H-Bridge
An H-bridge is a circuit that allows you to control the motor in both forward and reverse. Notice the "H"-looking configuration in the next graphic. Relays (which are little switches activated by a control current) configured in this fashion make an H-Bridge.
The "high side drivers" are the relays that control the positive voltage to the motor. This is called �sourcing current�.
The "low side drivers" are the relays that control the negative voltage to sink current to the motor. "Sinking current" is the term for connecting the circuit to the negative side of the power supply, which is usually ground.
When relays A and D are activated, the current flows from the top left corner of the circuit to the bottom right, making the motor turn in the forward direction. When B and C are activated, current flows from the top right corner to the bottom left, making the motor turn in the backwards direction.
Transistors
The transistor was developed at Bell Laboratories in 1948. Large scale commercial use didn't come until much later owing to slow development. Transistors used in most early entertainment equipment were the germanium types. When the silicon transistor was developed it took off dramatically. The first advantages of the transistor were relatively low power consumption at low voltage levels which made large scale production of portable entertainment devices feasible.
Transistors work on the principle that certain materials e.g. silicon, can after processing be made to perform as "solid state" devices. Any material is only conductive in proportion to the number of "free" electrons that are available. Silicon crystals for example have very few free electrons. However, if "impurities" (different atomic structure - e.g. arsenic) are introduced in a controlled manner then the free electrons or conductivity can be increased. By adding other impurities such as gallium, an electron deficiency or hole is created. As with free electrons, the holes also encourage conductivity and the material is called a semi-conductor. Semiconductor material which conducts by free electrons is called n-type material while material which conducts by virtue of electron deficiency is called p-type material.
Transistors come in two �flavors�. NPN and PNP, which depict the location of the p-layers and the n-layers. The NPN transistor is more common. They have three legs, the collector, the emitter and the base. The leg sticking to the left in these schematic diagrams is the base, which acts as the �switch� that activates the transistor. And the arrow shows the direction of current in the emitter leg. This means the leg sticking up is the collector.
An H-Bridge Using Transistors
Transistor H-Bridges
It is also possible to create this same H-bridge effect using transistors. They are used as solid-state switches, instead of the relays in the earlier example.
These solid state circuits provide power and ground connections to the motor, as did the relay circuits. The high side drivers need to be current "sources" which is what PNP transistors are good at. The low side drivers need to be current "sinks" which is what NPN transistors are good at.
Note that if you turn on the two upper circuits, the motor resists turning, so you effectively have a breaking mechanism. The same is true if you turn on both of the lower circuits. This is because the motor is a generator and when it turns it generates a voltage. If the terminals of the motor are connected (shorted), then the voltage generated counteracts the motors freedom to turn. It is as if you are applying a similar but opposite voltage to the one generated by the motor being turned. Vis-�-vis, it acts like a brake.
And never, turn on the transistors on the same side. It is important that the four quadrants of the H-Bridge circuit be turned on and off properly. When there is a path between the positive and ground side of the H-Bridge, other than through the motor, a condition exists called "shoot through". This is basically a direct short of the power supply and can cause semiconductors to become ballistic, in circuits with large currents flowing. It could blow your transistors.
You should be nice to your transistors, so you would usually add diodes to catch the back voltage that is generated by the motor's coil when the power is switched on and off. This flyback voltage can be many times higher than the supply voltage! If you don't use diodes, you could burn out your transistors. The diagram on the right shows you where to put them.
Transistors, being a semiconductor device, will have some resistance, which causes them to get hot when conducting much current. This is called not being able to sink or source very much power, i.e.: Not able to provide much current from ground or from plus voltage.