2 opamp H-bridge

Discussion in 'General Electronics Chat' started by Jacopo D, Jan 16, 2016.

  1. Jacopo D

    Thread Starter New Member

    Jan 14, 2016
    3
    1
    Hello,
    i'm trying to design a H-bridge circuit using 2 operational amplifiers. I'd like to drive a DC motor, using a 0V/5V input (at 2.5V the motor should stop). This is my schematic:
    schemaTensione.jpg
    And this is the simulation result:
    graficoTensione.jpg
    The two probes measure the voltage on the two motor contacts. The DC motor has a 12.3 Ohm winding resistance. I'm finding some difficulties reading the results: will the motor run in both directions? I know, this is not a classic H-bridge scheme: i'd like to use a single supply (+28V), that's why the circuit is a little messy.
    Thanks in advance for your help
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Most opamps won't supply 28 volts to a 7 ohm load. For this, you would need a power opamp.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    @Brownout ,

    The mentioned opamp is a poweropamp:

    Bertus
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    Thanks. I can't read the fine print.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Actually, it is a semi-classic scheme. If the motor has brushes, it probably will run in both directions. However, depending on its internal design, it might not be happy going "backwards". If it is a brushless type (common in small cooling fans), it will not run with the standard voltage input reversed, and the reversal probably wil damage the internal electronics. What is the motor make/part number, and what is the application?

    ak
     
  6. PeterCoxSmith

    Member

    Feb 23, 2015
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    This is not an H Bridge. If you apply 0V with the left hand opamp and 28V with the right hand opamp a DC brushed motor will spin, lets say clockwise. If you apply 28V left and 0V right the motor will spin anti-clockwise. If you apply 14V left and 14V right then the motor will not spin. And at voltages in between you'll apply Vright-Vleft to the motor, either positive or negative and you can vary the speed. However the opamp will be operating in linear mode rather than the switching mode as you'd expect from a H Bridge and the dissipation will be higher.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    True. If you look at just the two opamp output stages and how they connect to the load, it has the same general structure as an H bridge, and the same intent. However, the output transistors are not intentionally saturating, the most common operation of a standard H-bridge.

    From the audio world, this kind of non-saturating output structure is called BTL - Bridge-Tied Load. Although, of course, that structure has its own exception, a BTL class-D output.

    ak
     
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  8. Jacopo D

    Thread Starter New Member

    Jan 14, 2016
    3
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    Thank you for all your answers. This is the motor:
    [​IMG]
    My motor is slightly different, the nominal voltage is 50 V, but it works very well at 1.5 V too. (!!)

    Yes, i want to vary the motor speed, in both directions. And i'd like to know if the following is correct:
    From the simulation result, if the voltage input is 0V, the first opamp gives 5V, and the second opamp 23V, so the motor voltage should be 5-23=-18V.
    If the voltage input is 5V, the first opamp gives 23V, and the second opamp 5V, so the motor voltage should be 23-5=18V (and the direction is the opposite).
    If the input is 2.5V, the motor should stop.
    (All the intermediate cases follow the same rules).
     
  9. PeterCoxSmith

    Member

    Feb 23, 2015
    148
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    It may work, I've never driven a motor with power opamps; good luck with your adventure. Watch out for thermal issues, inductive current flow and what happens when the motor acts as a generator/brake.
     
  10. Jacopo D

    Thread Starter New Member

    Jan 14, 2016
    3
    1
    Tomorrow i'll try the circuit, i've just soldered all components. Stay tuned!
     
    PeterCoxSmith likes this.
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It should work just fine. I've used LM386 in the same way. Both for audio and motors and a solenoid.
     
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