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H-Bridge

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by kal.a, Feb 6, 2016.

  1. kal.a

    kal.a Member

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    Hello Gents,

    I am putting together a circuit to run a 24VDC-3Amp motor with a 4 Amp transformer and borrowed bits and pieces of schematics I found on the web to put this one. The switches will most likely be SSRs or I may opt for Opto-Couplers but that will be determined in time.

    So will this work? And how can I do better?

    Thanks
    Kal
     
    Last edited: Feb 7, 2016
  2. gophert

    gophert Active Member

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    kal.a

    What happens when Q1 is turning on or off? It doesn't happen instantly so, when the collector of Q1 is at 12 volts for example (as it is turning on or off). What do you think is happening when the gates of M4 and M3 are at 12 volts at the same time. That can but some significant strain on those two MOSFETS. Depending on the operating conditions, there may be some durability issues.

    Also, you might want to put some free-wheel diodes across each mosfet (reversed bias, of course).
     
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  3. kal.a

    kal.a Member

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    Back to the drawing board :D
     
  4. dave

    Dave New Member

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  5. gophert

    gophert Active Member

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    You are really close, the problem is that you need a Microcontroller or a delay circuit of some type to prevent shoot-through (conduction down one leg of the H.

    You can also buy dedicated motor control chips.

    No speed control needed?
     
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  6. kal.a

    kal.a Member

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    Of course I can buy one but that wouldn't be so much fun. I wanted the circuit to only supply power to the motor and the speed will be pulsed via a PWM generated by a PLC.
    I bought one and burnt it a few weeks ago but it did not have control inputs it just had a POT to control the speed.
     
    Last edited: Feb 6, 2016
  7. kal.a

    kal.a Member

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    Is that what the zener diode in this schematic for, to pull up the gate quickly? I'm sure I asked spec that at one point, gotta go back and read that thread?
     
  8. gophert

    gophert Active Member

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    For Pwm, the best method is to bring the gate of the p-channel mosfet low, then pulse the N-channel mosfet to control speed.

    P-channel MOSFETS are generally slower than N-channel so less heat and loss is experienced when only the N-channel is switched by the Pwm signal.

    Another problem with your circuit, the P-channel will be pulled low (very low) when turned on. Actually close to 24 V. Check the datasheet. Usually 10V is preferred and 15 V or so s the maximum. Therefore, you need a little trick with a Zener diode to prevent over stressing the gate.
     
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  9. kal.a

    kal.a Member

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    I think that was it. I knew there was a very good reason for using it but couldn't remember.
    Thanks
     
  10. kal.a

    kal.a Member

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    How about this, does the 10K resistor pull the N-Channel gate any faster this way?
     
    Last edited: Feb 7, 2016
  11. spec

    spec Well-Known Member Most Helpful Member

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    This schematic has errors: see later post #13

    Hy again kal,

    Here is a circuit that you may be interested in.
    Rectifying 24V AC with a bridge rectifier will produce (24 * 1.414)-2V =32V. If you want a 24 supply line the AC voltage should be (24 +2) *0.707= 18V approx
    If clock input is made 18V or more the motor will turn clockwise
    If anticlock input is made 18V or more the motor will turn anticlockwise.
    If both inputs are either open circuit or taken down to 300mV or less the motor will be off.
    Clock and Anticlock inputs must never be enabled at the same time or the MOSFETs will pass excessive current and blow.
    A safety function could be added so the if both inputs were high the MOSFETs would not turn on and the circuit would be safe

    ETO_2016_02_07_Iss01-00_H_BRIDGE.png
     
    Last edited: Feb 7, 2016
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  12. kal.a

    kal.a Member

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    Hello spec, thanks again and as always I'm enjoying looking at your schematic and dissecting it to understand how it works.

