H-Bridge

An H-Bridge is a circuit which uses transistors to connect a pair of outputs either to the positive or negative rail. This can be used, for example, to control the direction of a DC motor, or as part of an inverter circuit or for other purposes where it is useful to be able to run the voltage in either direction.

They can be built from discrete transistors, or by using small ICs for low-power applications (such as the L293 or L298). These driver ICs are generally only really suitable for operation in the milliamp range, and will tend to "release the blue smoke" if used to drive "too much motor" (such as a motor which requires several amps).

Most discrete power transistors, however, have built-in resistors which will cause them to go into "thermal shutdown" if running too much current (and getting excessively hot). However, provisions may still need to be made to deal with flyback due to inductive kick or similar.

Various transistors can be used, such as BJT, Darlington Transistors, MOSFETs, or IGBTs. Each has different tradeoffs:
 * BJT
 * Reasonably cheap
 * Can handle moderate amounts of current (such as 10A with a MJE3055T).
 * However, heat and driving current are considerations.
 * Generally reasonably robust.
 * Darlington BJT
 * Also reasonably cheap (between the price of BJTs and MOSFETs)
 * Can also drive moderate current (for example, 5 A with TIP122 and TIP127)
 * Can be built from discrete transistors (such as 2N2222A+MJE3055T or 2N3907+MJE3055T)
 * Need far less drive current, so are used more like MOSFETs.
 * MOSFET
 * More expensive than BJTs or Darlingtons
 * Can drive considerably more current (say, 30A with an IRF540N)
 * discrete MOSFETs are available which can drive up to 180 amps.
 * Driven by voltage rather than current.
 * Generate much less heat at saturation than BJTs.
 * However, very voltage sensitive
 * Going outside of the voltage ranges will destroy the device immediately.
 * Very problematic with higher voltages or inductive loads.
 * For example, the IRF540N does 100V at Vds, and 20V at Vgs.
 * For an H-Bridge, this effectively limits its safe range to below about 18V.
 * IGBT
 * Comparably very expensive.
 * Can handle crazy load current and voltages.

In addition to transistors, electromechanical relays or contactors may also be an option:
 * They can be cheaper than transistors for switching large loads.
 * Contactors are available which can handle hundreds or thousands of amps.
 * However, switching is comparably pretty slow.
 * Generally, switching times are well into the millisecond range.
 * Switching on/off also degrades contacts, so they are not well suited for rapid switching (or PWM).
 * They are not generally an economical option for small or moderate loads.
 * Solid state options tend to be cheaper at this point.

Simple BJT H-Bridge:



A slightly more elaborate BJT H-Bridge: