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Need help with DC Motor Circuit

  1. Jan 30, 2015 #1
    Good day. I was wondering if anyone could help me with the following problem. As you can see in the diagram below, I want to set up a circuit with 4 DC Motors in parallel, each with a push button switch. And a variable resistor to control the speed manually. I would like a LED strip to be connected as well as shown. The LED should always be on and the motors would only run one at a time as long as the switch is pressed. I need to know the following:
    If this circuit is do-able?
    What my power source should be? (Preferably a wall plug).
    The LED won't pull that much current that the motors don't work will they?

    Here is a list of the Items I am using. The motors I already have but the rest must still be ordered.

    M: Motors: 4x RS-360SH
    R1: Variable resistor: Either This one or this one.
    S1-S4: Switch: Tactile Switch B1735

    And my schematic
    http://imageshack.com/a/img661/4373/7nFhRB.jpg [Broken]
    Thanks in advance!

    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Jan 30, 2015 #2


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    Your circuit will not work at all. There is no voltage applied to any component.
  4. Jan 30, 2015 #3
    Then what is the problem?
  5. Jan 30, 2015 #4


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    Greetings MIke

    one side of the battery isn't connected to anything
    both sides of the motors are connected to the same side of the resistor, once side directly, the other via the switch
    therefore the motors wont work and the LED wont light either as only one side of it is connected to the battery

    Am just a little busy at the moment ... I will try and get a chance later to redraw the circuit for you :)

  6. Jan 30, 2015 #5


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  7. Jan 30, 2015 #6
    Thanks davenn and Baluncore. Not to good with the schematics... I see there is a resistor in with the LED's? Why would that be? Each motors operating range is 3-12V and at maximum efficiency uses 1.76A. So I could use a 12V 2A adapter for this? And is that the reason for the resistor in with the LED(To bring down the current)?
  8. Jan 30, 2015 #7


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    It appears you have a very limited understanding of ohms law and circuits in general. That's ok, just take the time to learn it. Any diode, LED or otherwise, will only ever drop a fixed voltage across it before it destroys itself. This means that an LED that is conducting .02 amps for instance will drop about 2 volts. The same LED that is conducting .03 amps will also drop about 2 volts. The current and voltage do not have a linear relationship as a resistor does. That is the reason for the resistor. It limits the current. A 12 volt supply running an LED in the above schematic will have the resistor dropping around 10 volts assuming the LED has 2 volts across it. Do you know enough about ohms law to compute the required resistor size based on the spec of the LED?
  9. Jan 30, 2015 #8


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    Thanks Baluncore

    I got tied up with work till last nite, didn't get finished till 1030pm

    was going to do the diag this morning but you have filled in the gap :)

    and one other thing .... to the OP
    controlling motors with a variable resistor isn't a good way to go ... Yes you slow it down
    by dropping the voltage, but you also remove much of its current supply which means it
    cannot develop the same power at the slower speed

    You may want to read up on PWM and H-Bridge driving of motors

  10. Jan 30, 2015 #9


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    No. If each motor draws 1.76A then a 2A supply can only supply one motor (two motors will draw 1.76*2A and so on). Also, the max current draw will likely be greater than the draw at max efficiency, the draw at stall could be double that. The actual current draw will depend on the load and the motor curve. If you draw a nice diagram of your intended plan we can offer some design input. Smaller motors may meet spec.
  11. Jan 30, 2015 #10

    jim hardy

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    Max efficiency and 1.76 amp point on operating curve is per the datasheet at very low load.
    The motor can pull over 8 amps at stall.

    http://www.mabuchi-motor.co.jp/cgi-bin/catalog/e_catalog.cgi?CAT_ID=rs_360sh [Broken]

    What do you intend to spin with the motor?
    Last edited by a moderator: May 7, 2017
  12. Feb 2, 2015 #11
    Thanks for all the posts. Did a bit of research on the Ohm's law. Starting to understand it now. Probably should have done my research before I posted. Sorry guys. My plan is to have only 1 motor running at any time. never more than 1. It is a project that will just circulate various solutions of water. The motor comes with a centrifugal attachment. So I don't think I would need as much current. The water just needs to move as the button is pressed. Don't need a strong motor. Those are the smallest motors available that I can find in RSA. I came across a similar project using an Raspberry Pi and motor controller. That would ultimately be the best option I think? I reckon I will go with that tutorial and do it from there. Thanks again
  13. Feb 4, 2015 #12


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    Those switches will not be able to handle the amount of current required.

    Look into using a mosfet or bjt and using the switch to turn on the collector or base of the transistor.

    You will also want to add a flyback diode to protect the transistor.

    I also highly recommend you follow the advice given before and put some thought into your motor controler
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