Design
In an ideal flyback diode selection, one would seek a diode which has very large peak forward current capacity (to handle voltage transients without burning out the diode), low forward voltage drop, and a reverse breakdown voltage suited to the inductor's power supply. Depending on the application and equipment involved, some voltage surges can be upwards of 10 times the voltage of the power source, so it is critical to not underestimate the energy contained within an energized inductor.
When used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is removed, due to the continued circulation of current in the relay coil and diode. When rapid opening of the contacts is important, a low value resistor can be placed in series with the diode to help dissipate the coil energy faster, at the expense of higher voltage at the switch.
Schottky diodes are preferred in flyback diode applications, because they have the lowest forward drop (~0.2 V rather than >0.7 V for low currents) and the fastest (but still "soft") reverse voltage recovery.
Low-cost applications unconcerned with efficiency often use 1N4004, 1N4005 or 1N4007 power diodes as flyback diodes.
Read more about this topic: Flyback Diode
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