Motor start

Discussion in 'General Electronics Chat' started by xnihilo, Dec 1, 2008.

  1. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    Hi,

    I need to start a DC motor from 9V battery (PIC MCU controled, using 5V relay and BJT).
    It has to start as fast as possible and if possible revolve only one time or in the worst case, spin very shortly (<100ms). There is a little weight attached to the axis so it behaves as a rumble motor which adds some extra inertia.
    From what I know, a DC motor that starts requires a lot of current in the first moment so I was wondering if using a large capacitor (25V 100UF?) near the motor could help to supply current for the short period of time when motor is starting.

    Would you have same suggestions?

    Thanks :)
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes, it will help. If possible mount the capacitor directly on the motor's terminals. This will reduce unwanted noise voltages to be induced in the whole circuit due to the inductance of the wires.
     
  3. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
    33
    0
    Thanks.
    So I will follow your kind advice.
    I also read that one need to add a resistor for when the moment where the motor starts as it will need a lot of current and it may affect other components of the circuit, is that right?

    In fact I need to add a recoil effect to a lasergame gun equipment as it is the project subject.

    I thought of different systems: gas valve, solenoid,... but none seems to meet my needs... I'm still stuck, so I was thinking of a rumble motor but then I'm facing the problem of the starting of the motor.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Well, if you need a high starting torque then you done have to use the resistor. If not use it to have a lower starting current but after the motor speeds up short circuit it. If you want you can use a separate power supply for the motor as not to affect other components.
     
  5. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
    33
    0
    Hello

    Well, if you need a high starting torque then you done have to use the resistor.

    > Torque means force or speed at which it will start to spin?
    The resistor has to be used only when the motor is tarting, right?
    Then I wonder how I will implement that... maybe using transistors to route current to either a wire or a resistor.

    If not use it to have a lower starting current but after the motor speeds up short circuit it.

    > You mean use another power supply or one with a current limiter then supply power at normal level?

    If you want you can use a separate power supply for the motor as not to affect other components.

    > Actually I will be using a 9V battery with 7805 for the main circuit and another 9V battery for the motor. I Noticed that when motor starts my MCU reboots (brownout?) and the LCD goes dimmer... The motor sucks quite a lot of current when it starts...
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    Because you use a relay to drive the motor and the motor takes power from a different battery it would not cause problems to the PIC. Maybe the relay is the one which causes problems. If you want to use a resistor to start the motor, you can use another relay to short it after the motor speeds up. Post a schematic to see what's going on.
     
  7. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    0
  8. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
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    I wrote "nichrome" instead of motor but the principle is the same. I have a lasergame project called TANGO. I need to simulate gun recoil.
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    You supply the relay coil from the gun. Disconnect it and supply the relay coil from the external 9V connector.
    Also, you supply the relay coil, which is rated 5V, with a higher voltage and this causes a larger current to flow in the circuit. You have to use a series resistor with the relay to limit the current.
    Also, add a diode across the motor like the one across the relay to absorb the back EMF created by the motor when it is switched off.
     
  10. xnihilo

    Thread Starter Active Member

    Nov 20, 2008
    33
    0
    I will check the schematics. the coil is supposed to be controled by the pic (5v). The pic switches a bc182l bjt so the 5v supply coming from the gun pcb can flow or not to gnd through the bjt and activate the 5v relay.
     
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