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DC motor

Discussion in 'Electronic Basics' started by Raul, Dec 3, 2004.

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  1. Raul

    Raul Guest

    First, let me say hello to everyone as I am new to the group. My
    question is how can I wire a DC motor to be reversible. Can I use a
    potentiometer? It's for a remote control fan. The switch will be
    connected to a servo. What I want is: The more I turn to the right the
    faster the fan goes clockwise. The more I turn to the left the faster
    the fan goes counter clockwise.
    If anyone can help that would be great.
    Thanks, Raul
     
  2. Ian Stirling

    Ian Stirling Guest

    Are you sure it's a simple DC motor?
    Most small fans are not, and cannot be reversed.
     
  3. Clarence

    Clarence Guest

    If, as you said this is a motor for an RC race car. A potentiometer will not
    be a reasonable way to control speed. Such motors are wound with as few as ten
    turns per phase and draw a LOT of current. A solid state controller for that
    type of motor may have as many as 6 MOSFET's to switch a PWM for speed control.
    Reversing is just a mater of reversing the leads. However it will cause
    additional wear on the Brushes.

    I would recommend getting a more suitable motor rated for a somewhat higher
    voltage (say 24VDC) surplus, and working with a lower current requirement and a
    slower speed due to an already reduced voltage vs. rating.
     
  4. Brian

    Brian Guest

    You didn't give any voltage or current ratings. If it is a low power DC
    motor, I drew up something that would probably work for you. Check it out at
    http://www.fncwired.com/MotorExample/
    Brian
     
  5. Raul

    Raul Guest

    -"You didn't give any voltage or current ratings. If it is a low power
    DC
    I'm sorry, it's a small DC I bought at a shop. It's 6v. I might change
    it to a 3v because physical space limitations. Thanks for the drawing,
    I'll see if I can get it to work.

    Raul
     
  6. Don Kelly

    Don Kelly Guest

    ------
    This depends on how the motor is wound. If it has a wound field- then you
    need to get into it to separate the field and armature windings. Then you
    can use a reversing switch to reverse either the field or armature. This
    requires surgery on the motor. If it has a permanent magnet field then
    reversing the connections should do it. Note that often a motor will rn
    better in one direction than the other. A center tapped potentiometer could
    do it, with the right connections but this is horribly wasteful of energy.
    Better to use a reversing switch along with your speed control which could
    be electronic rather than a potentiometer.
     
  7. Raul

    Raul Guest

    I'm also wondering about the parts in your dual powersource drawing.
    You have a NPN and a PNP transistor. Will any do or do they need to be
    specific?
    What do I use for the amplifier? I apologize for my ignorance. I'm a
    newbie who builds junkbots, if I don't have the part number, it's hard
    for me. This is for a small underwater ROV. The motor will control the
    verticle movement.
    Raul
     
  8. Raul

    Raul Guest

    Yes, by switching the leads the motor will run either clockwise or
    counter clockwise. So I guess I just need a switch to swap the
    voltage. That's good. I can have the servo connected to the switch and
    the speed control at the same time. That might be the easiest way.
    Thanks, Raul
     
  9. Brian

    Brian Guest

    The parts aren't that fussy. The op-amp could be something like a LM741.
    What transistor to use, depends on how much current your motor takes to run.
    I would use a transistor that could handle the stall current of the motor.
    To find the right transistor, I would suggest you go to
    http://www.digi-key.com/ and use their search engine. Their search engine is
    very good. After you select "transistors", it will give you some parameters
    to choose from. Based on that, it will give you a list of transistors to
    choose from.
    Brian
     
  10. Brian

    Brian Guest

    The op-amp should work under the voltages that you are using and supply
    enough drive current for the transistors. The transistors need to work with
    the voltages being used (I like to have them work with 4 times the max.
    voltages required). The transistors must handle the max. current and have
    enough current gain for the drive current supplied.

    The 741 op-amp supplies 0.02 amps of drive current for the transistors. If
    the transistors have to supply maybe 0.8 amps to the motor, then the current
    gain (Beta) of the transistor has to be 0.8 / 0.02 = 40. So if you are using
    a +/- 6 volt supply, under normal working conditions each transitor will
    need to handle 12 volts, so choose a transistor that can handle 48 volts,
    with a minimum Beta of at least 40 or more.
     
  11. Raul

    Raul Guest

    I got all the parts but it doesn't work yet. I'm sure it's some stupid
    thing I'm doing wrong. Here is a shot of the bread board setup. What
    do you think the problem is?

    http://www.asianhound.com/motor.html

    Thanks, Raul
    p.s. I must add I can't believe how great the response has been for my
    problem. If I could only get people that actually work for me to be
    this enthusiatic about stuff.
     
