H-bridge motor driver
Four transistors around a motor form an “H”. Turning on one diagonal pair drives current through the motor one way; the other pair reverses it. Here each side is a complementary PNP-over-NPN half-bridge whose bases are tied together, so one logic input sets that side high or low.
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How it works
| IN1 | IN2 | bridge | motor |
|---|---|---|---|
| 0 | 0 | both sides low | brake (motor shorted) |
| 0 | 1 | left low, right high | reverse |
| 1 | 0 | left high, right low | forward |
| 1 | 1 | both sides high | brake |
On each side the inverter drives both bases. A low base turns the PNP on and the NPN off, pulling that motor terminal to 5 V. A high base does the opposite. The bases are tied, so the two transistors on one side can never both be on, which avoids “shoot-through” (except briefly while switching).
Motor voltage ≈ ±(5 − 2 VCE(sat)) ≈ ±4.76 V, current ≈ 781.6 mA.
Try it on the bench
- Flip IN1 and IN2 on the DIP module: the motor runs forward, reverses, or brakes.
- PWM on one input and a steady level on the other gives speed and direction control: that is what an L298N or DRV8833 does.
- Change R1 to 4.7 kΩ: Q1 no longer saturates, the motor voltage drops and Q1 heats up.
Parts on the bench
- Bench power supply
- DIP switch module
- 74xx04
- 4 × Resistor
- 2 × PNP transistor
- 2 × NPN transistor
- 2 × Digital multimeter
- DC motor
In the course
Zen’s Electronics Design Mastery, module “Transistors”.