changed.

    Then I switched motors to one that I had disassembled and got similar results. This motor is different though in that it has a diode. I assume that is a big part of the reason for the discrepancy, but I was surprised to find that the voltage was still quite different for each side. Switching the motor around, the voltages were reversed.

    I guess that really answers my question about the circuit. The circuit is fine, but the motors ont behave the same in both directions. Is this typical?


    Also, I assume the diode is there to give current someplace to go when the motor is switched off. How does one deal with that or a motor that is meant to go in both directions?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Usually DC brushed are bidirectional, unless some component has been added, the other reason which you can come across in some treadmill motors is that they are uni-directional due to the brushes being offset to one side of the commutator rather than being directly opposite in a line through the comm centre.
    These will run in the opposite direction but with less performance.
    Max.
     
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  3. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    With the CD tray, I suspect there may be different physical forces in different directions. With the other motor, the diode is doing it. Now that I think about it, the diode isn't doing what I was thinking since the inductor is constantly switching in a motor. Maybe the diode isn't serving a purpose and I should just remove it to make the motor bidirectional.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Depending on where or how the diode is connected it is going to have some affect on a DC motor in some fashion.
    Have you checked the motors and do they appear to be ordinary brushed motors?
    Max.
     
  5. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    It looks ordinary. This is from a tape player. It's a 12 volt motor, and the diode is across the terminals.

    I should probably change my experiment to a resistor since I'm really only trying to characterize the H-bridge. It's just less satisfying to not have movement.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Normally a diode across the motor would be almost dead short if you change polarity (direction).
    Max.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    If you reversed the supply polarity with the diode still connected across the motor then you most likely killed the diode.
     
  8. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    Oh I see now. The diode isn't connected across the terminals. I have no idea what it is connected too, since I would have to take apart the casing to see. That kind of stinks since it will limit my ability to use this motor.
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
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    If you want to diagnose whether the difference in speed is cause by the motor or by your circuit, disconnect the leads to the motor and reverse them, so "forward" and "backward" are inverted. Then if the condition stays the same, the motor (or something that is drives) is obviously not symmetrical. If the condition now affects the other direction of travel, then you must have some kind of imbalance in your electronics.
     
  10. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    I think thats true only if the diode were connected in series. As it is, I don't know how it is connected. The motor hasn't changed behavior. I thought diodes were often connected accross inductors to prevent the counter EMF from going haywire when the current was cut. I have no idea if this serves a similar purpose.

    Reversing the polarity reverses the voltage issue precisely so I'm sure this discrepancy is caused by the motor.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    No. In parallel, in series it just would not run in one particular direction (blocked).
    Max.
     
  12. GunsNTulips

    Thread Starter New Member

    Nov 22, 2013
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    Another issue I found is that apparently the joystick varies it's resistance pretty significantly. It just isn't as good a controller as I had hoped. It goes from 50 - 150 Ohms and I think there may be a bias for one direction, but my multimeter is sucky. All I know is I connected both buttons to the same terminal with a 10K resistor in place of the motor and still got the variance of .2 V out of 9 V across the 10K resistor depending on he side I pushed. That's annoying.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I wouldn't have thought it, but is it a logarithmic pot by any chance, instead of linear track?
    Or is this a N.O. contact type we are talking?
    If so with a sensitive meter the contact resistance may be influencing it?
    Max.
     
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