The H-Bridge is the link between digital circuitry and mechanical action. The computer sends out binary commands, and high powered actuators do stuff. Most often H-bridges are used to control rotational direction of DC motors. And unless you buy a potentially expensive motor-driver, you need an H-bridge to control any robot with a motor.
This is a quickly sketched H-Bridge circuit with supporting circuitry.
Use PNP transistors for the top two MOSFETs, and NPN MOSFETs for the bottom two (the ones connected to ground). Otherwise bad stuff could happen.
First lets talk about what a transistor is. These nifty chips revolutionized the electronics industry and you would be hardpressed to find something electronic that does not have at least a few thousand of these in them. So what do they do? They can control a flow of electrons by applying a voltage to them. The plumbing equivalent would be a water valve. By rotating the valve, a very large flow of water can easily be controlled.
So to operate a MOSFET, you apply a voltage to the gate (from your microcontroller), and suddenly a current of electrons passes through the other two pins. Connect a motor (M) in line with one of the pins and your robot is set to go. In the above schematic you will notice the letters A and B. These are your two control lines which you apply this logic voltage to. Since you have two pins, and only a binary control, there are four possible things that can happen.
A=0 B=0 : Nothing happens, the motor is turned off
Here is a ghetto visual graphic of the H-bridge logic chart:
So now lets talk about how to operate the MOSFET's. Basically all you need to do is attach the gate to your digital output of your controller. When the digital output is turned on, 5V will be applied to the gate, turning the MOSFET on. However it is better to amplify that 5V to a value higher and I will explain why. The gate voltage controls the MOSFET internal resistance. Zero voltage makes the resistance too high for it to work. A very high voltage has a very low resistance. Resistance leads to loss of energy thermally. This means your MOSFET will heat up and possibly burn out. Take a look at the MOSFET picture above and you will notice my finger print in it. That is what happens when you touch a hot MOSFET - pain! So although you do not need to amplify the gate voltage, it is best to do so. You should also put a heat sink on it.
Make sure the MOSFET you have has built in protection diodes. If not, install them on your circuit as shown. This is to prevent back currents from your DC motor. Also do not forget to put a small capacitor across the leads on your motor to reduce electronic noise and increase motor life. You might also want to refer to the tutorial on robot power regulation to help you design a better power source for your H-bridge.
It is also recommended to put a slow blow fuse after the power supply, resistors of a few 100 ohms on the gate logic, and the additional capacitors on your circuit as shown. This will prevent melting, large voltage surges, and high frequency emission.
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