H-bridge for servo motor control (beginning design)

• Why an H-bridge? Because the DC motor needs voltage to be applied across a load in multiple directions to change motor rotation (to move forward and backward).

• What does an H-bridge consist of? H-bridges can be built from both discrete or bought as an integrated circuit (from companies such as Texas Instruments or Analog Devices).

•  What does the schematic look like?

BOM consists of: 

4 bipolar or FET transistors

4 schottky diodes (to keep current flowing in a particular direction, preventing back EMF)

1 motor in between the four diodes

• How does it work?

If Q2 and Q3 are turned on, the left lead of the motor will be connected to ground, while the right lead is connected to the power supply. Current starts flowing through the motor which energizes the motor in (let's say) the forward direction and the motor shaft starts spinning. If Q1 and Q4 are turned on, the converse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning in that way. If less than full-speed (or torqe) operation is intended one of the switches are controlled in a PWM fashion. The average voltage seen by the motor will be determined by the ratio between the 'on' and 'off' time of the PWM signal.

Current flow in the forward direction	Current flow in the backward direction

• Example design and implementation notes? (courtecy of http://www.robotroom.com/HBridge.html)

The example provided on this page features:

• TTL/CMOS compatible Microchip/Maxim 4424/4427A or IXYS IXDN404 MOSFET driver chips that protect the logic chips, isolate electrical noise, and prevent potential short-circuits inherently possible in a discrete H-bridge.
• Schottky diodes to protect against overvoltage or undervoltage from the motor.
• Capacitors to reduce electrical noise and provide peak power to the driver chip.
• Pull-up resistors that prevent unwanted motor movement while the microcontroller powers up or powers down.

A diode-less version of this circuit successfully drove Bugdozer to mini-sumo victory. The more robust (diode protected) version is used on Sweet and Roundabout.

Warning: Direct motor driving with this chip is only possible for motors that draw less than 100 mA (TC4427A), 150 mA (TC4424), or 340 mA (IXDN404) under load. To determine if your motors qualify, use a multimeter to measure how much current your motor uses under load (for example, when actually driving your robot around) when the motors are connected directly to the battery (not through these chips).This chip is not really supposed to drive a motor by itself. If you find the chip gets very hot and the motor doesn't spin (or barely spins or stalls when loaded) then you need to have the chip drive some real power MOSFETs like it is supposed to. It's not that much more difficult and it really makes a huge difference in performance.