An H-Bridge is a circuit of 4 switches allowing to select the direction of current flowing through a part. The simplest H-Bridge consists of four manually controlled switches:
When using a simple H-Bridge it is important to prevent switches to short the supply voltage. In several commercially available H-Bridges additional circuits prevent accidental short circuits - beware!
Often "Half-bridges" are found where you'd expect an H-Bridge. They are almost the same - but only the left (or right) part of the Bridge (2 switches in series with their middle as output. Out of two half-bridges you can build an H-Bridge (also often referred to as Full-Bridge)
Use
- H-Bridges are often used to switch DC-motors in speed and direction.
- Generate AC-Currents where AC is needed.
- Turn a bi-stable relay on/off
- Dual-H-Bridges (or quadruple half bridges) Are useful to control bipolar stepper motors
Popular H-Bridges and Half-Bridges
L293D quad half-bridge: one of the cheapest part to build one or two H-Bridges. Has built in diodes, comes in a standard DIP package: conveniently useable in a Breadboard. 600 mA continuous current. Since it's a Half-Bridge there's no circuitry preventing shorting outputs.
SN754410: built in diodes, DIP package - very similar to the L293D but more power (1.1A per channel) and allowed pwm frequencies are higher. According to its dataseheet the SN754410 is usable as a replacement for the L293. (even better).
L298N: Well known and widely used Dual H-Bridge (2 H-Bridges in One IC). Comes as Powerdip IC - doesn't fit into breadboard without modifications. 2 A continuous current. Even if it's designed to be an H-Bridge it has no additional logic preventing shorts.
Links
H-Bridge using darlington Transistors - 5A http://www.bobblick.com/techref/projects/hbridge/hbridge.html
Figure-1 on this page from The Armory displays a circuit of an h-bridge using MOSFET transistors.
German Tutorial, connecting 2 Motors to an L293D: http://wiki.ctbot.de/index.php/L293D
English Tutorial, connecting one Motor to an L293D: http://itp.nyu.edu/physcomp/Labs/DCMotorControl
Circuit of an H-Bridge entirely made of discrete components like transistors, free wheeling diodes. (including NO shortage prevention): http://www.mikrocontroller.net/articles/H-Br%C3%BCcken_%C3%9Cbersicht