I've been simulating some stuff on Proteus 7 and have been struggling to get something to work that, as far as I'm concerned, works on paper - a simple H-bridge. The H-bridge itself isn't really the point of interest for me, but it's part of the circuit and it simply doesn't work as I expected it too and for my own life I can't see why not.
This is a screen shot from Proteus, mid-simulation. Now, if I'm going crazy and I'm missing something obvious then great - shout it at me, but why on earth are there the huge voltage drops across Q1 & Q4!? They should be switched on hard should they not?
Interestingly I tried swapping out the MOSFETs for BJTs and found the same thing. (note the change in how R7 & R8 are connected and their value)
So in this case, there is a huge voltage drop across Q2 (which is the top right transistor, label's hiding behind R2 unfortunately), but more current is flowing through the motor. If I turn off Q2 & Q3, and turn on Q1 & Q4 however, then the circuit behaves as I expected it too originally with the MOSFETs (note there is no R3 in this case).
So what exactly am I missing here? Am I wrong to think that the H-bridge with MOSFETs will work as expected? I've attached the .dsn files to this post so that anyone else with Proteus can experiment themselves.