# Help required understanding hbridge

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

1. ### Spence Thread Starter Member

Apr 23, 2010
49
3
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.

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2. ### Bosparra Member

Feb 17, 2010
79
3
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
2,803
595
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
49
3
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
5,699
912
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.

John