  12. Steve Evans

    Steve Evans Guest

    that's your main problem solved. some motors only run one-way whatever
    the polarity attached (automobile starter motors, for example).
    you want a power mosfet in to drve the motor with a PWM signal; that's
    most efficeint. there are some simple and really cheap circuits out
    there that you can use. just do a search on the web.
     
  13. You don't identify the op-amp, but whatever it is, you seem to have
    several pins unconnected. In fact it looks to me as if you only have
    pins 5 and 6 connected! In particular, you have not supplied power to
    the opamp. If that's a 741 or similar, +V should go to pin 7, and 0V
    (ground, or battery negative) to pin 4. And what is pin 6 connected to
    via the red and yellow links? Looks like they just go to strips 18 and
    19 of your stripboard, but then nowhere! And what transistors are
    those, and what pins are which?

    Why not *draw* the schematic, i.e. show what wires connect what pins
    etc.
     
  14. Your schematic should show pin numbers on the op-amp, and should also
    show the power supply connections for the op-amp.

    I don't see any power supply connections on the breadboard - without
    that, it is hard to say what is wrong.

    I don't see the feedback connection on the op-amp (output to -input),
    and one lead of the pot is not connected.

    One of the transistors is inserted in the breadboard incorrectly, I
    think. The flat sides should both face the same way, not towards each
    other.

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
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  15. Raul

    Raul Guest

    Ok, I re-did the bread board but still no dice. Here are pics of the
    current setup.

    http://www.asianhound.com/motor.html

    If I swap the transistors the motor runs in one direction but one of
    the transistors gets really hot.

    Thanks for all the help.
    Raul
     
  16. Raul

    Raul Guest

    Ok, I re-did the board but still no dice. Here are pics of how it sits
    now.

    http://www.asianhound.com/motor.html

    If I swap the transistors the motor runs but one of the transistors
    gets really hot.

    Now what am I doing wrong?
    Thanks for the help.
    Raul
     
  17. Several things, including my suggestion that you *draw* the schematic
    so that we can be clear what you are doing. It doesn't have to be a
    work of art. Just identify all connections clearly, and photograph
    *that* instead of your breadboard.

    Also, if that's 3V you're using for the 741, it's too low a voltage.
     
  18. You are still only using 2 connections on the op-amp chip.

    It needs at least 4 connections, input, output, plus power and minus
    power.

    The power connections are not shown in some schematics, but the op-amp
    always need power to work.

    One wire from the pot is not connected to anything..?

    Your pictures are very good, and they make it easy to help you. It is
    easy to see how the circuit actually is connected.

    You need to check all connections of all components on the schematic and
    in the real breadboarded circuit.
     
  19. John Fields

    John Fields Guest

    ---
    Probably nothing, since if the motor runs, one of the transistors
    _has_ to get hot! The transistors just look like variable resistors,
    and since you're dropping some of the supply voltage across and
    allowing current to flow through the one that's running the motor, the
    amount of current flowing through the resistor multipled by the
    voltage dropped across the resistor will be equal to the power it's
    wasting in order to slow the motor down.

    Unfortunately, there's not a lot you can do with the motor even if you
    do get it to run back and forth, since at low speeds you'll get very
    little torque out of it and there'll be a pretty wide "dead band"
    around the center of the pot's range where the motor won't do anything
    except sit there.

    The best way to do what you want to do is to pulse-width modulate the
    motor, which means that if you want it to run slow you turn it all the
    way on but only for a short time every once in a while, and if you
    want it to run faster you leave the switch on a little longer every
    once in a while. The slick thing about doing it that way is that
    since the switches are only ever either all the way on or all the way
    off they don't get very hot. I'm working on a simple circuit to do
    that, and I'll post it when I'm done if you're interested.
     
  20. You have the negative sides of the two batteries connected together,
    so you have two positive supplies, not one positive,m and one
    negative.

    Please show pin numbers for the op-amp on your schematic. As far as I
    can tell, the op-amp is not connected corretly - it appears that you
    misunderstand how the pins are numbered, but without pin numbers on
    the schematic, it is hard to say how you think the pins are numbered.

    The dimple on top of the IC is close to pin 1, and the pin numbers run
    counterclockwise from there - just as shown on the pinout drawing.

    You appear to have the output (pin 6) connected to V- (pin 4), and the
    wiper of the pot connected to V+ (pin 7)

    You still don't have any power supply on the op-amp, so it won't work.
     
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