Help; micro controlled motor (PWM)

Discussion in 'The Projects Forum' started by ShanAruba, Mar 7, 2010.

  1. ShanAruba

    Thread Starter New Member

    Mar 4, 2010
    4
    0
    Yes,

    I am currently working on a project of which a motor has to be controlled (0 to 50) Hz by a PIC18F.

    I'm planning on using IRF840 N-channel mosfet, controlled by an IR21094 gate driver.

    I'm currently testing these components with the model given with the datasheet of the gate drivers.

    Problem.

    I am currently having problem with getting the mosfets to work, after hours of searching on the internet and this forum i concluded that it must be the Bootstrap circuit, i'v tried different capacity (around 4.7 uF) but nothing works.

    Can anyone give a help or guide me in the right way? many thanks in advance
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Would you be able to post your schematic so that our members can see what your connection to the MOSFET looks like?


    hgmjr
     
  3. ShanAruba

    Thread Starter New Member

    Mar 4, 2010
    4
    0
    Here you go,


    [​IMG]

    I am planning on using this schematic, then upgrading to a full bridge...
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the voltage rating of your capacitors?

    What is your Vdd?

    You say nothing works, but you're just showing the schematic from the datasheet, which is quite generic.

    Unless you can show us exactly how YOU have it wired, and insert all values of what you are using, about all we can do is shrug our shoulders and say, "Well, you have a problem now, doncha?" which isn't very helpful.

    Post how you have the thing wired to the uC, show us some code, tell us what signals you've been applying where, what voltages you're using, what you are seeing on the inputs and outputs currently.
     
  5. ShanAruba

    Thread Starter New Member

    Mar 4, 2010
    4
    0
    Yes i'm sorry for the late reply, I've checked the circuit and noticed that i had it wrong. After some tweaking i've managed run a Half bridge @3200HZ 30% duty cycle without any problem with a resistive load of 1K . The bootstrap capacitor was 4.7 uF. Vcc is equal to 15 V.

    The next plan is to expand it to an full bridge, but sadly enough i don't have any more gate driver at hand. So i'll have to wait.

    i'm planning to run an AC motor (220V) 1A max with this circuit,

    The specs of the IRF840 looks good for the application.

    If you guys have any idea on things that i need to worry about, when running the motor, please let me know.

    Thank for the replies,

    Shan
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You cannot control an AC motor by PWM using MOSFETs directly.

    Did you notice that the IRF840 has a body diode? What do you think will happen when you try to connect AC across such a device?

    If it is a universal-type (brushed) motor, you might use TRIACs to control the speed. If it is not a universal-type motor, you can't control it's speed - that is fixed by the input AC frequency, and if you vary the input frequency out of it's designed range, you will burn it up.
     
  7. ShanAruba

    Thread Starter New Member

    Mar 4, 2010
    4
    0
    Forgot to mention the H-bridge(mosfet) will be connected on a DC bus 220V. i know that i cannot get a sinus (AC) out of the h-bridge, but as long as i can get a current that can run both ways through the motor i'll be happy.

    In other words a bidirectional DC motor controller, but trying to control h-bridge in a way that i can hopefully run AC motor.

    Shan
     
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