Arduino Motor Control

Discussion in 'The Projects Forum' started by cl10Greg, Sep 19, 2012.

  1. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
    49
    0
    Hello Everyone,

    I have posted on this project previously but I am done to the last steps and figured a new post would be better.

    I am working on using a TTL MOSFET (PSMN3R4) to drive a motor with a LSD configuration (picture attached). I am building my own riding scooter that is a 24V, 500W motor that is rated for 27A. The example I am showing has two in parallel but it may not have to. So I did this original setup and put an LED for a load and nothing happened. I shorted the FET and the LED lit up so I know the LED it setup correctly and the FET is not turning on. I checked the FETs with a multimeter and they are operational (I checked the FET with diode settings and resistance). So with that being said I am stuck and want to throw some ideas out there.

    My control is a 500Hz PWM from an arduino. If i do a duty cycle of 1% that is a on time of only 2 microSeconds. Now the arduino can only source 40mA. So with that amperage and the resistance of the gate and current limiting resistor I will only source about 20mA. So my initial thought is that it may not be enough current to charge (or time) and discharge the FET all the way. So that leads me to two options.

    1.) PNP to drive the gate. I can implement a 2n3906 PNP transistor to supply 200mA to the gate. There will only be a .4 voltage drop so the gate will see 4.6V and should be fully on for switching. With this setup I also can leave the gate with no resistor on it (or at most a resistor to limit the draw to 200mA to protect the transistor). I will assume to add a 25 ohm resistor to limit the pnp. This means that the gate time constant is 25*3907pF = 101ns. so with that number I can compare that to the necessary gate charge which is 31nC. If I wanted to wait 3 time constants (303ns) to charge the gate fully I would only need 101mA (31nC = A*s = 31nC/304nS) which I can easily supply. The discharge time would be a bit different though since I am using a bigger resistor to discharge. So I guess the whole point of asking this is does this seem plausible or do I need more power to switch at 500Hz? Has anyone done this with a TTL MOSFET to drive a motor?

    2.) H-bridge to drive the gate. I have an SN754410 that I could use to drive the gate pretty easily. This would provide lots of regulated current to charge the gate and be easy to control with TTL logic. This would break the previous calculation times since it can easily provide up to 1A.

    This is the last issue I have. Once I am able to control the low side the motor can be tested with the software and I am home free. I am going to try this PNP design tonight to see if I can control what I am talking about but I would prefer some other eyes on it as well


    Any other suggestions on this circuit in general or has someone created something similar? I have the full sloppy schematic attached as well.
     
  2. cl10Greg

    Thread Starter Active Member

    Jan 28, 2010
    49
    0
    So doing the math to drive it directly from the arduino at 40mA yields a time constant of 775ns. So I guess I don't understand right now why I can't drive what I have in the circuit. Maybe since I am running them in parallel and doubling the capacitance it is not having enough time to charge and discharge. Hmm interesting though.
     
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