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Re: Low logic voltage - high motor voltage H-bridge with brake

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  • firnagzen
    Mmm, I see. Incidentally,can I ask this: Doesn t the 6-transistor biomech h-bridge (Ie. Junkbots, bugbots and bots on wheels) have the same functionality? ...
    Message 1 of 7 , May 31, 2006
      Mmm, I see. Incidentally,can I ask this: Doesn't the 6-transistor
      biomech h-bridge (Ie. Junkbots, bugbots and bots on wheels) have the
      same functionality?

      --- In beam@yahoogroups.com, "Joseph Charles" <jodicalhon@...> wrote:
      >
      > firnagzen wrote:
      >
      > > One question on a related topic: I'm considering MOSFET H
      bridges
      > > for my minisumo, if I want to run the logic circuits on 5 volts
      and
      > > the motors on 7.2 volts, would I have the same problem? (Ie.
      Can't
      > > turn on the MOSFETs)
      >
      > I imagine you would have a similar problem. My problem was turning
      off
      > the PNP high-side drivers.
      >
      > > Would using a LB1630 chip as in interface work?
      >
      > Sorry. I don't know how this chip functions.
      >
      > > Oh, and can your Low logic voltage - high motor voltage H-bridge
      > > with brake be implemented with MOSFETs? (If yes, it's gate=base,
      > > drain=emitter, source=collector compatability, right?)
      >
      > I'm sure there would be similarities. You would have to use P-
      channel
      > MOSFET's as your high-side drivers, I think, but I ain't no expert!
      > I think you have your drain and source compatabilities reversed.
      >
      > Be aware of Wilf's review of my circuits.
      >
      > Cheers,
      >
      > Jo
      >
    • Joseph Charles
      Good question, firnagzen. I never thought of that bridge, even though I d been playing with it, for another reason, just a couple of days before I started
      Message 2 of 7 , Jun 1, 2006
        Good question, firnagzen. I never thought of that bridge, even though
        I'd been playing with it, for another reason, just a couple of days
        before I started thinking about the bridge I posted. How the obvious
        can pass you by.

        The biomech h-bridge doesn't have a brake, which is what I was after,
        and it will smoke if you switch both sides on.

        But, I've just been out to the batcave and set up a biomech bridge
        with NPN input transistors and 100k input resistors and, yep, it can
        be driven from 5 volts with a motor voltage of 12 volts. You really
        only have to overcome a couple of diode drops to turn it on. Duh.

        The input transistors had around 10 volts across them, and were
        passing around 14mA of base current for the drive transistors. So the
        input transistor is acting as a ~700 ohm base resistor for the drive
        transistors. Power dissipation was, therefore, about 140mW and, as
        most small signal transistors are good for around 300mW or so, that's
        alright. Actually, the base resistor could probably be raised a bit.
        The BC337/327's I'm using as drive transistors don't need that much
        base current to stay saturated with the motor I'm driving.

        But, I'm blathering...

        More investigations to come. Fire extinguisher at the ready...

        And I've still got Wilf's H-bridge challenge to contend with. Why did
        I sign up for this? (Shambles off, muttering to self....)

        Jo





        firnagzen wrote:

        > Incidentally,can I ask this: Doesn't the 6-transistor
        > biomech h-bridge (Ie. Junkbots, bugbots and bots on wheels) have the
        > same functionality?
      • Joseph Charles
        Thanks firnagzen, just the nudge I needed. Because I was after a bridge with (1) a brake and, (2) the ability to drive high voltage motors from lower voltage
        Message 3 of 7 , Jun 2, 2006
          Thanks firnagzen, just the nudge I needed.

          Because I was after a bridge with (1) a brake and, (2) the ability to
          drive high voltage motors from lower voltage control signals, I
          started with a bridge that had (1), and added (2).

          You reminded me I could have gone the other way.

          So I've stolen the brake from Wilf's Quasibridge and added it to the
          biomech bridge, and it seems to work well. Just two more transistors
          and two more resistors gives a bridge with what I needed. Has all four
          useful functions, too. And it looks like the inputs can be left to
          float without danger. No smoke, yet, anyway!

          I still have my fire extinguisher close to hand, though. I'll see how
          it goes.

          http://groups.yahoo.com/group/beam/files/Jo%20C/biomech%20bridge%20with%20brake.\
          GIF

          Cheers

          Jo



          --- In beam@yahoogroups.com, "Joseph Charles" <jodicalhon@...> wrote:

          > The biomech h-bridge doesn't have a brake, which is what I was after,
          > and it will smoke if you switch both sides on.
          >
          > But, I've just been out to the batcave and set up a biomech bridge
          > with NPN input transistors and 100k input resistors and, yep, it can
          > be driven from 5 volts with a motor voltage of 12 volts. You really
          > only have to overcome a couple of diode drops to turn it on. Duh.

          >
          > firnagzen wrote:
          >
          > > Incidentally,can I ask this: Doesn't the 6-transistor
          > > biomech h-bridge (Ie. Junkbots, bugbots and bots on wheels) have the
          > > same functionality?
          >
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