without an H bridge you can only control the speed of the motor in one direction
Actually, on the Milton Bradley Big Trak, they used a weird "half h-bridge" design with a dual ended power supply; only two transistors used per motor.
Regarding h-bridges: on another thread, someone mentioned that using only NPN transistors for such a bridge could be a bad design; it wasn't said, but I got the gist that it was better to use NPN on one half, and PNP on the other. I have designed an h-bridge using 2n3055 NPNs (TO-3 cases), but after hearing that information, I looked into other designs - I found this:
http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html
If I used the 2n3055 complement (MJ2955) for the other half, is this really a better design? It seems safer (ie, no possibility for SEDs/FEDs), and would allow me to implement coasting and braking (something I can't do with only NPNs?).
Am I looking at this wrong? Is there a way to use only NPNs and still get the safety from shorts? I am already using opto-couplers, and only drive one half at a time - but if both halves are high, fireworks (I imagine!) could appear...
Is an NPN only h-bridge safe, or -must- you use complementary pairs? If I have to re-design, no big deal - I am just looking for some feedback (PM me if you want).
