Help required understanding hbridge

Discussion in 'General Electronics Chat' started by Spence, May 25, 2010.

  1. Spence

    Thread Starter Member

    Apr 23, 2010
    The PNP conducts without base voltage, so how can a hbridge work? The PNP's conduct with 0v and with 5v, but one of the pair needs to switch off while the other switches on.

    Attached hopefully are images of circuits I am trying to simulate in ltspice.
  2. Bosparra


    Feb 17, 2010
    Perhaps you could rephrase your question, what do mean by "The PNP conducts without base voltage"?

    The 1's and 0's in the truth table represents a voltage. Looking at the first line in the table, A and D is on. This means that current flows from A, through the motor then through D. This cause the motor to turn in the forward direction.

    In order to reverse the direction, B and C is switched on.
  3. Markd77

    Senior Member

    Sep 7, 2009
    The PNPs will conduct with 0 and 5V input. You would need to give them around 12V (at least 11.3) to stop them conducting. That is how they work.
  4. Spence

    Thread Starter Member

    Apr 23, 2010
    O.K. perhaps you are right.

    If the zero in the truth table represents 0v then the pnp (B) is conducting

    B is also on with 0v at the base.

    To switch B off I need to apply a base voltage equal or higher than rail.

    If the zero in the truth table represents another voltage then OK, I follow. I can get the bridge working with another driver transistor controlling the PNP's and the simulation has very little quiescent current, it's just that I didn't want to overcomplicate things.
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    I think the zero in the table means "not conducting." That is "off." The one (1) means conducting (i.e., on). That is, it represents a status of the corresponding transistor, not any particular voltage on the base.