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building an H-bridge

Discussion in 'Electronic Basics' started by [email protected], Jun 26, 2007.

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  1. Guest

  2. BobG

    BobG Guest

    An H-bridge usually is used to get fwd and reverse direction with pwm
    speed control on a brushed dc motor. A triac is usually used to switch
    on the AC line to a load like a lamp part way through the half cycle.
    Perhaps you have a new idea to combine these two normally entirely
    different applications in a unique and unobvious way? Tell us more.
  3. I can imagine using four SCRs in an H-bridge fed from a full-wave
    rectifier to drive a permanent-magnet motor in either direction. You
    could also use two triacs and a center-tapped transformer.
  4. Guest

    Probably not that unique and unobvious... just wondering if 4
    identical triacs can be used instead of 2 npn and 2 pnp transistors.


  5. Gareth

    Gareth Guest

    AC or DC?

    Once switched on, triacs stay on until the current falls below the
    holding current, so in a DC application they would not switch off unless
    the current is interrupted.

    Triacs are normally used in AC applications where the current drops to
    zero twice in every cycle.

    Maybe you could use one of these?

  6. John Fields

    John Fields Guest

    With a DC source, it would be difficult to turn the TRIACs off since
    once they've been triggered ON that's how they want to stay until
    the current through them goes close to zero.

    With an AC source it would be possible to use the TRIACS to
    full-wave rectify the mains and then phase control the input to the
    motor, but it wouldn't really be PWM since the sine wave "tails"
    would persist after the TRIAC was triggered until the mains went
    through zero volts. Then there's the snubber which would (more than
    likely) have to be used to make sure the current waveform through
    the TRIACS would go through zero and allow the TRIAC to turn off.
  7. Guest

    Ok, thanks. Guess that's the deal killer right away, then. I wasn't
    aware of this fundamental feature of triacs.

    Just to clarify: when you say "once they've been triggered ON that's
    how they want to stay until
    the current through them goes close to zero", you don't mean the
    current through the gate, do you? (You mean the current through A2
    and A1, right?)

  8. Jamie

    Jamie Guest

    GTO's ?
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