I'm getting ready to do some playing with some DC motors and some stepper motors, and I happen to have tons of NPN transistors and Darlingtons, but fewer PNP transistors, and no PNP darlingtons. I saw online a mention that an H-bridge can be made with only NPN transistors, but could never find a schematic for one.

I'm totally aware there are better final solutions, but I always begin my learning with breadboard projects that help me learn the tech and try different things. (After goofing around learning, yes, I'm smart enough to use tried and true designs for final projects)

For whatever reason, the idea of building an all NPN H-bridge has been intriguing me, and I'd like to see if I could hack one out in the next few weeks. Anyone have any ideas on how it could be done?

 

The best way is probably to use a small PNP transistor in a compound connection with the power NPN to make the upper two transistor pairs of the bridge act as a PNP (sometimes called a a Sziklai pair or a complementary Darlington). You do this by connecting the emitter of the PNP to the collector of the NPN and the collector of the PNP to the base of the NPN.

The transistor pair then acts similar to a power PNP with the NPN collector connected to the plus supply voltage, the NPN emitter going to the bridge output, and the PNP base being the input control (connect to ground through a resistor to turn the pair on).

The main disadvantage is that the saturation voltage of the pair is higher than a single PNP, being equal to the NPN base-emitter voltage plus the PNP saturation voltage.