Transistor Help

Discussion in 'General Electronics Chat' started by bsfrye, May 17, 2009.

  1. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    I am pretty new at all of this. Still don't have my head around it all. I want to use a Transistor to turn a DC motor on and off. Really simple... Nothing special. Motor can draw a decent amount of current (say about 10 amps when stalled, 1.2 amps while running). I will control this with a Micro controller (Arduino). So my question is simple - can I use this Transistor to do it? (MJ300, http://www.farnell.com/datasheets/89477.pdf)

    Can a micro controller turn this transistor on directly? (40 ma max per IO pin) Or do I need another transistor in there somewhere? Also - if I controlled the Base with a 12 volt source - will I blow it up? (I suspect yes) Thanks,
    Brandon
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Don't forget a diode across the motor to protect the transistor.

    I give this schematic only by way of example, other than the diode it doesn't apply...

    [​IMG]

    There are types of transistors that have much higher gain, which is what you need. You can buy Darlington Pairs that are prepackaged.
    Think TIP100/TIP101/TIP102.
     
    Last edited: May 17, 2009
  3. mik3

    Senior Member

    Feb 4, 2008
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    A MOSFET will dissipate less power when it is fully turned on because it drops less voltage (Vds) than a BJT (Vce).
     
  4. bsfrye

    Thread Starter Member

    Feb 20, 2009
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  5. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    I have a bunch of these laying around - FQP17P10

    Can someone help with a really simple circuit design to help my power a motor? I have 5 volt 40 ma output directly from my micro controller - I also have 12 volt 1 amp output from another section of the board (controlled also by my micro controller). I think with this particular MOSFET I would need to use the 12 volt output...?
    Thanks for all the help,
    Brandon
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    Yes, it will work.
    Note, that you need to connect a 125 ohm resistor between the output of the uC and the MOS gate. Also, connect a 10K resistor between the gate and the source of the MOS.

    The motor has to be connected between the drain and 12V and with a diode in parallel with it as to absorb back EMF. The diodes cathode has to be connected to 12V.

    Note that, if you want to decrease the switching time you can use a BJT or a MOSFET driver IC to drive the MOSFET's gate.

    Another suggestion, to protect the uC in case the protection diode fails, is to use an opto-coupler to drive the MOS.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    Have you had any experience programming the Ardiuno board to generate a PWM output signal?

    hgmjr
     
  8. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    I have been able to use PWM on the arduino powering a LMD18200T but I get almost NO torque out of the motors. I have NO idea why - so. Also the LMD18200T can only drive 3 amps. Which may be enough... but the real problem for me is that even with a high PWM freaquency (~32kHz) I can't get enough torque from the motor. I *THINK* it is because when the PWM signal is LOW the LMD18200T is 'breaking' the motor. If I could get this to work I would be in good shape too.
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    Post your circuit to make sure it is correct.
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Once we have the schematic we can probably help you out.

    hgmjr
     
  11. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    I will put something together to post. I just got a MOSFET I had laying around and tried this. It seems to work (thanks mik3 for the MOSFET suggestion). My intention at this point is to pulse the signal to the MOSFET to get about 9 volts from my 12 volt supply. Hopefully I don't run in to the same problem I had with the LMD 18200T.
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    The LMD 18200T is an H bridge MOSFET driver, it cannot drive motors directly. Its job is two drive the MOSFETs which control the motor.
     
  13. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    0
    Sorry for this terribly simple schematic - not very good with Eagle yet. [​IMG]

    I have 3-4 EN at +5 from one of the IO pins on the Arduino. Input 4A I plan to PWM with 75% duty cylce. That SHOULD give me 9 volts to the output 4Y. I *HOPE* that this will give me enough power on my motor. The FQP50N06L says it already has a protection diode in it. The big question for me is - Can I PWM the power to the FQP50N06L? I assume yes?
     
  14. bsfrye

    Thread Starter Member

    Feb 20, 2009
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  15. mik3

    Senior Member

    Feb 4, 2008
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  16. bsfrye

    Thread Starter Member

    Feb 20, 2009
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    I assume though that I CAN PWM the input to the MOS?
     
  17. mik3

    Senior Member

    Feb 4, 2008
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    If you build the circuit correctly you can PWM the MOS and control the motor.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    The diode protects the transistor, any transistor. If the motor runs without it there is a chance the transistor will come away damaged.
     
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