Fig. 1 Basic transistor H-bridge circuit.

NPN Only Transistor H-Bridge Circuits

by Lewis Loflin

Here we look at construction of all NPN transistor h-bridge circuits. This is being done to learn further transistor theory and to deal with the problem of hard to find and often expensive PNP power transistors.

Fig. 1 shows an example h-bridge circuit using bipolar transistors. Note that if one is using transistors without internal diodes for absorbing counter-electromotive force they will have to be supplied externally.

My examples that follow will use NPN Darlington transistors such as the TIP120 with built in diodes and we work on the assumption the circuit will be controlled by a standard 5-volt micro-controller such as Arduino or PIC.

If one really requires a very large drive current say over 4 amps, then consider using 2N3055s with say a TIP31 in a Darlington pre-driver configuration. For background on these issues see:

NPN transistor switching circuit.
Fig. 2

Here in fig. 2 we have replaced the PNP transistors Q1 and Q2 in Fig. 1 (one of these circuits for each transistor) with a power NPN transistor Q3 and two driver transistors Q1 and Q2. Q1 acts as a switch to supply base current (Ib) to Q3 switching it on. Q2 is used to switch on Q1 and to protect the 5-volt Arduino I/O from the high voltage on the base of Q1.

4N25 opto-coupler turning on a NPN transistor.
Fig. 3

In Fig. 3 we have replaced Q1 and Q2 with a single 4N25 opto-coupler whose photo transistor Q2 switches on to supply base current for Q3. The 4N25 provide voltage isolation between the micro-controller and 12-volt motor power supply.

See Opto-Isolated Transistor Drivers for Micro-Controllers

NPN transistor h-bridge with partial opto-coupler input.
Fig. 4

Here we have redrawn the circuit using all the same NPN power transistors with opto-couplers for Q1 and Q2. The circuit in Fig. 2 works just as well. All 4 inputs are a direct 5-volt TTL connection.

NPN transistor h-bridge with partial opto-coupler input.
Fig. 5

In Fig. 5 we switch on D1 which switches on Q1 and we also switch on Q4 creating a current path for the motor to run clock-wise.

NPN transistor h-bridge with partial opto-coupler input.
Fig. 6

In Fig. 6 we switch on D2, Q2, and Q3 creating an opposite current path for the motor now running counter-clockwise.

This completes our brief introduction to NPN transistor h-bridge circuits.