    Here is what I understand so far:
    1- I the solution to the solution to the concern gophert's concern is to turn both + and - on and off at the same time so that there's no overlap and chance of shoot through.
    2- When "clock" input has 18V+ or more (let's say 24) Q19 will turn on pulling the right side of M to ground.
    3- And at the same time "clock"input will turn on Q21 as well and here is where I get lost. I can't figure out the voltage at the gate of Q16.
    R13 and R14 form a voltage divider and with 32V+ supply line there will be almost 13V+ at the gate. The source of the mosfet is at 32 so the gate will be 19 volts more negative than the drain.
    Is that enough to turn on the mosfet though the data sheet specifies absolute maximums of +-25V?
    If it is enough, how do I come up with that number?
    What is the function of the zener diode in this case? I tried simulating this part of the circuit and the value at R13 and R14 doesn't change with or without the zener diode. Of course I could measuring incorrectly.
    4-What happens when Q21 is off and there's no current path for R13 and R14, what is the potential difference then at the gate/source of Q16


    Cheers
    Kal
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    kal,

    You have spotted my errors. No wonder you had difficulty in trying to understand the circuit function. I will have a rethink and get back to you :banghead:
     
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  14. spec

    spec Well-Known Member Most Helpful Member

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    Give this circuit a try kal:

    (1) If clock input is made 24V the motor will turn clockwise
    (2) If anticlock input is made 24V the motor will turn anticlockwise.
    (3) If both inputs are either open circuit or taken down to 0V the motor will be off.
    (4) Clock and Anticlock inputs must never be enabled at the same time or the MOSFETs will pass excessive current and blow.
    (5) A safety function could be added so that if both inputs were high the MOSFETs would not turn on and the circuit would be safe
    (6) To be controlled by a micro, this circuit would require some mods to the input. This would be fairly simple

    ETO_2016_02_07_Iss02-00_H_BRIDGE.png
     
    Last edited: Feb 7, 2016
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  15. kal.a

    kal.a Member

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    Thanks spec,
    I'm going to play with this a bit and post my understanding of how it works. Am trying to decipher the why not just the how.

    Cheers
    Kal
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    No probs kal,
    apologies for errors- you did well to analyze the circuit and find the problems.
    There is nothing complicated about the circuit. It just turns opposite NMOSFETs and PMOSFETs on so as to reverse the polarity of the voltage across the motor.
    You don't need catching diodes to handle the inductive load of the motor, because they are built into the MOSFETs, as shown on the schematic symbols, as a function of their fabrication.

    keep up the good work
    spec
     
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  17. kal.a

    kal.a Member

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    Hi spec,

    Alright I think I understand what's going on. In the first schematic if I use IRFZ44N for P-Channel MOSFETS and NTE159 for N-Channel MOSFETS the +-20VDC (which I will be using) with a 24VAC rectified to 32DC then it's all good. (minus the zener diodes)
    The second schematic is for the MOSFETS specified in it. (Except you changed the input voltage to 18VAC/24VDC)

    So this is what how I would like to do it and I would like a hint on how to protect against both inputs being on at the same time. I will also be using optocoupler and was looking at your diode AND gate on the the other schematic and tried my best to do an XOR but it's not happening. Are there optocoupler with a built in XOR logic or something of the sort?

    Thanks
    Kal
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    Hi kal,

    Would you be prepared to use some integrated circuits in the design or would you rather stick with simple discrete components like, resistors, capacitors, diodes, and transistors?

    spec
     
  19. kal.a

    kal.a Member

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    I'm willing but not sure if I'm prepared. What's involved? Wiring, programming or both?

    I forgot to ask about rectification. I have a GBU605 which I will be using and I connected the transformer to it, the measurements were 28.18VAC out of the transformer and 25.13VDC out of the bridge rectifier. Do they put something in there other than diodes and capacitor to regulate the voltage? Would using 25VDC instead of 24 work alright or should I recalculate the resistor values?

    Kal
     
  20. kal.a

    kal.a Member

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    I just realized that I have to use some sort of a flip flop as two inputs will basically be one for pules and the other for direction. I think that will make things easier.
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    Only simple integrated circuits, thru hole, not surface mount. Yes wiring but no programming.

    It sounds like you are measuring the voltage out of the bridge rectifier without the reservoir capacitor connected. You need to connect the reservoir capacitor (4m7F in circuit), then you should get (28.18V * 1.414)-2V = 37.85V DC off load.

    It depends on the application. You can put a voltage regulator circuit after the reservoir capacitor to provide a stabilized (constant) voltage if required

    Yes, 25VDC would be fine. Absolute accuracy is not necessary with a circuit like this.
     

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