PWM Motor Controller Review

Discussion in 'The Projects Forum' started by jwilk13, Jul 22, 2011.

  1. jwilk13

    Thread Starter Member

    Jun 15, 2011
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    As some of you know, I've been working on a motor controller using a PIC, and I've got a design that seems to be working. The theory behind it seems fine, but I'm not sure about some of the practical aspects behind circuit design to make sure the longevity of my design is high.

    I posted a schematic of what I currently have constructed. The concept behind it is pretty straightforward. Essentially, the PIC monitors 4 analog signals via the internal A/D converter. These 4 analog voltages are controlled by the voltage dividers created by potentiometers R1, R2, R3 and R5. R1/2/3 are all 10 kohm pots. R5 is a center-tapped (not center-detent) potentiometer, and as configured will swing from 0-5000 ohms when moved from center to either direction (5 kohms when turned CW, 5 kohms when turned CCW).

    The PIC outputs a PWM signal (3.3 Vpeak) at RC2 at 250 Hz, which is then switched by the BSP762T at 24 VDC (basically taking the place of a PMOS high side switch). The two other IC's are voltage regulators providing fixed voltage references (3.3V and 5V) for the PIC and for the voltage dividers.

    My questions are as follows:

    1) What are some practical design considerations I should be taking into account? To be honest, I feel like my design is too simple to work for an extended period of time.

    2) Should I use a zener diode to make sure Vdd at the PIC does not exceed 3.3V?

    3) Any other input (this won't work, this will work, add this, replace this, etc.) to help me out.

    I'm confident in the theory of what I have since it all works as is. I have tested it and I can say with confidence that it works, I just don't know how long that will last. What I'm looking for is to pair my theoretical knowledge with the practical knowledge of people here to create the best possible project.

    Thanks everyone!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, you're not saying what you're powering with the high-side driver. Its' specs say it's capable of supplying up to 2A. It's best to operate semiconductors at well under their maximum specifications, but we don't know how much current your load might draw.

    You're using linear regulators to get 5v and 3.3v. While that's an easy way to do it, the efficiency when the input is so much higher than the output, is really poor.

    Your 5v regulator will be under 20.8% efficient, and your 3.3v regulator will be less than 14% efficient. If your project will be battery powered, that should concern you. At any rate, anything powered by 5v will automatically have over 4 times it's power wasted in the regulator alone, and 3.3v powered items will have over 7x it's power wasted in that regulator.

    You would be better off efficiency-wise use a switching regulator at least for the 5v supply, and then use a low-dropout 3.3v regulator from the 5v output. That could boost your efficiency by a large margin - but then again, we don't know what the loads are on those two regulators.
     
  3. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    Thanks SgtWookie, I always welcome your input. I'll check into a switching regulator for the 5V supply. The 3.3V regulator I am using is a low dropout regulator, and looking at the datasheet, it seems like I should still be able to use this regulator off of the 5V output.

    The load I am driving is <1A, so I figured a nominal continuous current rating of 2A should be sufficient.

    Anything else I'm missing?
     
  4. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    Another question:

    The BSP762T has a turn-on threshold close to the output of my microcontroller output (uC outputs 3.3V signal). Is there a good way to shift the 3.3V output PWM to 5V so I can be sure the high side switch turns on when it's supposed to?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This is not correct.
    That IC has a max turn-on voltage of 2.2v, and a minimum turn-off of 0.8v.

    Your 3.3v uC should be able to supply that without problems.
     
  6. jwilk13

    Thread Starter Member

    Jun 15, 2011
    228
    12
    You're right, it should (and it does). I'm thinking if I eventually want to supply a much larger current than what I am now (I may want to increase it from the 1A now to 7-10A later), I can't find any high side switches like the one I'm currently using that have such a low turn-on threshold. Most of them are 3v or more, and that's cutting it a bit close.
     
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