what is the freewheeling diode

 

http://lmgtfy.com/?q=what+is+a+freewheeling+diode?

 

The current in an inductor (a relay coil, for example) is a bit like the motion of a heavy flywheel. Hard to get it going but once it's going it doesn't want to stop.

In an electronic circuit, the free-wheel diode gives this current somewhere to flow when the thing that controls the current switches off. While the current is slowly reducing.

 

There are various functional reasons for having free-wheel diodes, but one of the most basic is avoiding excessive rise of voltage or heating when a controlling or switch device turns off abruptly.

The tendency of a coil to maintain current flow results in an induced voltage proportional to the rate of change of current (e = -Ldi/dt), so attempting to reduce the current very quickly could give rise to a large and possibly damaging voltage. Even if the voltage were not great enough to be destructive in itself, a large proportion of the energy stored in the coil's magnetic field could be dissipated in the switching device, leading to excessive heating.

A free-wheel diode can let the coil current decay at a reasonable rate without excessive voltage being developed, while allowing the coil energy to be dissipated elsewhere than in the switch.