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34185Re: [beam] H_Bridge

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  • Dwayne Reid
    Mar 1, 2003
      At 10:38 AM 3/1/03 -0600, Duane C. Johnson wrote:

      >With frustration with the L298 I looked for a better
      >H-bridge. I came up with this circuit:
      >http://www.redrok.com/actuator.htm#act01
      >OK, it has some warts, but works well if high switching
      >speeds are not needed.

      This discussion resembles one that took place recently on the Open Source
      Motor Controller (osmc) group recently. The following is a copy of a post
      I made there:

      At 09:23 AM 2/24/03 +1100, Brett Paulin wrote:
      >
      >I'm planning on having just a single "leg" of PWM'ed Fets on the ground
      >side of a relay based H-Bridge made with Hi Current SPDT relays. This
      >removes the need for special hi-side gate drivers, Anti-shoot through
      >logic, and cuts the number of Fets to 1/4 of normal, while still allowing
      >nice smooth PWM control.

      We did a design similar to that many years ago (controlled gear motor
      linear actuators) but had a lot of problems with relays failing (welding)
      while braking the motor.

      The design that worked used 2 relays and 2 FETs - the FETs switched the
      ground side, the relays switched the hot side (24V). A little tiny PLD
      (EP320) controlled the whole works.

      We basically had 3 modes with our version: stopped (with active braking),
      forward, reverse.

      Forward and Reverse worked similarly: turn off both FETs (turn off
      braking), close 1 relay, wait, verify relay closed (loop back to wait if
      necessary), turn on 1 FET.

      Stopping was the same for both modes: turn off FET, wait short time, open
      relay, wait, verify relay opened (loop back to wait if necessary), turn
      both FETs on for dynamic braking.

      The relays were P&B T90 series (I'd use American Zettler AZ-2150 form-A
      relays now). The FETs were IRCZ44 with a simple PWM over-current
      detector/limiter.

      There was a simple pull-up from each motor lead to 1/2 supply voltage that
      was used to test for shorted relays or FETs.

      We built several thousands of these for one client (variable pitch fans
      used in Caterpillar earth moving equipment) with great success.

      <end of original post>

      I'd consider something similar for this application, except using Aromat
      JS1E relays. These relays are relatively small and very robust for their
      size: up to 10A rating.

      The problem with SCRs in the top leg is still voltage drop - a typical
      forward voltage for an SCR is much more than a saturated bipolar transistor
      or MOSFET.

      The ultimate in tiny motor drivers still might be complimentary smt MOSFETs
      driven hard and fast by HC/HCT/AC/ACT invertors. I'll have to give it a
      go, I guess.

      dwayne

      --
      Dwayne Reid <dwayner@...>
      Trinity Electronics Systems Ltd Edmonton, AB, CANADA
      (780) 489-3199 voice (780) 487-6397 fax

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