H-bridge and PWM

Discussion in 'General Electronics Chat' started by Dritech, Jul 13, 2013.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi,

    In a project that I will be doing, I will need to control the direction and speed of a linear actuator which is rated at 12V, 30A stall torque.
    For this reason I need to design an H-bridge and a PWM that can handle that amount of current. These will be controlled through a PIC microcontroller.

    Now for the H-bridge I was planning to use relays. Is that a good idea?
    For the PWM, the pulses will be given from the PIC, which then will switch a high current transistor.

    A schematic is attached below.

    Different options and other suggestions will be highly appreciated. My aim is to make this project efficient and reliable.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Using relays is a terrible idea.

    1) Relays are too slow to handle PWM efficiently.

    2) Your relays will be very noisy.

    3) Your relays will have short lifespan.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Are there transistors which can handle such current without problems?? In another project I did use an H-bridge using transistors to drive a 3A motor, but it gave me hell lot of trouble. So I can assume what will happen to drive a 30A max actuator with a transistor based H-bridge.
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
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    You could use relays for an "always on" h-bridge which does nothing more than establish current direction through the motor, then a single mosfet to control the PWM. There are transistors for just about every power level there is. You just have to look at bigger devices. You probably think of TO-220 package when you think of transistor, there are many, many mor packages.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    There are many MOSFET transistors that allow 10s of amps to flow. additionally, they can be designed in parallel to handle even more current. Use appropriate heat sinks and pick a reasonable PWM frequency.

    Since you are using a PIC, you can easily add a current sensing feature to shutdown if you reach your stall torque.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Thanks for the replies.

    OK lets assume I use MOSFETs transistors for the H-bridge, can I eliminate the upper transistor (referring to the diagram in post #1) and instead connect the PWM directly to the H-bridge transistors? (something similar to the attached diagram)
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    @tubeguy, can I use N-channel MOSFETs only?
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you use N-Channel Mosfets only you will need a high-side driver or an additional higher voltage supply to drive the gates of the upper Mosfets.

    EDIT: This is because standard Mosfets gates require around 10 volts gate-source voltage to fully turn-on.
    Logic-level devices require less voltage depending on the device. It's critical when controlling large currents to fully turn on Mosfets to reduce device dissipation to the lowest level possible.

    With the availability of inexpensive higher current P-channel Mosfets you can use those for the top devices and simplify the circuit.
     
    Last edited: Jul 13, 2013
  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    I didn't see a problem with your original idea. It is unconventional, but I'd be interested in how it works out. I've thought of doing something similar.
     
    GopherT likes this.
  11. wayneh

    Expert

    Sep 9, 2010
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    You may have missed the OP's point. If I understand it, the relays would switch only when the direction is changed, which we assume is not so often. The PWM signal would still be under control of a single large transistor (or bank of them).

    This may still be a terrible idea. I think the relays would have to be mechanically linked to prevent any chance of timing problems (shoot-through). I guess you could make sure the PWM is off for an envelope of time around the switching of the relays. That would protect against any timing mismatches between the relays.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    Shoot through is easily addressed by either employing "break before make" techniques (assuming you have a couple milli-seconds to change direction to allow the relays to settle) or by limiting current to a safe level for a short duration.
     
  13. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Following the relay/single transistor (or mosfet) idea, you could use a single DPDT relay for direction control.
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    That's what I envisioned when I read it, bit then I read Mr chips post and realized there could be different ways to interpret the post, so I explained the method you just described in my reply.

    Now that I think about it, this method could be employed much more efficiently with a single 4PDT relay than 4 separate relays.
     
  15. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi again,

    Thanks for the replies and suggestions:)

    @Wayneh, yes that's what I meant.

    The idea of using a DPDT reply is actually good. This way I eliminate 3 relays; less problems, less expensive and less noise :)

    I did a diagram using DPDT relay. Is it ok if I wire it this way? or it is still better to use four MOSFET transistors?
     
  16. tubeguy

    Well-Known Member

    Nov 3, 2012
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    As mentioned, the relay method is OK if you don't intend on switching directions often.

    Regarding the circuit in post 15, I would suggest using an N Mosfet device between the motor relay contact's negative (blue) end and ground in a sinking configuration, instead of the sourcing position. Drive the relay coil the same way using an NPN transistor. And add reverse protection diodes across the relay coil and the motor relay supply connections.

    A high current N Mosfet is easier to drive than a high current power transistor.
     
    Last edited: Jul 13, 2013
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